sauvegarder, conserver, transmettre au public ...... gather on mobile pack-ice to breed, while Weddell seals are a southern phocid that deal ..... analysis function (a Hamming window with a sampling rate of 2048 points/1024 bins was used).
CALLING DEPTH AND RELATED ATTRIBUTES OF HARP (PAGOPHILUS GROENLANDICUS) AND WEDDELL (LEPTONYCHOTES WEDDELLII) SEAL UNDERWATER VOCALIZATIONS by Hilary Bernice Moors
B.Sc. University of New Brunswick, Saint John
A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science
Supervisor: John M. Terhune, Lie. Scient. Biology Examining Board:
Dr. L. Best Dr. J. Kieffer
This thesis is accepted by the Dean of Graduate Studies
THE UNIVERSITY OF NEW BRUNSWICK October, 2004 © Hilary Bernice Moors, 2004
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ABSTRACT
Harp (Pagophilus groenlandicus) and Weddell (Leptonychote weddellii) seal calling depth was investigated using a small vertical hydrophone array. Both rough depth approximations (< 35 m, ~ 35 m or > 35 m) and more precise calling depth estimates (e.g., ± 10 m) were obtained. Both species were found to vocalize throughout the water column, although they called predominately from shallower depths (generally < 35 m). The seals also avoided sea-ice interference to some extent by diving well below the ice surface (to depths > 10 m) to emit the majority of their vocalizations. The vocalizations did not change over depth with respect to call types, subtypes, number o f elements, rhythm patterns or total call duration, (or with respect to season and photoperiod for the Weddell seals). Frequency (kHz) of the calls did not increase with depth, indicating that the seals vocalize likely using the vocal cords of the larynx.
I would like to thank the Chateau Madelinot for their hospitality and logistical support during the harp seal field work, and the Australian Antarctic Division for providing logistical support and funding for the Weddell seal fieldwork. The National Science and Engineering Research Council (NSERC) provided funding for this research with a Discovery Grant to Dr. Terhune and a CGS-M to myself. This project was also supported by a Board o f Governors Merit Award for Graduate Studies (UNB), a Vaughan Graduate Fellowship (UNB), the J.S. Little Fellowship (UNB) and a Scholar Award (Golden Key International Honours Society). Thanks to my SAEL lab partner, Phil Rouget, for getting me some great recordings, for the conceptual support, and for his never-ending wit. A special thanks to Ian Butts for all of the support, and to all of the other UNBS J faculty and graduate students who helped me along the way. Many thanks to Caryn Thompson for her statistical advice, and to my supervisory committee, Dr. Alexander Wilson and Dr. Steve Turnbull, for their comments and suggestions. Thanks to my examiners for taking the time to review this thesis. Finally, a BIG thanks to my supervisor, Dr. Jack Terhune, for all of his support, advice, idea’s and, of course, for giving me the once in a lifetime experience of being able to walk on water with the seals.
ACKNOWLEDGEMENTS............................................................................................. iii TABLE OF CONTENTS................................................................................................. iv LIST OF TABLES......................................................................................................... viii LIST OF FIGURES........................................................................................................... x I. INTRODUCTION..........................................................................................................1 1. Breeding behavior and underwater vocalizations................................................. 1 la. Harp seals......................................................................................................... 1 lb. Weddell seals...................................................................................................2 lc. Difficulties in determining specific functions of underwater vocalizations...................................................................................................5 2. Impact of sea-ice on communication................................ .................................... 5 2a. Characteristics of pack-ice and implications for harp seal vocal behavior............................................................................................................ 5 2b. Characteristics of fast-ice and implications for Weddell seal vocal behavior............................................................................................................ 6 2c. Sea-ice interference with signal transmission............................................... 7 3. Calling depth and frequency (kHz')........................................................................ 8 4. Acoustic localization techniques...........................................................................10 5. Objectives..............................................................................................................13 II. MATERIALS AND METHODS...............................................................................16
1. Underwater recordings.......................................................................................... 16 la. Harp seal recordings...................................................................................... 16 lb. Weddell seal recordings................................................................................16 lc. Recording equipment and set-up.................................................................. 20 2. Acoustic analysis...................................................................................................21 2a. Depth estimates............................................................................................. 21 2b. Sampling protocol......................................................................................... 26 2c. Measurements obtained.................................................................................27 3. Statistical analysis................................................................................................. 28 3a. Harp seal analysis.......................................................................................... 29 3b. Weddell seal analysis.................................................................................... 33 III. RESULTS................................................................................................................... 36 1. Limitations of point depth measures....................................................................36 2. Harp seal results.....................................................................................................39 2a. Sample sizes and types of depth measures...................................................40 2b. Overall depth results.....................................................................................40 2c. Call type categories/subtypes and depth......................................................45 2d. Number o f elements (NOE) and depth.......................................................53 2e. Rhythm patterns within multiple element calls and depth......................... 56 2f. Total call duration and depth.........................................................................56 2g. Frequency (kHz) and depth...........................................................................58 3. Weddell seal results.............................................................................................. 63 3a. Sample sizes and types of depth measures...................................................66
3b. Overall depth results.....................................................................................66 3c. Call type categories/subtypes and depth......................................................71 3d. Number of elements (NOE) and depth........................................................83 3e. Rhythm patterns within multiple element calls and depth......................... 86 3f. Total call duration and depth.........................................................................87 3g. Frequency (kHz) and depth...........................................................................90 IV. DISCUSSION............................................................................................................ 96 1. Limitations in determining point calling depth using Cato’s calculations....... 96 2. Types of depth measures obtained and analyses types.....................................101 3. Trends in call types, subtypes, NOE, rhythm patterns and total duration with depth.................................................................................................................... 102 3a. Harp seals.....................................................................................................102 3b. Weddell seals.............................................................................................. 104 3c. Implications for vocal behavior................................................................. 106 4. Trends in frequency (Hz) over depth................................................................. 107 4a. Harp seals.....................................................................................................107 4b. Weddell seals.............................................................................................. 108 4c. Implications for how the seals produce underwater vocalizations.......... 109 5. Overall calling depth trends................................................................................110 5a. Harp seals.....................................................................................................110 5b. Weddell seals.............................................................................................. I l l 5c. Comparisons of calling depths of the two species....................................I l l 6. Conclusions.......................................................................................................... 113
LITERATURE CITED................................................................................................. 114 APPENDIX 1................................................................................................................. 125 VITA............................................................................................................................... 127
Table 1. Harp seal call type categories and subtypes................................................... 46 Table 2. Summary o f ANOVA results for harp seal calls examining the mean number o f calls of each call type category and subtype emitted within each depth category of the BROAD, ROUGH, CLEAR1 and CLEAR2 analyses............48 Table 3. Summary of ANOVA results for harp seal calls examining the mean number of elements, rhythm patterns and total call duration of calls emitted within each depth category of the BROAD, ROUGH, CLEAR1 and CLEAR2 analyses................................................................................................................ 54 Table 4. Summary of ANOVA results for harp seal calls examining the mean frequency (kHz) of calls emitted within each depth category and frequency (kHz) of calls o f each call type and subtype emitted at each depth category of the BROAD, ROUGH, CLEAR1 and CLEAR2 analyses................................61 Table 5. Weddell seal call type categories and subtypes............................................. 72 Table 6. Summary of ANOVA results for Weddell seal calls examining the mean number of calls of each call type category emitted within each depth category of the BROAD, ROUGH and SINGLE analyses.............................................. 76 Table 7. Overall numbers of uncommon Weddell seal call types emitted at each depth category of the BROAD, ROUGH and SINGLE analyses...............................77 Table 8. Summary of ANOVA results for Weddell seal calls examining the mean number of calls of each subtype category emitted within each depth category of the BROAD, ROUGH and SINGLE analyses.............................................. 80
Table 9. Overall numbers of uncommon Weddell seal call subtypes emitted at each depth category of the BROAD, ROUGH and SINGLE analyses.................... 82 Table 10. Summary o f ANOVA results for Weddell seal calls examining the mean number of elements within the calls and total call duration emitted within each depth category o f the BROAD, ROUGH and SINGLE analyses.................... 84 Table 11. Summary of ANOVA results for Weddell seal calls examining the overall mean frequency (kHz) of calls and the frequency (kHz) of calls of each type emitted within each depth category of the BROAD, ROUGH and SINGLE analyses................................................................................................................ 91 Table 12. Summary o f ANOVA results for Weddell seal calls examining the mean frequency (kHz) of calls of each subtype emitted within each depth category o f the BROAD, ROUGH and SINGLE analyses.............................................. 95
Figure 1. Diagrammatic representation of the method of determining the location of an underwater sound produced by an animal, as described by Cato (1998).............................................................................................................. 14 Figure 2. Map of harp seal recording sites.................................................................... 17 Figure 3. Map of Weddell seal recording sites.............................................................18 Figure 4. Spectrograms of a harp seal vocalization on the 10 and 60 m hydrophone channels illustrating ATD and AD differences............................................ 23 Figure 5. Uncertainty in the point depth estimates when ATD is altered by ± 1 msec................................................................................................................. 37 Figure 6. Uncertainty in the point depth estimates when AD is altered by ± 1 dB.................................................................................................................... 38 Figure 7. Mean number of ROUGH, SINGLE and CLEAR calls analyzed from the harp and Weddell seal recordings................................................................. 41 Figure 8. Mean number of harp seal calls made at each depth category of the BROAD, ROUGH, CLEAR1 and CLEAR2 analyses.................................42 Figure 9. Point depth estimates obtained from harp seal CLEAR calls...................... 43 Figure 10. Mean number of harp seal calls of each call type emitted within each depth category and overall percent composition of each call type within
each depth category for the BROAD analysis.............................................. 49
Figure 11. Mean number of harp seal calls of each call subtype emitted within each depth category and overall percent composition of each subtype within each depth category for the BROAD analysis.............................................. 52 Figure 12. Mean number of harp seal calls of each number of elements category emitted within each depth category and overall percent composition of each number of elements category within each depth category for the BROAD analysis............................................................................................................ 55 Figure 13. Mean number o f harp seal calls of each rhythm pattern emitted within each depth category of the BROAD and ROUGH analysis........................ 57 Figure 14. Scatterplot and regression results o f total call duration and point depth estimates of harp seal CLEAR calls.............................................................. 59 Figure 15. Mean frequency (kHz) of harp seal calls of each call type emitted within each depth category of the BROAD analyses.............................................. 62 Figure 16. Scatterplot and regression results of frequency (kHz) and point depth estimates of harp seal CLEAR calls.............................................................. 64 Figure 17. Scatterplot and regression results of frequency (kHz) and point depth estimates of harp seal call CLEAR calls of type 14 (grunts)..................... 65 Figure 18. Mean number of Weddell seal calls made at each depth category o f the BROAD, ROUGH and SINGLE analyses................................................... 68 Figure 19. Overall number of Weddell seal calls made at each depth category of the CLEAR1 and CLEAR2 analyses.................................................................. 69 Figure 20. Point depth estimates obtained from Weddell seal CLEAR calls............ 70
Figure 21. Mean number of Weddell seal calls of each call type emitted within each depth category and overall percent composition of each call type within each depth category for the BROAD analysis.............................................. 75 Figure 22. Mean number of Weddell seal calls of each call subtype emitted within each depth category and overall percent composition of each subtype within each depth category for the BROAD analysis..................................81 Figure 23. Mean number of Weddell seal calls of each call number of element category emitted within each depth category and overall percent composition of each number of element category within each depth category for the BROAD analysis................................................................. 85 Figure 24. Mean number of Weddell seal calls of each rhythm pattern emitted within each depth category of the BROAD, ROUGH and SINGLE analyses........................................................................................................... 88 Figure 25. Scatterplot and regression results of total call duration and point depth estimates of Weddell seal CLEAR calls........................................................89 Figure 26. Mean frequency (kHz) of Weddell seal calls of each call type emitted within each depth category of the BROAD analysis.................................... 93 Figure 27. Scatterplot and regression results of frequency (kHz) and point depth estimates of Weddell seal CLEAR calls........................................................97 Figure 28. Scatterplot and regression results of frequency (kHz) and point depth estimates of Weddell seal call CLEAR calls of type WD (whistledescending calls)........................................................................................... 98
I. INTRODUCTION
1.
Breeding behavior and underwater vocalizations Harp (Pagophilus groenlandicus) and Weddell (Leptonychotes weddellii) seals
are gregarious, pagophilic phocids that gather annually on sea-ice to form whelping and breeding groups. Although the two species do share some similar characteristics with each other in regards to their annual life cycles, different social behaviors and reproductive strategies are exhibited by each. Harp seals are a northern phocid that gather on mobile pack-ice to breed, while Weddell seals are a southern phocid that deal with stable fast-ice conditions during their breeding season. The life cycle of both is closely associated with seasonal ice formation and the seals have adapted behaviors that allow them to live and reproduce in their respective sea-ice habitats (Kooyman 1981; Lavigne and Kovacs 1988).
la. Harp seals Each year, harp seals of the Northwestern Atlantic population migrate south from their summer feeding grounds off the western coasts of Greenland and within the eastern Canadian Arctic, to their winter breeding grounds in pack-ice regions off of the eastern coasts of Newfoundland and Labrador and within the Gulf of Saint Lawrence (Lavigne and Kovacs 1988, Sergeant 1991). The females congregate on sea-ice in these areas by late February or early March to give birth to their pups, while the males spend most of their time in the water. The pups are weaned by mid to late March and the breeding period occurs immediately afterward. Harp seals tend to mate promiscuously without any
long-term pair bonding, although courtship behaviors have been observed. Both courtship and copulation take place in the water (Lavigne and Kovacs 1988; Sergeant 1991). The seals remain in the pack-ice regions until the ice begins to melt (by late March to early May), at which time they begin migrating back to their summer feeding grounds (Lavigne and Kovacs 1988; Sergeant 1991). Vocal communication appears to play an important role in the harp seal life cycle. The seals have a large vocal repertoire and increase vocalizations with the onset of breeding season (M0hl et al. 1975). Underwater recordings of harp seals made during this time display a rich variety of sounds and at least 26 different underwater call types have been described (M0hl et al. 1975; Terhune 1994; Serrano 2001). Both males and females vocalize (Serrano 2001). The seals are thought to be relatively quiet during the remainder of the year (M0hl et al. 1975). Harp seal underwater vocalizations appear to be emitted in conjunction with herd formation and courtship behavior. It has been suggested that they may present a double function wherein longer range calls assist in herd formation by attracting conspecifics, while shorter range calls are thought to be used for courtship and mating purposes (Terhune and Ronald 1976, 1986). More specific functions of the different calls the seals produce are unknown.
lb . Weddell seals Weddell seals have a circumpolar distribution around Antarctica and surrounding islands (Bertram 1940). Fast-ice begins to form around the Antarctic continent with the onset of the Austral winter (in June). The distribution of Weddell seals during this time
has been widely debated. Some studies propose that the majority of the seals move to areas outside the fast-ice regions during the winter to avoid the problems of living under the solid fast-ice (Smith 1965; Lugg 1966; Kooyman 1968; Green and Burton 1988). Other studies suggest that the seals remain within the limits of the fast-ice throughout the winter in close proximity to their breeding grounds (Bertram 1940; Rouget 2004). Weddell seals are known to maintain breathing holes in fast-ice using specially adapted teeth to abrade the ice throughout the winter (Kooyman 1981). During the Austral spring (October-November), female Weddell seals gather in small groups at traditional whelping and breeding grounds to give birth to their pups. Males spend the majority of their time in the water during the pupping period (Kooyman 1981; Thomas and Kuechle 1982). The pupping period lasts from October to mid-November and the pups are weaned when they are approximately six-weeks old (Kooyman 1981). The stable fast-ice environment enables males to establish fixed underwater territories in the vicinity of breathing holes (Kooyman 1981). The males defend their territories aggressively during the breeding season and fighting is often observed (Stirling 1969; Rouget 2004). The establishment of underwater territories has resulted in a polygynous breeding system for Weddell seals (Kooyman 1981). Courtship and copulation take place in the water near the end of lactation (Stirling 1969), and the breeding groups disappear in late December when the ice begins to melt (Green and Burton 1988). Like harp seals, Weddell seals have an extensive vocal repertoire and many different call types produced by the seals have been described (Thomas and Kuechle 1982; Thomas et al. 1988; Pahl et al. 1997; Abgrall et al. 2003). Studies of in-air closed mouth calls made by Weddell seals indicate that males and females produce similar types
of sounds (Terhune et al. 1994). Geographical separation between different breeding groups is reflected by differences in the call types emitted by the seals (Thomas et al. 1988; Abgrall et al. 2003). The underwater vocalizations of Weddell seals are thought to play an important role in social communication during the breeding season and underwater calls are made frequently near whelping colonies (Kooyman 1981; Thomas et al. 1988). Calling rates of the seals decrease sharply after mating has occurred (Green and Burton 1988; Thomas et al. 1988). Territorial males are vocal underwater both prior to and during the mating period (Thomas et al. 1988; Pahl et al. 1997; Rouget 2004). Some call types, such as trills (long, frequency (kHz) sweeping calls), are thought to be male specific territorial defense calls (Oetelaar et al. 2003). It has also been suggested that trills may be associated with advertisement of breeding condition (Thomas et al. 1983). The function of the different types of calls emitted by Weddell seals have been investigated using playback experiments. Chug calls (pulsed, decreasing frequency (kHz) calls) are thought to have an aggressive function. The different timing patterns within multiple element chug calls appear to be indicative of aggression level. Slow chugs with a constant timing pattern and consistent frequency (kHz) throughout the calls appear to be low intensity threat calls; while chugs that increase in timing and frequency (kHz) throughout the calls appear to be high intensity threat calls (Thomas et al. 1983). Chirp calls (descending whistle calls) probably have a submissive function and/or may be used for navigational purposes (Watkins and Schevill 1968; Thomas et al. 1983). Little is known about the specific functions of other call types produced by Weddell seals.
lc. Difficulties in determining specific functions of underwater vocalizations In order to fully understand social interactions involving signal and response, studies of animal communication and social behavior ideally use methods where signals and actions can be associated with individuals (Thomas et al. 2002). In aquatic environments with limited visual range, sender detection is almost always a problem and studies often link calls to a general group of animals rather than to specific individuals (Janick et al. 2000). This is true of harp and Weddell seal underwater communication studies, which typically are only able to connect underwater vocalizations to a group of seals in a general area, and not to individuals. Due to the fact that neither sender nor receiver can be identified from underwater acoustic recordings alone, the exact function of the different harp and Weddell seal calls is unknown. Specific vocalizations cannot be linked to individual seals, to a particular sex (but see Oetelaar et al. 2003), or to any specific behaviors. Because of this, little is known about the behaviors associated with harp and Weddell seal underwater calls.
2. Impact of sea ice on seal communication 2a. Characteristics of pack-ice and implications for harp seal vocal behavior Pack-ice is a very unstable medium. It consists of large free-floating sheets of seaice (floes) formed annually, that move with wind and water currents (Lavigne and Kovacs 1988). The ice sheets often shift position, causing floes to break apart or collide together and creating rafted areas and other inconsistencies along the ice surfaces. Because the ice is continually moving and may drift several kilometers in a single day, harp seals are subjected to constantly changing surroundings. The herd does not have a
stable location throughout the whelping and breeding season, and breeding success must be influenced to some extent by the ability of the seals to locate one another. Sound is the only effective means of long distance communication underwater. Distant seals could likely find the herd using acoustic cues (Terhune and Ronald 1986). It would be valuable for the seals to emit calls in ways that would maximize sound transmission, not only for effective communication to individuals within the herd, but to help in the herd formation process itself.
2b. Characteristics of fast-ice and implications for Weddell seal vocal behavior The Antarctic fast-ice consists of firm sea-ice (usually > 2 m thick) formed annually, that remains solidly fastened to the land throughout the Austral winter and spring (June-November). The fast-ice in areas around Antarctica can extend several hundred kilometers out into the ocean where they are bordered by less stable pack-ice (Zwally et al. 2002). Because of the stability of fast-ice, the locations of Weddell seal breeding groups do not change throughout the breeding season and males are able to establish underwater territories. Many of the vocalizations emitted by the seals appear to be related to territorial defense and submissive behaviors (Watkins and Schevill 1968; Thomas et al. 1983) and are probably intended for short-range transmission. However, underwater vocalizations may still be important signals to distant seals indicating the location of a breeding group, and some calls probably serve long-range communication functions as well.
2c. Sea-ice interference with signal transmission Sea-ice produces ambient noise in the ocean through various processes in including ice sheets rubbing together, ridge-building events, thermal cracking or fracturing, and ice break-up events (Milne and Garlton 1964; Payne 1964; Greening et al. 1992; Xie and Farmer 1992; Lewis 1994; Ye 1995). Most sounds produced by sea-ice are singular events that vary in frequency and duration and typically propagate from a point source. Thus, most underwater ice noise would be loudest directly beneath the sound source and would decrease in amplitude with depth. High levels of background noise produced by the sea-ice have the potential to mask underwater acoustic communications emitted by the seals. Sea-ice can also cause a high degree of signal degradation and may pose a problem for seals vocalizing underwater. Sea-ice is not a homogenous medium and its sound transmission properties have not been well characterized (Farmer and Xie 1994). Although sea ice is only frozen seawater, its lower density and higher sound speed result in an acoustic impedance twice that of seawater, causing sound energy to be reflected at the sea/ice interface (Albers 1965). If a seal is vocalizing near the water surface in a narrow tide crack or just under the ice in a rafted area, heterogeneous undersurfaces of the ice could potentially interfere with the horizontal propagation of the sound through water (Diachok 1976). Heterogeneous surfaces on the sea floor may also present similar limitations for sound transmission at deeper depths. Seals could therefore maximize transmission range of their signals by vocalizing well below the undersurface of the ice or well above the bottom substrate.
We do not know if ice-breeding seals such as harp and Weddell seals have adjusted their calling behavior to overcome the potential problem of ice interference. Specific call types or call attributes, such as the number of elements in a call, total call duration or rhythm pattern within a call, may be emitted within certain depth ranges depending on their function and whether they are meant for short or long distance communication. It would be beneficial for the seals to emit long distance communications, (such as those intended for attracting conspecifics to the herd/group), at depths that would maximize transmission range of the signals (well below the sea/ice interface). Calling depth would not be as important for communications that only need to be transmitted over a short range (such as territorial defense or courtship calls). Determining at what depth the seals are calling from in the water column may give valuable insight to the function of the different calls they produce.
3. Calling depth and frequency (kHz) As seals dive underwater, their body structure changes to accommodate high pressure. Air spaces within the body of deep-diving animals (such as penguins, whales and seals) are known to decrease in volume with increasing pressure, even to the point of collapse (Kooyman and Ponganis 1998). Compression of air chambers in the bodies of Weddell and elephant (Mirounga angustirostris) seals has been observed with increasing pressure. The respiratory aveoli of the two species showed considerable decreases in size at depths of less than 30 m (pressures of less than 4 atmospheres of pressure; ATM). The more rigid trachea began compression at depths of less than 50 m (less than 6 ATM of
pressure; Kooyman et al. 1970). Complete alveolar collapse occurred in Weddell seals at depths exceeding 28 m (about 3.7 ATM of pressure; Falke et al. 1985). Seals are thought to vocalize by vibrating vocal cords and resonating sound out through air spaces in the throat (Pabst et al. 1999). Terhune et al. (1994) observed that sound was produced from the larynx area of the throat in Weddell seals producing in-air closed mouth calls. Pierard (1969) presents anatomical evidence from a Weddell seal that suggests sound is produced by air moving between the larynx and trachea, and through vibrations of the air-filled trachea (Kooyman et al. 1970). The frequency (kHz) of many mammalian and avian vocalizations is influenced by the size of resonating chambers present in the body (Bradbury and Vehrencamp 1998). Sound production systems such as these are known as response-driven systems because frequency (kHz) of the vocalizations is not controlled by the vibrations of the sound production organ itself, but by the properties of surrounding air chambers and structures that the vibrations excite. Generally, smaller chambers resonate at shorter wavelengths and thus higher frequency (kHz) sounds are emitted (Bradbury and Vehrencamp 1998). If harp and Weddell seals produce sounds by means of a response-driven system, their vocalizations would increase in pitch over depth as air cavities in the body become compressed under increasing pressure. This phenomenon occurs in sperm whale (Physeter macrocephalus) clicks, which increase in frequency (kHz) as depth of the animal increases, due to compression of air cavities. Frequency centroids of clicks increased from approximately 2 and 10 kHz to 3 and 15 kHz as the depth of the animal increased from 0-700 m (Thode et al. 2002). It is possible that frequency (kHz) of the harp and Weddell seal vocalizations may be controlled mainly by the vocal cords. This is
known as a source-driven system in which vibrations of the sound production organ itself controls the pitch of the vocalization and surrounding structures and air chambers have little effect on the frequency (kHz) produced (Bradbury and Vehrencamp 1998). If this is the case with harp and Weddell seals, frequency (kHz) of their calls would not change over depth. By determining if frequency (kHz) of vocalizations increase with increasing depth of calling seals, the mechanism behind how harp and Weddell seals produce their calls can be better understood.
4. Acoustic localization techniques Several solutions have been proposed to solve the problem of locating an animal vocalizing underwater including isolating animals, identifying vocalizing animals by bubble stream emissions, attaching tags to animals, video photography and acoustic localization (Thomas et al. 2002). Acoustic localization is a non-invasive, readily applied method that can be used to monitor the vocal activity of several individuals at once (Janick et al. 2000). Various methods of locating an underwater signal source using sounds detected by hydrophones have been described (e.g., Spiesberger and Fristrup 1990; Cato 1998; Aubauer et al. 2000; Clarke and Ellison 2000; Wahlburg et al. 2001). The most commonly used methods require measurements of differences in the arrival time of a sound on several accurately placed hydrophones of a large aperture array. Arrival time differences provide information about the direction (bearing) from which a sound originates. By determining the arrival time difference of a sound on three or more hydrophones pairs, intersecting cross bearings can be used to determine the location of
the sound source (Cato 1998). The location accuracy using this method is dependant on the precision of sound velocity measurements and measurements of the time of arrival difference, as well as receiver position and source-array geometry (Walburg et al. 2001). Some studies have reported sound source locations to be determined within a few meters (e.g., Thomas et al. (2002) reports localization errors of less than 1.5 m), while other studies report large errors (e.g., Wahlburg (2002) reports localization errors exceeding 100 m). Acoustic localization of animals producing underwater signals using large aperture arrays have been widely documented, particularly for whale and dolphin studies (e.g., Cummings and Holliday 1985, 1987; Freittag and Tyack 1993; Norris et al. 1999; M0hl et al. 2000; Van Parijs et al. 2000; Thomas et al. 2002; Wahlberg 2002). Most of these studies use horizontal hydrophone arrays and typically do not give information on the depth of the vocalizing animal (although three dimensional dive trajectories of sperm whales have been mapped using acoustic localization from large vertical hydrophone arrays; Wahlberg 2002). There are few studies that examine the calling depth of pinnipeds. Evans et al. (2004) used time-depth recorders and video-photography to investigate the depth of vocalizing Weddell seals. The majority of the vocalizations examined were produced by two individuals located far away from breeding colonies. The vocalizations (which included chirps, chugs, clicks and trills), were associated with approaching breathing holes, looking at the under-ice surface and examining a novel object (Evans et al. 2004). M0hl et al. (1975) report that the depth of clicks produced by harp seals (examined by determining arrival time differences of clicks and their echos on a single hydrophone), occurred anywhere between the water surface and bottom of the
sea. No precise depth estimates were provided and depth of other call types was not examined. This is the only study that has mentioned depth of harp seal calls. There are some inherent problems with using large hydrophone arrays to localize sound. These methods are expensive and often complex (e.g., M0hl et al. 2000). They are also impractical for use in the unstable pack-ice environment where harp seals are found and the isolated fast-ice areas where Weddell seals are located. Cato (1998) describes a method of determining source levels of sounds that may present a more practical method of locating animals found in environments less ideal for the deployment of large hydrophone arrays. This method requires an array of only two hydrophones and involves looking at the time of arrival and amplitude measures of a signal recorded on the two channels. If two hydrophones (Hi and H 2) are placed in the water to form a small vertical array and an animal (a seal) produces an underwater sound, assuming that the sound spreads spherically, it will reach the hydrophone closest to the animal first and at a higher amplitude. By comparing the relative arrival time difference (ATD) and amplitude difference (AD) of a call on the two hydrophone channels (by determining the channel on which the call occurs first and loudest), a rough approximation of the animal’s depth can be obtained; the animal is either closer to Hi or closer to H 2. If an accurate measure of the ATD and AD of the call on both hydrophone channels can be obtained, the distance of the animal from the closest hydrophone (r0 and the angle (or bearing) of the animal from the axis of the two hydrophones (0) can be calculated based on the distance between the two hydrophones (S), the speed of sound in the water (C0), and the assumption that the sound is spreading spherically (Cato 1998). It is important to note that the calculations
given by Cato (1998) do not allow for discrimination between left and right, therefore the exact location of the calling animal cannot be determined. Rather, the ri and 0 values describe a circle and the animal can be calling at any point along the circumference (Cato 1998). In the case of a vertical hydrophone array, the circle of possible positions will be horizontal in the water column and thus the depth of the calling seal can be obtained (Figure 1). This method of determining distance and bearing of a sound source location becomes limited as ATD and AD measures approach zero. This occurs as a sound source moves farther away from the hydrophone array or as a sound source approaches the midpoint between the two hydrophones. The accuracy of the ATD measures, and to a much greater degree the AD measures, also affect the accuracy of the resulting source location (Cato 1998). Cato (1998) notes that these methods of obtaining a source distance and bearing are accurate over a much smaller range than reported for more conventional methods of acoustically locating sound, however, the value of the method lies in its simplicity as acceptable levels of accuracy can be obtained by using only two hydrophones.
5. Objectives The first and primary objective of this study was to determine at what depth harp and Weddell seals vocalize in the water column. The second objective was to determine if the vocalizations produced by the seals changed over depth with respect to the call types and subtypes emitted, the number of elements in the calls, the rhythm patterns within multiple element calls, and the total duration of the calls.
Figure 1. Diagrammatic representation of the method of determining the location of an underwater sound produced by an animal, as described by Cato (1998). By measuring the arrival time difference (ATD) and amplitude difference (AD) of a sound recorded on two hydrophones (HI and H2) separated by a known distance (S), the distance of the animal (seal) from the closet hydrophone (ri), and the angle of the animal from the axis of the two hydrophones (0) can be determined. These values describe a circle and P (the sound source location) can be located anywhere along the circumference (Cato 1998). In the case of a vertical hydrophone array, as pictured above, this circle is horizontal in the water column and thus represents depth of the calling animal.
Little is known about the behavior of harp and Weddell seals underwater due to the logistical problems of trying to view the animals. For both of these objectives, examining calling depth of the seals would give insight to the behavior of the animals under the ice, and would give some indication as to what extent they avoid sea-ice interference. Trends in the calling depth of harp seals were investigated from a single breeding location/group (the Gulf herd), over a single biological period in the seals’ life cycle (the pupping/breeding period), and during daylight hours only. For the Weddell seal portion of the study, trends in calling depth were determined from two different breeding locations/groups, during two biological periods (the pre-pupping period and pupping/breeding period), and over two different light conditions (light and dark). This allowed seasonal and diurnal trends to be considered in the Weddell seal analyses. The third and final objective of this study was to determine if frequency (kHz) of the seal calls increased with depth. By examining frequency (kHz) trends over depth it can be determined if the seals produce sound by means of a predominately source-driven system or response-driven system. The functional significance of most harp and Weddell seal vocalizations is unknown. By determining where the seals vocalize in the water column and what call types they use at different depths, we may be able to better understand the behavioral function of the calls. Examining frequency (kHz) changes of the calls over depth would indicate the mechanism by which seals vocalize.
II. MATERIALS AND METHODS
1. Underwater recordings la. Harp seal recordings Underwater digital audiotape (DAT) recordings of harp seals were obtained near the Magdalen Islands within the Gulf of Saint Lawrence, Canada (Figure 2), during the 2000 and 2003 harp seal pupping and breeding season (February-March; Sergeant 1991). A single recording made on 8 March 2000 and five recordings made from 1-8 March 2003 (all from different locations) were examined. Recording sessions varied from 1-4 h and were all made during daylight hours (8 am - 4 pm). All recordings were made on ice floes occupied by harp seals. The hydrophones were placed at least 10 m away from the nearest seals and lowered into the water through breathing holes or through large cracks or leads in the ice. Thousands of seals, both adults and pups, were observed on the ice during the recordings, but it was not possible to determine the number of seals vocalizing underwater. The sex and age of the vocalizing seals also could not be determined, but it is known that both mature males and females are vocally active (Serrano 2001). No other species were observed on the floes during the recording sessions.
lb . Weddell seal recordings Underwater DAT recordings of Weddell seals were obtained from two breeding groups (SEAL MO and SPA) located in Holme Bay near Mawson Station, Eastern Antarctica (Figure 3). The two sites were separated by a distance of 7 km. The recordings
Figure 2. Map of harp seal recording sites. The arrow indicates the Magdalen Islands and the stars represent the approximate areas where recordings were obtained (A = recordings made on 1-4 March 2003, B = recordings made on 5-8 March 2003). The exact location of the March 2000 recording is unknown, although it was made in the same general area as the March 2003 recordings.
Figure 3. Map of Eastern Antarctica showing Mawson Station (indicated by the star) and Holme Bay (indicated by the arrow). The Weddell seal recordings were obtained from two sites (SEAL MO and SPA) located 7 km apart within the Holme Bay region.
were made prior to and during the 2002 Weddell seal pupping and breeding season (the Austral winter and spring; Kooyman 1981), between 10 July to 27 November 2002. Each recording session consisted of one hour of recording made every other hour over a 24hour period (resulting in 12 hours of underwater recording per session). One 24-hour recording was made during each month (from July to November) at each site. Therefore, there were ten recording sessions in total; five from each site. The first Weddell seal pup sighted on the ice in Holme Bay in 2002 occurred on 9 October (Rouget 2004), thus the start of the pupping season was presumed to begin in October. All recording sessions prior to October (June-September) were considered to be part of the pre-pupping period (PRE-PUP). This included a total of six recording sessions; three from each site. All sessions that took place in October and November were considered to be part of the pupping and breeding period (PUP), which included the remaining four recording sessions (two from each site). The photoperiod at the recording sites fluctuated over the months which sampling took place, ranging from two hours of twilight in mid-June to 24-hour light in November. For all recordings, hydrophones were placed at least 20 m away from the nearest seals. The hydrophones were lowered into the water through small holes (20 cm in diameter) drilled using either a manual ice auger or a motorized Jiffy drill. The two sites sampled were frequented by Weddell seals. The presence of seals at the sites during the times during which recordings took place was confirmed either through direct observation of seals on the ice or at breathing holes, or through indirect evidence of seal activity in the area such as presence of underwater vocalizations on the recordings. It was not possible to determine the exact number of seals vocalizing underwater, nor was it
possible to determine the sex and age of the vocalizing seals (but it is known that both mature males and females are vocally active; Thomas and Kuechle 1982; Oetelaar et al. 2003). No other species were observed on the ice during the recording sessions.
lc. Recording equipment and set-up A two-channel hydrophone array was used to record harp and Weddell seal underwater vocalizations. Recordings were made using Vemco VHLF omni-directional hydrophones (frequency response of ± 4 dB from 0.03-22 kHz) connected to a Sony TCD-D100 DAT recorder (frequency response of ± 1 dB from 0.02- 22 kHz). The system had a combined frequency response of ± 4 dB from 0.03-22 kHz. A single 10 min recording session with both hydrophones lowered 10 m below the water surface was made during the 2003 harp seal recordings and was used to compare the frequency (kHz) response of the two hydrophones. For all other harp seal recordings, the hydrophones were lowered to 10 and 60 m depth (forming a vertical hydrophone array with hydrophones separated by a distance of 50 m). In the 2003 harp seal recordings, a calibration tone (equivalent to 139.6 dB re 1 |aPa) was recorded on the 60 m hydrophone channel prior to each recording. For the Weddell seal recordings, the 10 m hydrophone was lowered 4-10 m below the ice surface and the 60 m hydrophone was lowered 52-60 m. Hydrophone depths were measured and recorded for each recording session. The distance between the two hydrophones varied from 46-50 m. A calibration tone (equivalent to 139.6 dB re 1 jxPa) was recorded on either the 10 or 60 m hydrophone channel prior to each recording.
2. Acoustic analysis Multispeech 2.2 (Kay Elemetrics Corp. Model 3700) and Ishmael 1.0 (Integrated System for Holistic Multi-channel Acoustic Exploration and Localization, D. K. Mellinger 2001) spectrogram analysis programs were used for all acoustic analyses. A sampling rate of 44 kHz was used when playing back the recordings into the sound analysis programs, therefore all vocalizations of 22 kHz or less could be analyzed. The majority of harp and Weddell seal vocalizations are typically emitted between frequencies of 0.08-20 kHz (M0hl et al. 1975; Thomas and Kuechle 1982). To ensure that the frequency (kHz) response of both hydrophones was identical, the 10 min section of tape recorded with both hydrophones at 10 m depth was analyzed using the Multispeech LTA (long-term analysis) function. The amplitude response of the two hydrophones was compared and no significant differences were found.
2a. Depth estimates Rough (approximate) depth estimates were obtained from calls that occurred on both hydrophone channels but from which no accurate ATD and AD measures could be obtained (ROUGH calls), or from calls that occurred on only one hydrophone channel (SINGLE calls). For simplification purposes, for these rough depth estimates it will be assumed that the hydrophones were always placed at 10 and 60 m and the midpoint between the two was at 35 m depth. Although this is not entirely the case for the Weddell seal recordings, the positions of the two hydrophones were always close to 10 and 60 m depth and the midpoint ranged from 29-35 m. Therefore, for SINGLE and ROUGH calls of the Weddell seals, the number of calls estimated to be made at < 35 m depth may be
slightly underestimated and the number of calls estimated to be made at the > 35 m depth may be slightly overestimated. For ROUGH calls, depth approximations were made by comparing the relative ATD and AD of the call on the two hydrophone channels. Calls made somewhere in the water column closer to the 10 m hydrophone (< 35 m deep) were louder and occurred first on the 10 m hydrophone channel (Figure 4). Calls that were made closer to the 60 m hydrophone (> 35 m deep) were louder and occurred first on the 60 m hydrophone channel. Calls that occurred at approximately the same time and amplitude on both channels were made close to the midpoint between the two hydrophones (~ 35 m deep). It should be noted that the ~ 35 m depth category was not as precise an estimate of calling depth as the < 35 m and > 35 m depth categories. Calls estimated to be emitted at depths of < 35 m and > 35 m clearly occurred within these categories. Calls estimated to be made at ~ 35 m include calls that were produced at exactly 35 m depth, and calls that could not be accurately assigned to the < 35 m or > 35 m depth categories because of very small ATD and AD’s. In a few cases, the channel on which the call occurred first was not always the channel on which the call was loudest. In these situations, amplitude loss of the signal may have been caused by directionality in the sound beam emitted by the seals (Schevill and Watkins 1971), or due to sea-ice interference or some other factor interfering with the transmission of the signal. Sea-ice can cause significant alterations in the amplitude of a sound as it propagates through the ocean due to fluctuations in microstructure, interference of boundary reflections, and refraction. The transmission time of a signal would suffer little perturbation due to sea-ice interference (Cato 1998). Thus,
Frequency (kHz)
Amplitude (dB)
I
0
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ppo i i
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i i
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Figure 4. Figures obtained from a spectrogram analysis program of a harp seal Knock (call type 15) recorded on the 10 m (top figure) and 60 m (bottom figure) hydrophone channels. Both waveforms (upper portion of each figure with time on the x-axis and amplitude on the y-axis) and spectrograms (lower portion of each figure with time on the x-axis and frequency (kHz) on the y-axis) of the knock on the two channels are shown. The knock clearly occurs first on the 10 m channel and arrival time difference (ATD) can be obtained from the spectrogram. Amplitude differences (AD) between the two channels can be obtained from the waveforms or through Fast Fourier Transform (FFT) techniques.
measurements involving time are a more reliable means of obtaining information from a call in comparison to amplitude measures. If the arrival time of the call on both channels was obvious, the call was still included in the analysis and depth approximations were based on ATD alone. For SINGLE calls, it was assumed that the call was produced somewhere in the water column closer to the hydrophone that detected the call. Therefore SINGLE calls were only categorized as either < 35 m or > 35 m deep. More precise depth estimates were obtained from calls for which accurate ATD and AD measurements could be made (CLEAR calls). For these calls, ATD was measured from both Multispeech (directly from a spectrogram; Figure 4) and Ishmael (using the Phone Pair Bearing function which determines ATD of a signal on multiple channels using cross-correlation techniques). AD was measured from a Multispeech Fast Fourier Transform (FFT) power spectrum using the Multispeech FTA analysis function (a Hamming window with a sampling rate of 2048 points/1024 bins was used). The sound speed in the water (C0) was assumed to be 1439 m/sec for both the harp and Weddell seal recordings based on salinity and temperature of the water. This estimate of C0 is very similar to estimates of C0 used in other underwater sound studies in areas covered by sea-ice (e.g., Langley 1989; Greening et al. 1992). The actual hydrophone separation was used for S (Figure 1). ATD and AD measures were used to calculate the ri and 0 values (Cato 1998), which in turn were used to estimate the depth of the calling seal. The accuracy of the precise depth estimates was examined by calculating the ATD and AD expected for a sound produced at a known depth and distance away from
the hydrophone array, given the assumption of spherical spreading, and then varying these values slightly to determine how the resulting depth estimate is affected (Appendix 1). For example, by varying the ATD measures by ± 1 msec or the AD measures by ± 1 dB, and then calculating the depth using the altered ATD and AD measures, the difference between the actual depth of the sound source and the calculated depth of the sound source can be determined. Through these calculations an estimate of how the precision in time and amplitude measures affected the resulting depths could be obtained (Appendix 1). It is important to note that not all calls originally considered CLEAR calls could give accurate depth measures. In some situations, ATD and AD measures resulted in very large ri values (> 100 m). It is known that the accuracy of the ri calculations tend to decrease as the sound source moves further away from the hydrophone array, particularly as the source approaches a distance greater than twice that of the hydrophone separation (Cato 1998). Therefore, CLEAR calls estimated to be > 100 m from the hydrophone array were not considered accurate and were examined under the ROUGH call analysis type only. Some of the calculated depth values were impossible. The maximum depth in the areas where the harp seal recordings were made was approximately 80 m. Harp seal calls which were estimated to be deeper than 90 m or which estimated the seal to be more than 10 m above the ice surface (having an estimated depth of < -10 m) were considered inaccurate and were examined under the ROUGH call analysis type. Cut-offs of -10 m and 90 m were used instead of 0 m and 80 m because of uncertainty levels involved with the depth measures. Calls made close to the ice surface
have a higher probability of interference effects decreasing the amplitude of the call before it reaches the hydrophones, thus decreasing the AD values. Because the ATD measures would not change or would be affected only slightly, the ri value calculated would be larger than expected, resulting in the estimated position of the seal to be actually above the ice surface. There is a high probability that the calls estimated to be made from above the ice surface (> 0 m) were actually produced in the water column close to the undersurface of the ice. This same phenomenon may occur with calls emitted near the bottom substrate; amplitude losses due to interference would cause ri values to increase, resulting in an overestimation of the depth of the seal. The depth at both of the Weddell seal recording sites was greater than 300 m, therefore a cut-off of -10 m was used for the Weddell seal calls. As stated previously, all calls greater than 100 m away from the hydrophone array were not considered in the CLEAR analysis, therefore the depths of the calls examined had to be 160 m or less (which are valid depth estimates for the Weddell seal sites).
2b. Sampling protocol For the harp seals, the six tape recordings using the 10 and 60 m hydrophone array, each from a different site, were analyzed. From each tape, two 10 min segments were randomly chosen for analysis. For the Weddell seals, 2 hours of recording (one hour from peak daylight, typically around 1-3 pm sun-time and one hour from peak dark hours, typically around 1-3 am sun-time were chosen from each 24-hour recording session for analysis. From each hour of recording chosen, one 10 min segment of time was randomly sampled. All seal calls from which some kind of depth estimate could be
obtained (either through rough approximations or more precise estimates) were sampled from each 10 min segment. These only included calls with sufficient signal to noise ratio to allow for clear identification of the beginning and end of the vocalization on either hydrophone channel. For the Weddell seal recordings only, due to the small sample size of CLEAR calls available from the 10 minute segments, CLEAR calls from the entire hour of recording from which the 10 min time segment was randomly chosen were analyzed.
2c. Measurements obtained from the calls The following features were noted for each call analyzed: (1.) Type of depth estimate that could be obtained from the call (SINGLE, ROUGH or CLEAR) (2.) Call type category (following the classification scheme in previous studies; M0hl et al. 1975; Thomas and Kuechle 1982; Terhune 1994; Pahl et al. 1997; Serrano 2001; Rouget 2004). (3.) Call subtype (calls were further classified into more descriptive subtypes that allowed vocalizations most similar to each other to be grouped together). Calls of each call type category were classified as distinct subtypes based on relative differences in waveform, spectral shape, duration and frequency (kHz). The criteria used to classify the calls into distinct subtypes in this study were arbitrary and based on differences distinct to the observer, and may not necessarily follow the subtype classification scheme reported by other authors.
(4.) Number of elements (NOE) in the call. (5.) Total call duration (msec). (6.) Rhythm pattern within calls consisting of three or more elements (following rhythms identified in previous studies; Moors and Terhune 2003, 2004). (7.) Start, middle and end frequency (kHz) of the first element in the call. (8.) Rough (or approximate) calling depth estimate, based on relative differences between ATD and AD measures (< 35 m or > 35 m for SINGLE calls, 35 m for ROUGH and CLEAR calls). (9.) Point depth estimate (m), for CLEAR calls only.
3. Statistical analysis The harp and Weddell seal calls were examined under several different analysis types: (1.) BROAD analysis: data from the SINGLE, ROUGH and CLEAR calls obtained from the 10 min segments of tape analyzed were pooled together and rough depth categories of < 35 m and > 35 m were examined. ROUGH and CLEAR calls estimated to be ~ 35 m deep and Weddell seal CLEAR calls obtained from the rest of the hour of recording were omitted from the analysis. (2.) ROUGH analysis: ROUGH calls only analyzed, depth categories of < 35 m, ~ 35 m and > 35 m examined. (3.) SINGLE analysis: SINGLE calls only analyzed, depth categories of < 35 m and > 35 m examined.
(4.) CLEAR1 analysis: CLEAR calls only analyzed (including all CLEAR calls examined from the entire hour of recording for Weddell seals), point depth estimates divided into broad depth categories of < 25 m, 25-45 m, and > 45 m were examined. (5.) CLEAR2 analysis: CLEAR calls only analyzed (including all CLEAR calls examined from the entire hour of recording for Weddell seals), point depth estimates divided into narrower depth categories of 10 m intervals (< 10 m, 10-20 m, 20-30 m, 30-40 m, 40-50 m, 50-60 m, 60-70 m and > 70 m) were examined. (6.) CLEAR 3 analysis: CLEAR calls only analyzed (including all CLEAR calls examined from the entire hour of recording for Weddell seals), point depth estimates themselves examined. It should be noted that SINGLE calls were not examined separately for the harp seals because of their small sample size. The CLEAR 1 and CLEAR2 analyses were not considered for the majority of the Weddell seal analysis due to the small number of CLEAR calls obtained.
3a. Harp seal analysis Because all six tape recordings were made during a single period within the harp seal life cycle (pupping and breeding season), during the same photoperiod (daylight hours) and were all recorded from a single herd (the Gulf herd), there is no biological reason to believe that the seals were behaving differently with regards to their underwater acoustic communications during the different recording sessions. Studies have shown that harp seals of the Gulf herd have a very stable underwater vocal repertoire and the sounds
produced by the seals remain the same from year to year (Serrano and Terhune 2002). Preliminary examination of the data indicated that major trends in the depth of the vocalizations occurred across all 10 min segments sampled. Also, any two 10 min segments analyzed from a single tape were not any more similar to each other than two 10 min segments analyzed from different tapes (based on visual inspection of the raw data). Therefore, the twelve 10 min segments analyzed were considered repetitions, or the statistical unit in the analysis. Analysis of variance (ANOYA) models were used to analyze the data. For all analyses, it was assumed that the calls were independent of one another, although this assumption was probably violated. Count data were used to examine categorical factors (depth categories, call types and subtypes, NOE in the calls and rhythm patterns of the calls). Because the count data residuals generally followed a skewed distribution, a log transformation was applied to allow the residuals to fit the ANOVA model assumptions of normality and homogenous variance. The normality assumption was not always completely satisfied by the log transformation although the transformation always brought the data residuals close to a normal distribution. ANOVAs are generally robust to small violations of normality (Gupta and Richards 1997) and so were still used to examine the data (although data which violated the normality assumption were noted and should be interpreted with caution). The assumption of homogenous variance was always satisfied by the log transformation. Total duration and frequency (kHz) data residuals were highly skewed due to a large number of very short calls and a large number of low frequency (kHz) calls, and very few long or high frequency (kHz) calls. The duration and frequency (kHz) data residuals could not be normalized through any simple
transformation, although a log transformation did make the residuals close to normal and satisfied the assumption of homogenous variance. Again, ANOVAs were still used to analyze the data, however, results should be interpreted with caution. It should be noted that all ANOVAs were initially run using the raw, untransformed data. The results from these ANOVAs were very similar to the results obtained from the ANOVAs analyzing the transformed data, however, because the transformed data better satisfied that ANOVA assumptions, they are the results presented. Two and three-way mixed ANOVAs were used to analyze the data using the SAS PROC MIXED procedure (SAS Statistical Software 2002). Mixed ANOVA models were used because repetition was considered a random factor (since the 10 min segments were randomly chosen from the recordings). All other variables analyzed (analysis type, depth category, call type category, subtype, NOE and rhythm pattern) were considered fixed factors because the levels within each were repeatable and statistical inference was only drawn on the levels examined (Newman et al. 1997). The mixed ANOVA model recognizes that variability likely exists between the levels of the random factors (the repetitions in this case), and takes this into account when testing for significant effects of the fixed factors within the model, even though it does not actually test for differences between the levels of the random factor (Newman et al. 1997; Hicks and Turner 1999). PROC MIXED uses restricted maximum likelihood (REML) for estimation of the variance components and model parameters, and a Satterthwaite approximation (Satterthwaite 1946) to estimate the denominator degrees of freedom (SAS Institute Inc. 1999). For all tests, p-values of < 0.05 were considered significant and Tukey’s
multiple comparisons post-hoc tests were used to determine between which means significant differences occurred. The mean number of SINGLE, ROUGH and CLEAR calls analyzed from the 10 min segments, and the mean number of calls from the 10 min segments emitted within each depth category of the BROAD, ROUGH, CLEAR1 and CLEAR2 analyses, were analyzed using two-way mixed ANOVAs (with repetition considered a random factor). For the BROAD, ROUGH, CLEAR1 and CLEAR2 analyses, three-way mixed ANOVAs were used to examine the mean number of calls (within the 10 minute segments) of a particular call type category or subtype emitted within each depth category. Differences in the mean number of calls having a specific NOE or particular rhythm pattern within each depth category were also examined using three-way mixed ANOVAs. The influence of depth on call duration and frequency (kHz) was examined using two and three-way mixed ANOVAs with repetition considered a random factor. Mean call duration at each depth category, mean frequency (kHz) of calls at each depth category, and mean frequency (kHz) of calls of each call type and subtype at each depth category were examined for the four analyses. Simple linear regressions were used to determine the influence of depth on call duration and frequency (kHz) using the precise depth estimates (CLEAR3 analysis). Log transformed duration and frequency (kHz) data were used for the regressions. Chi-square contingency tables were used to examine differences in the relative frequency of occurrence (proportion) of each call type category, subtype or NOE within each depth category of the BROAD, ROUGH, CLEAR1 and CLEAR2 analyses. This test
was used to determine if the proportional usage of calls of a particular call type, subtype, or NOE varied across the depth categories.
3b. Weddell seal analysis The Weddell seal statistical analysis was similar to the methods described for the harp seals, although in addition to the factors considered in the harp seal statistical analyses, site (SEAL MO and SPA), season (PRE-PUP and PUP) and photoperiod effects (LIGHT and DARK) were also considered in the Weddell seal analyses. ANOVA models were used to analyze the data. For all analyses, it was assumed that the calls were independent of one another, although this assumption was probably violated. Count data were used to examine categorical factors (depth categories, call types and subtypes, NOE and rhythm patterns of the calls). The count data residuals generally followed a skewed distribution and a log transformation was applied to allow the data residuals to fit the ANOVA model assumptions of normality and homogenous variance. Again, the normality assumption was not always completely satisfied by the log transformation, although the transformation always brought the residuals close to a normal distribution. Total duration and frequency (kHz) data residuals were highly skewed and could not be normalized through any simple transformation, although a log transformation did make the residuals close to normal. ANOVA tests were still used to examine the data, however, tests which violated the normality assumption were noted and should be interpreted with caution. The assumption of homogenous variance was always satisfied by the log transformation.
Similar to the harp seal analyses, all ANOVAs for the Weddell seal data were initially run on the raw, untransformed data. The results obtained were very similar to those obtained from the ANOVAs examining the transformed data. Because the transformed data better satisfied the ANOVA assumptions, only these results were presented. Multifactor mixed ANOVAs using SAS PROC MIXED procedures were used to analyze the data. Mixed ANOVA models were used because both repetition and site were considered random factors. Repetitions were considered random because the 10 min segments were randomly chosen from the recordings. Sites were considered random because it was assumed that they behaved similarly to a random selection from all possible sites in Holme Bay (even though they were not chosen randomly), and the levels of the effect could not be exactly replicated by another experimenter (Newman et al. 1997; Rouget 2004). Therefore, it was recognized that sites and repetitions were likely a source of variability in the model, however, how they differed was not of interest in this study. All other variables analyzed (season and photoperiod, analysis type, depth category, call type category, subtype, NOE and rhythm pattern) were considered fixed factors because the levels within each were repeatable and statistical inference was only drawn on the levels examined (Newman et al. 1997). For all tests, p-values of < 0.05 were considered significant and Tukey’s multiple comparisons post-hoc tests were used to determine between which means significant differences occurred. The mean number of SINGLE, ROUGH and CLEAR calls analyzed from the 10 min segments were analyzed using a two-way mixed ANOVA (with repetition
considered a random factor and analysis type considered a fixed factor). Site, season and photoperiod effects were not of interest for this particular ANOVA test. The mean number of calls (from the 10 minute segments), within each of the depth categories of the BROAD, ROUGH and SINGLE analyses were examined using multifactor mixed ANOVAs which considered repetition, site, season, photoperiod and depth category as factors. Due to the low number of CLEAR calls obtained from the Weddell seal tapes, only the overall count data for each depth category were presented for the CLEAR1 and CLEAR2 analyses. Multifactor mixed ANOVAs were used to examine the mean number of calls (within the 10 minute segments) of a particular call type category, subtype or NOE emitted within each depth category for the BROAD, ROUGH and SINGLE analyses. Repetition, site, season, photoperiod, depth category and call type, subtype, or NOE effects were the factors considered in the ANOVA. Only the overall count data were examined for the number of calls with each rhythm pattern emitted at each depth due to low sample sizes. The influence of depth on call duration was examined using multifactor mixed ANOVAs which considered repetition, site, season, photoperiod and depth category effects. For the BROAD, ROUGH and SINGLE analyses, mean frequency (kHz) of calls at each depth category, and mean frequency (kHz) of calls of each call type and subtype at each depth category were examined using two and three-way ANOVAs that considered repetition as the random effect, and depth category and call type as fixed effects. Site, season and photoperiod effects were not of interest for these analyses.
Simple linear regressions were used to determine the influence of depth on call duration and frequency (kHz) using the precise depth estimates (CLEAR3 analysis). Log transformed duration and frequency (kHz) data were used. Chi-square contingency tables were used to examine differences in the relative frequency of occurrence (proportion) of each call type category, subtype or NOE within each depth category of the BROAD, ROUGH and SINGLE analyses. This test was used to determine if the proportional usage of calls of a particular call type, subtype, or NOE varied across the depth categories.
III. RESULTS
1.
Limitations of point depth measures The estimated accuracy of the precise depth estimates for the CLEAR calls was
found to vary with distance of the sound source from the hydrophone array, depth of the sound source, and the accuracy of the ATD and AD measures (Figures 5 and 6). Underestimating the ATD and AD measures had a greater impact on the depth calculations than overestimations of the two measures (Figures 5 and 6). Precision in both the amplitude and time measures had an effect on the depth values calculated (Figures 5 and 6). When ATD was altered by ± 1 msec, the resulting depth estimate was found to be within ± 8 m for calls produced at a distance of less than 50 m away from the hydrophone array, and within ± 10 m for calls made less than 100 m away (Figure 5). A precision of ± 1 msec for ATD is a reasonable estimate of the
Q. ± 100 m (Figure 9). Most of the calls examined were calculated to be at distances closer to the hydrophone array and thus most of the depth estimates should be fairly accurate (79% of CLEAR calls were estimated to be made within 40 m of the hydrophone array; Figure 9). When looking at the distribution of the precise depth estimates for the CLEAR calls, most of the calls (74 %) were estimated to be made at depths of < 35 m, which followed the same trend found in the BROAD and ROUGH analyses (Figure 8). It is important to note, however, that 73 % of those calls (or 54% of the total number of CLEAR calls) were estimated to be made at least 10 m below the undersurface of the ice. Even when just the most accurate CLEAR call depth estimates were considered, those calculated to be made within 40 m of the hydrophone array, this trend persists. Most of the calls (87%) were still estimated to be made between 10 and 35 m depth (Figure 9).
2c. Call type categories/subtypes and depth The calls were classified into 17 call type categories (Table 1). The most common call type was Type 14 (49% of the calls analyzed). Call types 2, 6 , and 15 were also common, making up 34% of calls. Call types 11, 1, 3, 13, 18, and 4 had sample sizes of more than 50 calls and comprised an additional 15% of all calls. The remaining seven call types were relatively rare, all having a sample size of less than 50 calls and collectively making up only 2 % of the calls analyzed (Table 1). For the BROAD and ROUGH analyses the seven rare call types were analyzed as a single group of calls designated “OTHER”. Therefore, for the BROAD and ROUGH analyses, 11 call type categories were considered (including the OTHER category). Only 12 of the 17 call types were represented by the CLEAR calls analyzed, most of which had relatively small sample sizes. The most numerous CLEAR call type was Type 14, with 195 calls (67% of the CLEAR calls) sampled. Call types 15 and 18 were also common within the CLEAR call data set, however, due to small sample sizes (less than 50 calls), they were examined as part of the “OTHER” call type category along with calls of types 1, 2, 3, 4, 6 ,11, 13, 16 and 20. The CLEAR call analyses therefore only considered two call type categories; Type 14 and OTHER. There were no significant interactions between call type and depth category for the BROAD, ROUGH, CLEAR1 or CLEAR2 analyses (Table 2). There were significant differences among the mean number of calls emitted within the depth categories (for each of the call types) of all four analyses (Table 2). In all cases, Tukey’s post-hoc tests indicated that significantly more calls were made at the shallower depth categories (e.g., Figure 10A). Generally, the same trends in depth as those presented in Figure 8 occurred;
Table 1. Number of calls analyzed of each of the harp seal call type categories and call subtypes. The call types followed the same classification system reported in other studies (M0hl et al. 1975; Terhune 1994; Serrano 2001). Call subtypes were determined from relative differences in calls of each type distinct to the observer. A brief description of the call subtypes is given. Call Type Category
Number of calls
Subtype Description
Number of calls
14NOR
Broadband, low constant frequency, grunt like sound
909
140M P
Broadband, low frequency, pulsed, “omp”like sound
363
14BEEP
Broadband, low frequency, pulsed, “beep”like sound
264
Subtype
14YIP 14 (Grunts)
1682 14DRIP
14FROG
6 (Chirps)
15 (Knocks)
609
1 (Sine wave)
31 12
14HIGH
Broadband, high frequency, “yip”-like sound
4
6NOR
Narrowband, high increasing/decreasing frequency, chirp-like sounds
543
6HAR
Narrowband, high decreasing frequency , harmonics present, shorter calls than 6NOR, chirp-like sounds
64
6STEEP
Narrowband, high decreasing frequency, rapid down sweeps
2
15NOR
Broadband, low frequency, short calls, knock/thunk-like sound
15SHORT
Broadband, low frequency, shorter calls than 15NOR, knock/thunk-like sound
297
276
120
43
Broadband, low frequency, pulsed, “purr”like sound
2NOR 11 (Passerine call)
56
14PURR
2REV 2 (Whistles)
Broadband, mid increasing frequency, “yip”-like sound Broadband, mid constant frequency ending in a steep frequency upsweep, drip-like sound Broadband, low frequency, pulsed, ‘croak”like sound
Narrowband, mid constant frequency call ending in a frequency upsweep, whistle like sound Narrowband, mid constant frequency call beginning in a frequency upsweep, whistle-like sound
242 55
194
82
11 NOR
Narrowband, rapid frequency upsweep
120
1HAR
Narrowband, long constant frequency tone, harmonics present
52
1NOR
Narrowband, long constant frequency tone
51
103
Table 1. (Continued) Call Type Category
3 (Morse call)
Number of calls
85
Subtype
Subtype Description
Number of calls
3NOR
Narrowband, short constant frequency tone, multiple elements
69
3HAR
Narrowband, short constant frequency tone, multiple elements, harmonics present
11
3PROG
Narrowband, short constant frequency tone, multiple elements, elements get progressively longer
5
13 (Squeaks)
73
13NOR
Broadband, low frequency, pulsed, creak-like sound
73
18 (Growls)
63
18NOR
Broadband, low frequency, growl-like sounds
63
4 (Trills)
58
4NOR
Broadband, low frequency, pulsed, long calls
58
10 (Distressed blackbird)
37
IONOR
Broadband, low frequency, u-shaped call, grunt-like sound
37
16 (Clicks)
34
16NOR
Broadband, very short, “click”-like sound
34
20
10
20NOR
Broadband, mid varying frequency, long call
10
25
3
25NOR
Broadband, low frequency, harmonics present, two-part call, “hic-cup” like sound
3
5 (Gull’s cry)
2
5NOR
Narrowband, constant frequency, then down sweep to a lower constant frequency
2
9 (Frequency shift keying)
2
9NOR
Narrowband, mid constant frequency followed by brief freq. down sweep then lower constant freq.
2
17
1
17NOR
Varying frequency, sine wave
1
TOTAL
3455
3455
Table 2. Summary of the three-way mixed ANOVA results testing the number of harp seal calls emitted within each depth category for each call type category and subtype of the BROAD, ROUGH, CLEAR1 and CLEAR2 analyses. The count data was log transformed for all analyses, and both F and p values are given. Significant values (pvalues < 0.05) are indicated by an asterix (*). Repetition (the 10 minute segments) was considered a random factor for all analyses.
ANOVA Results ANOVA
Effect BROAD
ROUGH
CLEAR1
CLEAR2
F 10,231 = 1 -05 p = 0.400
F20,352 = 1-25 p = 0.212
F2,55 = 0.41 p = 0.663
F 7,165 =1-26
Call type category
F io,23i = 46.86 p < 0.001*
F 10,352 = 75.74 p < 0.001*
Fi ,55 = 1 3.65 p < 0.001*
Fi,165 = 1 5.22 p < 0.001*
Depth category
F1,231 = 76.63 p < 0.001*
f"2,352 =
39.96 p < 0.001*
F2,55= 19.69 p < 0.001*
Interaction
F 17,385 = 1-07 p = 0.379
F34,583 = 1-35 p = 0.091
F2,55 = 0.09 p = 0.918
F7,165 = 0.62 p = 0.7405
Call subtype
F 17,385 = 26.37 p < 0.001*
F 17,583= 43.54 p < 0.001*
Fi ,55 = 0 .7 3 p = 0.398
F 1,165 = 0.53 p = 0.4688
Depth category
F l,3 8 5 =
113.34 p < 0.001*
F2,583 = 60.53 p < 0.001*
F2,55= 17.63 p < 0.001*
Interaction 3-way ANOVA for call type category
3-way ANOVA for call subtype
p = 0.274
9.99 p < 0.001*
F7,165=
= 12.21 p < 0.001*
F 7.166
3 O O c
(0 0) S
60 -i
14
6
15
2
11
1
3
13
18
4
OTHER
Call Type Category
Figure 10. Mean number of harp seal calls of each call type examined (A), and the overall percent composition of each call type (B), at the < 35 m (darker bars) and > 35 m (lighter bars) depth categories of the BROAD analysis. Error bars in graph A represent ± 1SD (of the mean count calculated for each call type within each depth category).
significantly more calls of each type were emitted at the < 35 m depth category for the BROAD (Figure 10A). ROUGH analyses, at the < 25 m depth category for the CLEAR1 analysis, and at the 35 m depth category (Figure 10). There were no significant differences in the proportion of calls of each type within each depth category for the CLEAR 1 and CLEAR2 analyses (x2 2 = 1.876, p = 0.391; x S = 13.937, p = 0.052). The 17 call type categories were subdivided into 31 subtypes (Table 1). Call type 14 was the most variable of all call types and was separated into 8 subtypes. The remaining call types were divided into only one, two or three subtypes (Table 1). The most common call subtype was 14NOR (27% of calls). Subtypes 6NOR, 140MP, 14BEEP, 15NOR, 2REV and 11NOR were also common, making up 50% of calls. Subtypes 2NOR, 3NOR, 13NOR, 6HAR, 18NOR, 4NOR, 14YIP, 15SHORT, 1HAR, and 1NOR all had sample sizes of greater than 50 calls and comprised an additional 18% of calls (Table 1). The remaining subtypes were relatively rare, making up only 5% of the
calls analyzed and all having a sample size of less than 50 calls. For the BROAD and ROUGH analyses 18 call subtypes were examined, with the 14 rare subtypes categorized as “OTHER”. Only 19 of the 31 subtypes analyzed were represented by CLEAR calls, most of which had relatively small sample sizes. The most numerous subtype was 14NOR, having a sample size of 130 calls (44% of the CLEAR calls). Subtypes 14BEEP, 140MP, 15NOR and 18NOR were also common within the CLEAR calls, however, due to small sample sizes (less than 50 calls) they were examined as “OTHER” along with the remaining subtypes. The subtype results were similar to results found for call types. There were no significant interactions between subtype and depth category, and all four analyses showed significant differences among the mean number of calls (of each subtype) emitted at each depth category (Table 2). In all cases, significantly more calls were made at the shallower depth categories (e.g., Figure 11A), following the same trends presented in Figure 8. The results of the CLEAR2 analysis should be interpreted with caution as the residuals of the log transformed data were not normally distributed. The proportion of calls of each call subtype within the depth categories again showed significant differences for the BROAD and ROUGH analyses (x n = 56.453, p < 0.001; x2 34 = 150.068, p < 0.001), although the proportions appeared to be similar across depth categories (e.g., Figure 11B). There were no significant differences found in the proportion of calls of each subtype within the depth categories for the CLEAR 1 or CLEAR2 analyses (x22 = 0.993, p = 0.998; x27 = 9.689, p = 0.207).
Figure 11. Mean number of harp seal calls of each call subtype examined (A), and the overall percent composition of each subtype (B), at the < 35 m (darker bars) and > 35 m (lighter bars) depth categories of the BROAD analysis. Error bars in graph A represent ± 1SD (of the mean count calculated for each subtype within each depth category).
2d. Number of elements (NOE) and depth The harp seal calls were made up of 1-28 elements. The majority of calls were double element calls (1622 calls; 47%) and single element calls (1033 calls; 30%). Only a small number of calls (800 calls; 23%) were made up of more than two elements (multiple element calls). Most of the multiple element calls were four elements in length and only two of the calls examined consisted of more than 20 elements. For the BROAD and ROUGH analyses, the number of calls having a particular NOE were analyzed in seven categories; calls having 1, 2, 3, 4, 5, 6-10, and >10 elements. The CLEAR1 and CLEAR2 analyses considered only three categories: calls having 1, 2 and > 3 elements. The three-way ANOVAs for all four analyses indicated no significant interactions between the NOE in a call and depth category (Table 3). In all cases significantly more calls were made at the shallower depth categories (e.g., Figure 12A), following the same depth trends presented in Figure 8. The CLEAR2 analysis once again violated the normality assumptions, although the log transformation did bring the resdiuals closer to a normal distribution. The proportions of calls having a specified NOE were not significantly different between depth categories of the BROAD, ROUGH and CLEAR1 analyses (x26 = 12.022, p = 0.061, x2 12 = 14.667, p = 0.260, and x% = 8.200, p = 0.085, respectively; Figure 12B). The CLEAR2 analysis could not be tested due to small count values for some of the depth categories.
Table 3. Summary of the two and three-way mixed ANOVA results testing the number of harp seal calls of each of the NOE categories and rhythm patterns emitted within each depth category of the BROAD, ROUGH, CLEAR1 and CLEAR2 analyses. The results of the two-way ANOVA tests for total call duration and depth category are also given. The data were log transformed for all analyses, and both F and p values are given. Significant values (p-values < 0.05) are indicated by an asterix (*). Repetition (the 10 minute segments) was considered a random factor for all analyses. “Not tested” indicates ANOVA tests not run for a particular analysis.
ANOVA Results ANOVA
Effect BROAD
ROUGH
CLEAR1
CLEAR2
Interaction
F6,i43=1-74 p = 0.117
Fl 2,220= 1-13 p = 0.335
F4,88 = 0.91 p = 0.461
F14,253= 1 -06 p = 0.398
Number of elements
F6,143 =107.67 p < 0 .0 0 1 *
F6,220 = 131.22 p < 0.001*
F2,88 = 8.54 p < 0.001*
F2,253 = 9.04 p < 0.001*
Depth category
F1,143= 106.84 p < 0.001*
F2 220 = 45.31 p < 0.001*
F2,88 = 19.84 p < 0.001*
F7,253 = 13.15 p < 0.001*
F2,55=4.77 p = 0.012*
F4,88 = 2.04 p = 0.096
Not tested
Not tested
Timing pattern
F2,55= 192.32 p < 0.001*
F4,88 = 207.20 p < 0.001*
Not tested
Not tested
Depth category
F, .a =3 3 .7 0 p < 0.001*
^2,88 = 15.04 p < 0.001*
Not tested
Not tested
2-way ANOVA for Pattern 1
Depth category
F, n = 33.22 p < 0.001*
Not tested
Not tested
Not tested
2-way ANOVA for Pattern II
Depth category
F i ,22= 0.93 p = 0.345
Not tested
Not tested
Not tested
2-way ANOVA for Pattern III
Depth category
F111 = 14.14 p = 0.003*
Not tested
Not tested
Not tested
2-way ANOVA for total duration
Depth category
F 1,2627 = 0.00 p = 0.992
F2,2964 = 3.03 p = 0.049*
F2,218 = 0.08 p = 0.928
F7,271 = 0.44 p = 0.879
3-way ANOVA for NOE
Interaction 3-way ANOVA for rhythm pattern
100 n
1
2
3
1
2
3
4
5
6-10
>10
50 -1
4
5
6-10
>10
Number of Elements
Figure 12. Mean number of harp seal calls of each NOE category examined (A), and the overall percent composition of NOE category (B), at the < 35 m (darker bars) and > 35 m (lighter bars) depth categories of the BROAD analysis. Error bars in graph A represent ± 1SD (of the mean count calculated for each NOE category within each depth category).
2e. Rhythm patterns within multiple element calls and depth The multiple element calls were found to occur with all three rhythm patterns described by Moors and Terhune (2003). The most common rhythm pattern was Pattern I (all short interval durations between elements) which occurred in 704 of the calls analyzed (88% of multiple element calls). Only 14 calls (2% of multiple element calls) were emitted with Pattern II timing (all long interval durations between elements), and 82 calls (10% of multiple element calls) had Pattern III timing (alternating short and long intervals between elements). A significant interaction occurred between rhythm pattern and depth category for the BROAD analysis (Table 3). The rhythm patterns were therefore tested using a series of two-way ANOVAs. There were significant differences among the mean number of calls emitted at each depth category for both the Pattern I and Pattern III calls, while no difference was present for the Pattern II calls (Table 3; Figure 13). It should be noted that the ANOVA test for the Pattern II calls was based on a small sample size of 14 calls and the data residuals were not normally distributed, (thus the test violated the ANOVA assumptions). There was no significant interaction between rhythm pattern and depth category for the ROUGH analysis (Table 3), and the Tukey’s post-hoc tests indicated that significantly more calls were made at the < 35 m depth category (Figure 13).
2f. Total call duration and depth Total duration of the calls ranged from 0.006-14.124 sec. The data were highly skewed due to a large number of very short calls (2799 calls, or 81%, were < 1 sec long) and few very long calls (29 calls were > 5 sec long; only 1 call was > 10 sec long). The
c
3 O o
c
(0 35 m (lighter bars) depth categories of calls analyzed within the BROAD (A) and ROUGH analysis (B). Error bars represent ± 1SD (of the mean count calculated for each rhythm pattern within each depth category).
results should be interpreted with caution because the duration data residuals could not be made normal through simple transformations for any of the analyses although a log transformation did make the distribution of the residuals closer to normal in all cases. The BROAD, CLEAR 1 and CLEAR2 analyses showed no significant differences among mean total duration values within each depth category (Table 3). A slight significant difference was found among the mean total duration of calls within the depth categories of the ROUGH analysis (Table 3). A Tukey’s test indicated that calls of the ~ 35 m depth category had significantly shorter mean total duration value than calls of the < 35 m depth category. The regression of total duration and depth using the point depth estimates of the CLEAR calls was not significant (Fi,292 = 0.00, p = 0.993) and no relationship was evident between the two variables (R = 0; Figure 14).
2g. Frequency (kHz) and depth The calls examined were typically emitted within the frequency (kHz) range of 0 .1 -1 0 kHz. In total, 1535 calls (46%) were < 1 kHz in frequency, and 1777 calls (53%) were emitted in the frequency range of 1-10 kHz. Only 49 of the calls analyzed (1%) were emitted at frequencies higher than 10 kHz and typically were high frequency chirps and whistles (call types 2 and 6). Like the total duration data, the data residuals for call frequency (kHz) were highly skewed due to the occurrence of a few high frequency (kHz) calls and could not be normalized. A log transformation did bring the data closer to a normal distribution and was used to analyze the data. Frequency (kHz) measurements of 94 calls (typically broadband calls such as clicks) could not be measured accurately and were excluded from the analysis. Although
LOG (DURATION) = -0.149 + 0.000007 DEPTH Fi,292 = 0.00 p = 0.993 R2 = 0 ♦
81
-10
0
10
20
30
40
50
60
70
80
90
Point Depth Estimate (m)
Figure 14. Point depth estimates and total call duration obtained from the harp seal CLEAR calls (N = 293 calls). Results of a simple linear regression (F, p and R 2 values) used to examine the relationship between the two variables is also given.
the start, middle and end frequency (kHz) of the first element in the calls were all examined, due to similar results between the three measures only the data for the element start frequencies will be presented. The BROAD analysis showed no significant differences among the overall mean frequency (kHz) of the two depth categories (Table 4). The ANOVAs examining differences among mean frequency (kHz) of calls of each type and subtype within each depth category also showed no significant differences (e.g., Figure 15; Table 4). A significant difference among the overall mean frequency (kHz) of calls emitted in each depth category was evident for ROUGH calls (Table 4). The Tukey’s post-hoc test indicated that calls made at the ~ 35 m depth category had a slightly higher mean frequency (kHz) than calls made at the < 35 m or > 35 m depth categories. When mean frequencies (kHz) of the call types or subtypes were considered for the ROUGH analysis, no significant differences between the depth categories were observed (Table 4). For the CLEAR1 analysis, a significant difference was found among the overall mean frequencies (kHz) emitted at each of the depth categories (Table 4) and the Tukey’s posthoc test indicated that the mean frequency of calls in the < 25 m depth category was significantly higher than the mean frequency of calls emitted in the > 45 m depth category. This result remained the same when frequency (kHz) of call types and subtypes were considered (Table 4). The CLEAR2 analysis showed a significant difference in the overall mean frequency (kHz) among depth categories (Table 4) due to a significantly lower mean frequency of calls within the < 10 m category and significantly higher mean frequency of calls within the > 70 m depth category. When call types or subtypes were
Table 4. Summary of the three-way mixed ANOVA results testing the mean frequency (kHz) of harp seal calls emitted within each depth category, and the mean frequency (kHz) of calls of each call type and subtype emitted within each depth category of the BROAD, ROUGH, CLEAR1 and CLEAR2 analyses. The data was log transformed for all analyses, and both F and p values are given. Significant values (p-values < 0.05) are indicated by an asterix (*). Repetition (the 10 minute segments) was considered a random factor for all analyses.
Results ANOVA 2-way ANOVA for start frequency
3-way ANOVA for start frequency and call type category
3-way ANOVA for start frequency and call subtype
Effect CLEAR1
CLEAR2
F2.161
= 6.89 p = 0.001*
F7,262 = 5.35 p < 0.001*
F21,2857 = 1-05 p = 0.397
F2,287 = 0.1 1 p = 0.898
F7,275 = 0.95 p = 0.4685
F 10,2572 = 286.27 p < 0.001*
F 12,2859 = 328.28 p < 0.001*
F 1,287 = 15.56 p < 0.001*
Fl ,275 = 2.61 p = 0.107
Depth category
F 1,2574= 0.05 p = 0.822
F2,2859 = 2.29 p= 0.101
F2,190 = 7.31 p < 0.001*
F7,262 = 4.18 p < 0.001*
Interaction
Fl 7,2555 = 1-37 p = 0.141
^35,2835 = 1-14 p = 0.257
F2,283 = 0.27 p = 0.764
F7,273 = 2.20 p = 0.034*
Call subtype
F 17,2558 = 180.25 p < 0.001*
F 19,2837 = 233.83 p < 0.001*
Fl,285 = 0 . 1 0
F 1,276 = 0 . 0 2 p = 0.894
Depth category
F 1,2562 = 0.23 p = 0.634
F2,2839 = 2.34 p = 0.097
BROAD
ROUGH
Depth category
F 1,2596= 0-27 p = 0.607
F2,2893 = 34.30 p < 0.001*
Interaction
F 10,2569= 1 -27 p = 0.241
Call type category
p = 0.756
F2,163 = 6.96 p = 0.001*
3.56 p = 0.001*
F7,258=
n 8 x £ ,7 1 >. £ 0) 3 C7 5 0)
A
n V) 3 c
(0 o
0) ^
in 14
15
t
11
a 1
3
13
f a B1 18
OTHER
Call Type Category
Figure 15. Mean start frequency (kHz) of the first element within a call of harp seal calls of each call type category for the < 35 m (darker bars) and > 35 m (lighter bars) depth categories of the BROAD analysis. Error bars represent ± 1SD (of the mean frequency (kHz) calculated for each call type within each depth category).
considered, the significant difference among mean frequencies (kHz) of the different depth categories still existed (Table 4) with a significantly higher mean frequency in the > 70 m depth category. The higher mean frequency (kHz) value of the > 70 m depth category was probably a result of two high frequency chirps estimated to be made at depths of > 70 m (Figure 16). The regression of frequency (kHz; log transformed) and depth using the point depth measurements of the CLEAR calls showed a significant positive relationship between frequency and depth (Fij292 = 24.37, p < 0.001). Although some of the variance was explained by the regression equation obtained (R = 0.077; Figure 16), the increase in frequency (kHz) would be much greater than the rate shown if it was due to the compression of air chambers within the seals’ body. The positive relationship between frequency (kHz) and depth is most likely due to two high frequency chirps emitted in the deeper depth range (Figure 16). When a regression using depth estimates of only a single call type (Type 14) was performed, again, a significant relationship between depth and frequency (kHz) was found (F 1,292 = 24.37, p < 0.001). Again, only a small amount of the variance in the model was explained by the regression equation (R = 0.091; Figure 17).
3. Weddell seal results Like the harp seals, the majority of the Weddell seal calls analyzed were made at shallow depths (< 35 m). Neither season nor photoperiod had an effect on the depth at which the seals emitted their calls. More calls of each call type and subtype, as well as each specific NOE pattern, were found in the < 35 m and ~ 35 m depth categories than in the > 35 m depth category. The proportional usage of call types, subtypes, and specific
LOG (FREQUENCY) = 2.67 + 0.00387 DEPTH Fi,292 = 24.37 p < 0.001 R2 = 0.077 7.0 A
▲
6.0 _ 5 .0 'n ' I ^ 4 .0 >»
o
§3.0 C7 Q) ^ 2.0
A A
1.0
B
B B B B B
u ¿m
0.0 ■10
■ B
■
m
■
B B B _ | B B B 9 B ^^H B BM P B B fl B B B I H I B IB B B B ! | D I I ^ B JDC ID B
0
■ B • a
10
BB
.
a
■
L "■ ra* _ r a a ■
■
■ -
■ •A _■
■ ■ ■ mm -iT B b BB ^ ^ H B + ,B ■ A H+ B " ji ■ ■ ■ 9
■
-
B
B■
20
30
40
50
60
70
80
90
Point Depth Estimate (m)
Figure 16. Point depth estimates and start frequency (kHz) of the first element within a call obtained from the harp seal CLEAR calls (N = 293 calls). Call types are indicated in the legend. Results of a simple linear regression (F, p and R2 values) used to examine the relationship between the two variables is also given.
LOG (FREQUENCY) = 2.63 + 0.00354 DEPTH Fi,2 9 2 = 19.22 p < 0.001 R2 = 0.091
2.0 1.8
1.6 N1.4 ^ 1 .2 > C 1 .0 0)
§ 35 m depth categories (Fi, 2.3 = 52.74, p =
0.012), and significantly more calls were made at the < 35 m depth category (Figure 18). Of the 2263 BROAD calls analyzed, 90% (2046 calls) were made at the shallower depth category. There was also a significant difference in the mean number of calls emitted at each depth category of the ROUGH analysis (F2,6.6= 11.70, P = 0.007). Significantly more calls were made at the < 35 m and ~ 35 m depth categories (although there was no difference between the mean values of the two; Figure 18). Overall, 47% of the ROUGH calls (664 calls) were made at the < 35 m depth category, 43% (615 calls) were made at the ~ 35 m depth category and only 10% (146 calls) were made at depths >35 m. The mean number of calls made at each depth category within the SINGLE analysis were significantly different (Fi)2= 28.05, P = 0.033), and more calls were found to be made at the < 35 m depth category (Figure 18). Overall, 95% of the SINGLE calls (1363 calls) were made at the < 35 m depth category. The overall numbers of calls made in each depth category of the CLEAR1 and CLEAR2 analyses are presented in Figure 19. For both analyses, very few calls were made in the deeper depth categories (Figure 19). The point depth estimates (CLEAR calls) ranged from -9.5 to 126.0 m (Figure 20). The majority of the calls calculated to be less than 40 m away from the hydrophone array have expected uncertainty levels of only ± 5-10 m, while calls calculated to be made further away from the hydrophone array have large uncertainty levels reaching values of more than ± 100 m (Figure 20). Most of the calls examined were calculated to be at distances closer to the hydrophone array and thus most of the point depth estimates should be fairly accurate (71% of CLEAR calls were estimated to be made within 40 m of the hydrophone array; Figure 20). When looking at the distribution of the point depth estimates for the CLEAR calls, most of the calls (88 %) were estimated to be made at
BROAD
180 160 140
F t,2.3 = 52.74, p = 0.012
12 0 10 0
80 60 40 20
B
0
< 35 m
c
3 O o
c
35 m
140
SINGLE
12 0
F1|2 = 28.05, p = 0.033
10 0
80 60 40
B
20 0
< 35 m
> 35 m
Depth Category Figure 18. Mean number of Weddell seal calls emitted at each depth category for BROAD, ROUGH and SINGLE analyses. Error bars represent ± 1 SD (from the mean calculated for each depth category). F and p values from the multifactor mixed ANOVAs (using the log transformed data) are given, and groups (depth categories) having different letter labels (A or B) are significantly different (as determined from Tukey’s multiple comparisons post-hoc tests).
40 30 20
Count
10
< 25 m
25-45 m
> 45 m
20
B
15 10
5 _!----
0
70m
-
10-
-
20A
h+/- 20m a+/- >100m
a +/-10 m
■ +/- 5 m • +/- 40 m
□ +/- 50 m
a
+/- 30 m
x Unknown
B
-30-
0) (0 E 5
-40-
£
-60-
-50-
LU
a a> Q
-70 -
c o
-80-
-90-
-100-
110-
-
120-
-1300
10
20
30
40
50
60
70
80
90
100
Calculated Distance From Hydrophone Array (m)
Figure 20. A scatterplot showing the point depth estimates obtained from Weddell seal CLEAR calls (N = 73 calls) and their estimated distance from the hydrophone array. A depth of 0 m represents the sea/ice interface and the dotted line represents a depth of 35 m. It should be noted that in this figure calls estimated to be made from underwater are represented by negative values while calls projected to be above the ice are represented by positive values. The expected accuracy in the depth estimates (indicated in legend) are based on an assumed accuracy of +/-1 dB in the AD measures. Note that the calculated distance from the hydrophone array is assumed to be correct for these accuracy estimates, however, there is uncertainty involved with these values as well.
depths of < 35 m, and all but two of the calls (97%) were made at depths of < 45 m (Figure 20). This follows the trends found in the BROAD, ROUGH and SINGLE analyses (Figure 18). It should be noted, however, that 60% of the CLEAR calls that were estimated to be made at depths of < 35 m were estimated to be made at least 10 m below the undersurface of the ice. Even when just the most accurate CLEAR call depth estimates were considered, (e.g., those made approximately within 40 m of the hydrophone array), this trend is still evident; most of the calls (71%) were still estimated to be made between 10 and 35 m depth (Figure 20).
3c. Call type categories/subtypes and depth The Weddell seal calls were classified into 11 call type categories (Table 5). Type WD calls were the most common calls analyzed from the 10 minute segments (53%). Types C and WA were also common and made up an additional 39% of the calls analyzed. The remaining eight call types were relatively rare, all having sample sizes of less than 100 calls and collectively making up only 8% of the calls analyzed (Table 5). The multifactor ANOVAs investigating the mean number of calls of each call type made at the different depth categories only examined call types WD, C and WA for the BROAD, ROUGH and SINGLE analyses. The “OTHER” category (which consisted of the remaining eight call types) was not considered for these analyses because it would lump call types that are thought to be functionally different from one another together (such as types T and P; trills and pulses). Only 5 of the 11 call types were represented by the CLEAR calls (call types WD, WA, C, T and K). The most common CLEAR call type was Type WD, with 45 calls (62% of the CLEAR calls). The remaining call types all had
Table 5. Number of calls analyzed (from the 10 minute segments) of each of the Weddell seal call type categories and call subtypes. The call types followed the same classification system reported in other studies (Thomas and Kuechle 1982; Pahl et al. 1997; Rouget 2004). Call subtypes were determined from relative differences in calls of each type distinct to the observer. A brief description of the call subtypes is given. Call Type Category
WD (Whistle descending)
C (Chugs)
Number of calls
1528
758
Subtype
389
Narrowband, consistently decreasing frequency, short calls
645
WD2
Narrowband, constantly decreasing frequency, longer calls
855
W D3
Narrowband, inconsistently decreasing frequency
28
C1
Broadband, low decreasing frequency, harmonics present, energy concentrated at end of call, short call, “thunk”-like sound
699
C2
Broadband, low decreasing frequency, no harmonics present, energy concentrated at end of c a ll, short call, “glug”-like sound
10
C3
Broadband, low decreasing frequency, energy concentrated at end of call, longer call, “glug”-like sound at end of call
49
Narrowband, consistently increasing frequency, short calls Narrowband, constantly increasing frequency, longer calls Narrowband, inconsistently increasing frequency
251
P1
Broadband, very short call, click-like sound
21
P2
Narrowband, mid frequency, very short call, click-like sound
61
T1
Broadband, low frequency begins with a whistle-like frequency down-sweep then gradually increasing in frequency, long call
35
T2
Broadband, low frequency, gradually increasing frequency throughout call, long call
9
T3
Broadband, begins with a whistle-like frequency down-sweep then increasing in frequency, short call
1
WA2 WA3
r (Pulse)
T (Trill)
82
45
Number of calls
WD1
WA1 WA (Whistle ascending)
Subtype Description
127
11
Table 5. (Continued) Call Type Category
S (Squeak)
0 (Tone)
K(Knocks) SWD (Step whistle descending)
G (Grunts)
Number of calls
Subtype
17
13
Narrowband, mid constant frequency, very short call (like a very short tone)
7
S2
Narrowband, mid constant frequency ending in a frequency up sweep, very short call
14
S3
Narrowband, mid constant frequency, short call (like a short tone), harmonics present, energy concentrated at beginning of call, beep-like sound
4
01
Narrowband, constant frequency, long call
9
02
Narrowband, mid frequency, slight frequency up sweep at beginning of call then constant frequency, “clank”-like sound
12
03
Narrowband, constant frequency, short call, usually multiple element
13
K1
Broadband, low constant frequency, short call, knock-like sound
17
SWD1
Narrowband, frequency decreases in distinct steps (looks like a series of tones, each lower in frequency than the previous, strung together), long call
13
G1
Broadband, low frequency, “glug” like sound
5
G2
Broadband, low frequency, pulsed, “croak”like sound
3
01
Narrowband, constant frequency with terminal up sweep, drip-like or “whoop”-like sound
2
8
Q (Whoop)
2
TOTAL
2891
Number of calls
S1
25
24
Subtype Description
2891
small sample sizes (of less than 20 calls). Due to the low sample sizes, the CLEAR! and CLEAR2 analyses were not considered. For the BROAD, ROUGH and SINGLE analyses, the ANOVAs which considered repetition, site, season, photoperiod, call type and depth category effects all displayed significant interactions between the call types and depth categories (all pvalues < 0.001). Therefore, ANOVAs that considered only repetition, site, season, photoperiod and depth category effects were run for each call type (WD, C and WA) individually. All significant effects obtained from these ANOVAs are presented in Table 6 . For call types WD and C, significantly more calls were made at the < 35 m depth
category for the BROAD (Figure 21 A) and SINGLE analyses, and at the < 35 m and ~ 35 m depth categories for the ROUGH analysis (following the same trends presented in Figure 18). The mean number of calls of Type WA emitted at each of the depth categories were relatively consistent; no significant depth category effects were present for the ROUGH or SINGLE analyses, and only a slightly significant effect was present for the BROAD analysis (Table 6 ; Figure 21A). In all situations where two-way interactions involving depth category were present, more calls were made at the shallower depth category however the magnitude of the difference between the depth categories varied (either between seasons for call type C or photoperiods for call type WA). The data for the Type C calls of the SINGLE analysis violated the normality assumptions and should be interpreted with caution. The overall number of calls emitted at each depth category for the BROAD, ROUGH and SINGLE analyses for the less common call types are presented in Table 7. The majority of the calls of types P, T and O appear to be made at the shallower depth categories.
100 90 80 ^
70
g
60
ü
c
50
S
40
S
30
20 10
0 WD
WD
WA
WA
OTHER
Call Type Category Figure 21. Mean number of Weddell seal calls of call types WD, C and WA (A), and the overall percent composition of call types WD, C, WA and OTHER (B), at the < 35 m (darker bars) and > 35 m (lighter bars) depth categories of the BROAD analysis. Error bars in graph A represent ± 1SD (of the mean count calculated for each call type within each depth category).
Table 6. Summary of the multifactor mixed ANOVA results testing the number of Weddell seal calls emitted within each depth category for call type categories WD, C and WA of the BROAD, ROUGH and SINGLE analyses. The count data was log transformed for all analyses, and both F and p values are given. Depth category effects and all other significant interactions and main effects (p-values < 0.05) from each ANOVA are given. “Not significant” indicates that an effect was not significant for a particular analysis. Repetition (the 10 minute segments) and site were considered random factors and season, photoperiod and depth category were considered fixed factors for all analyses.
ANOVA Results ANOVA
5-way ANOVA for WD
5-way ANOVA for C
Effect BROAD
ROUGH
SINGLE
Depth category
F, .a , = 87.52 p = < 0.001
F2,8.18 = 11-15 p = 0.005
Fi,i.75= 31.66 p = 0.040
Depth category
Fi,4.58= 74.45 p < 0.001
F24i= 12.99
F1,2.15 = 48.66 p = 0.017
Season
Not significant
Not significant
F1i26 = 10.87 p = 0.003
7.51 p = 0.011
Not significant
F1,26= 16.99 p < 0.001
Fi 2 22 = 16.01 p = 0.048
F2,5.5 = 3.23 p = 0.1181 Not significant
Fi,is =6.31 p = 0.1351 Not significant
F1,24.6 = 5.81 p = 0.024
Not significant
F i ,23 =6.97 p = 0.015
Depth category * Season
Depth category
F 1,23.8 =
5-way ANOVA for WA Depth category * Photo
p
35 m
25
6
19
< 35 m
27
11
15
~ 35 m
NA
11
NA
> 35 m
7
7
0
< 35 m
17
4
13
~ 35 m
NA
5
NA
> 35 m
3
3
0
< 35 m
17
7
10
~ 35 m
NA
4
NA
> 35 m
3
3
0
< 35 m
4
3
1
~ 35 m
NA
6
NA
> 35 m
7
5
1
< 35 m
3
0
3
~ 35 m
NA
10
NA
> 35 m
0
0
0
< 35 m
5
2
3
~ 35 m
NA
2
NA
> 35 m
1
1
0
< 35 m
0
0
0
~ 35 m
NA
2
NA
> 35 m
0
0
0
The proportions of calls of types WD, C, WA and OTHER (the eight rare call 'y
types) were significantly different across depth categories of the BROAD (% 3 = 139.158, p < 0.001), ROUGH (^ 6 = 89.831, p < 0.001) and SINGLE (Z2 3 = 75.138, p < 0.001) analyses (e.g., Figure 21B). Each of the 11 call type categories were subdivided into one, two or three subtypes, and 25 subtypes were examined (Table 5). Subtype WD2 was the most common making up 30% of the calls analyzed from the 10 minute segments. Subtypes C l, WD1, WA2 and WA1 were also common and made up an additional 59% of the calls analyzed. The remaining 20 call types were relatively rare, all having sample sizes of less than 70 calls and collectively making up only 11% of the calls analyzed (Table 5). Only call subtypes WD2, C l, WD1, WA2 and WA1 were considered for multifactor ANOVAs investigating the mean number of calls of each call subtype made at the different depth categories of the BROAD, ROUGH and SINGLE analyses. Again, an “OTHER” category was not considered for these analyses. Only 9 of the 25 subtypes were represented by the CLEAR calls (subtypes C l, C3, K l, T l, WA1, WA2, WD1, WD2 and WD3). The most common subtype was WD2, with 37 calls analyzed (51% of the CLEAR calls). The remaining call types all had small sample sizes (of less than 20 calls). Due to the low number of CLEAR calls analyzed, the CLEAR 1 and CLEAR2 analyses were not considered. For all the three analysis types examined (BROAD, ROUGH and SINGLE analyses), the ANOVAs that considered repetition, site, season, photoperiod, call subtype and depth category effects displayed significant interactions between the subtypes and depth categories (all p-values < 0.002). Therefore, ANOVAs that only considered
repetition, site, season, photoperiod and depth category effects were run for each subtype (WD2, C l, WD1, WA2 and WA1) individually (Table 8). In all cases where the mean number of calls emitted at each depth category were significantly different, and significantly more calls (of each type) were made at the < 35 m depth category for the BROAD (Figure 22A) and SINGLE analyses, and at the < 35 m and ~ 35 m depth categories for the ROUGH analysis (following the same trends presented in Figure 18). In all situations where two-way interactions involving depth category were present, significantly more calls were made at the shallower depth categories, however, the magnitude of the difference between the mean number of calls emitted at each depth category varied (either between seasons for subtype WD2, C l and WA1, or between photoperiods for subtype WA1). The data for the WA1 and WA2 calls of the ROUGH analysis and C l, WD1 and WD2 calls of the SINGLE analysis violated the normality assumption of the ANOYA tests and results should be interpreted with caution. The overall number of calls emitted at each depth category for the BROAD, ROUGH and SINGLE analyses of the less common call subtypes are presented in Table 9. The majority of the calls of the less numerous subtypes, (subtypes WD3, WA3, C3, P2, T1 and S2), appeared to be made at the shallower depth categories (Table 9). The proportions of calls of call types WD2, C l, WD1, WA2, WA1 and OTHER (the 20 rare subtypes) were significantly different across depth categories of the BROAD (X2 5 = 138.632, p < 0.001), ROUGH (%2 10 = 86.230, p 35 < 35 ~ 35 > 35 < 35 ~ 35 > 35 < 35 ~ 35 > 35 < 35 ~ 35 > 35 < 35 ~ 35 > 35 < 35 ~ 35 > 35 < 35 ~ 35 > 35
m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m
A n alysis Typ e B R O A D R O U G H SIN G LE
26 NA 0 10 NA 0 4 NA 1 20 NA 5 9 NA 5 41 NA 20 19 NA 7 7 NA 1 1 NA 0 7 NA 0
14 2 0 7 1 0 3 5 0 18 23 4 0 7 5 0 0 1 11 10 6 0 1 1 1 0 0 0 0 0
10 NA 0 3 NA 0 1 NA 1 2 NA 1 9 NA 0 41 NA 19 8 NA 0 7 NA 0 0 NA 0 7 NA 0
Call S u btype
S2
S3
01
02
03
K1
SWD1
G1
G2
Q1
A n alysis Typ e BROAD
ROUGH
S IN G LE
9 NA 1 1 NA 2 7 NA 0 10 NA 1 0 NA 2 0 NA 0 3 NA 0 3 NA 0 2 NA 1 0 NA 0
3 4 1 1 1 2 2 2 0 5 2 1 0 0 2 3 6 5 0 10 0 2 2 0 0 0 1 0 2 0
6 NA 0 0 NA 0 5 NA 0 5 NA 0 0 NA 0 0 NA 0 3 NA 0 1 NA 0 2 NA 0 0 NA 0
3d. Number of elements (NOE) and depth The Weddell seal calls consisted of 1-121 elements. The majority of calls were single element calls (2444 calls; 85%). Only 56 (2%) and 391 calls (13%), respectively, were double element and multiple elements calls. The majority of the multiple element calls (80%) were less than 20 elements in length. Ten of the calls examined consisted of 50-100 elements, and only a single call was made up of more than 100 elements. The majority of the longer multiple elements calls were chugs and pulses (Types C and P). For the BROAD, ROUGH and SINGLE calls, the number of calls having a particular NOE were analyzed in three categories; calls having 1, 2-10 or > 10 elements. Because of the low number of CLEAR calls analyzed, the CLEAR 1 and CLEAR2 analyses were not examined. For the BROAD analysis, the interaction between NOE category and depth category was not significant for the 6-way ANOVA considering repetition, site, season, photoperiod, NOE category and depth category effects (Table 10). A significant difference was present between the mean number of calls emitted at each depth category of the BROAD analysis, and significantly more calls of each NOE category were emitted at the < 35 m depth category (Figure 23A). A significant interaction between NOE category and depth category was present for the ROUGH and SINGLE analyses (p-values < 0.021), therefore, ANOVAs that considered repetition, site, season, photoperiod and depth category effects were run for each NOE category individually (Table 10). There was a significant difference in the mean number of calls emitted at each depth category for single element calls of the ROUGH analysis (Table 10). The Tukey’s post-hoc tests indicated that more calls were
Table 10. Summary of the multifactor mixed ANOVA results testing the number of Weddell seal calls emitted within each number of elements (NOE) category and total duration of calls emitted at each depth category of the BROAD, ROUGH and SINGLE analyses. The count data was log transformed for all analyses, and both F and p values are given. Depth category effects and all other significant interactions and main effects (p-values < 0.05) from each ANOVA are given. “Not significant” indicates that an effect is not significant for a particular analysis and “NA” indicates results that are not given for a particular analysis. Repetition (the 10 minute segments) and site were considered random factors and season, photoperiod and depth category were considered fixed factors for the 5-way ANOVAs. The 6-way ANOVA also considered NOE category as a fixed factor.
A N O V A R esults ANOVA
Effect BROAD
R O UG H
SIN G LE
Depth category
F-i 2 = 66.63 p’ = 0.015
NA
NA
NOE Category
• 2 3 22 “ 66.06 p = 0.002
NA
NA
5-way ANOVA for single element calls
Depth category
NA
5-way ANOVA for calls with 210 elements
Depth category
NA
F2.16.4 = 4.32 p = 0.031
F1,28= 54.82 p < 0.001
5-way ANOVA for calls with > 10 elements
Depth category
NA
F2.4.57 = 2 .7 0 p = 0.167 Not significant
F 1,1.82 = 3.77 p = 0.083 Not significant
5-way ANOVA for total duration
Depth category
F 1,45.3 = 0.03 p = 0.870 Not significant
F21 =0.41 p = 0.743 Not significant
= 0-61 p = 0.440 Not significant
6-way ANOVA for single element calls
= 12.79 p < 0.001
F 2,38
F 1 ,1.99 = 30.85 p = 0.031
Fi,53.8
2-10
>10
Num ber of Elements Figure 23. Mean number of Weddell seal calls of each number of elements (NOE) category (A), and the overall percent composition of the NOE categories (B), at the < 35 m (darker bars) and > 35 m (lighter bars) depth categories of the BROAD analysis. Error bars in graph A represent ± 1SD (of the mean count calculated for each NOE category within each depth category).
emitted at the < 35 m and ~ 35 m depth categories. A significant difference was also present between the mean number calls 2-10 elements in length emitted at each depth category of the ROUGH analysis (Table 10), and significantly more calls were emitted at < 35 m than at the > 35 m depth category. For the SINGLE analysis examining calls with 1 and 2-10 elements, significant differences were present between the mean number of calls emitted at each depth category (Table 10) due to significantly more calls being emitted at the < 35 m depth category. No significant difference between the mean number of calls emitted at each depth category for calls > 10 elements in length was observed for the ROUGH or SINGLE analyses (Table 10). The data for SINGLE calls 2-10 elements in length violated the normality assumption and should be interpreted with caution. The proportion of calls of each NOE category was significantly different across depth categories of the BROAD (x2 2 = 7.127, p = 0.028; Figure 23B) and SINGLE (x22 = 14.320, p = 0.001) analyses. The proportion of calls was of each NOE category was not significantly different across depth categories of the ROUGH analysis (x 4 = 7.259, p = 0.123).
3e. Rhythm patterns within multiple element calls and depth The multiple element calls were found to occur with five of the seven rhythm patterns described by Moors and Terhune (2004). The most common rhythm patterns were Pattern cC (constant element and interval durations) which occurred in 199 (51%) of the calls analyzed, Pattern cl (constant element durations and consistently increasing interval durations) which occurred in 146 (37%) of the multiple element calls, and Pattern
cD (constant element durations and consistently decreasing interval durations) which occurred in 37 (9%) of the multiple element calls. Only a single call of each of Patterns iC and il occurred, and nine (2%) of the multiple element calls had irregular timing patterns. The overall number of calls emitted with Pattern cC, cl and cD timing (for the BROAD, ROUGH and SINGLE analyses) are shown in Figure 24. More calls of Pattern cC and cl appeared to be made at the < 35 m depth category of the BROAD and SINGLE analyses, and at the 35 m (lighter bars) depth categories of calls emitted within the BROAD (A), ROUGH (B), and SINGLE (C) analyses.
LOG (DURATION) = -0.333 + 0.00027 DEPTH Fi,71 = 0.01 p = 0.933 R2 = 0
Point Depth Estim ate (m)
Figure 25. Point depth estimates and total duration of Weddell seal CLEAR calls (N = 73 calls). Results of a simple linear regression (F, p and R2 values) used to examine the relationship between the total duration and depth is also given.
3g. Frequency (kHz) and depth The Weddell seal calls examined were typically emitted within the frequency range of 0.1-10 kHz. In total, 77 calls (3%) were < 1 kHz in frequency, 2390 calls (83%) were emitted in the range of 1-5 kHz, and 247 calls (9%) were emitted in the range of 510 kHz. Only 185 of the calls examined (6%) were emitted above frequencies of 10 kHz and were typically high frequency descending whistles (WD calls). Like the total duration data, the frequency (kHz) data residuals were highly slewed and could not be normalized, and so the results should be interpreted with caution. Frequency (kHz) measurements of 24 calls (all broadband pulses) could not be accurately obtained and were excluded from the analysis. Although the start, middle and end frequency (kHz) of the first element in the calls were all examined, due to similar results between the three measures, only the data for the element start frequencies will be presented. The BROAD and ROUGH analysis showed so significant differences between the overall mean frequencies (kHz) at the different depth categories (Table 11). A significant difference was present for the SINGLE analyses (Table 11) and the mean frequency (kHz) of calls in the < 35 m depth category was significantly higher than the mean frequency (kHz) of calls emitted within the > 35 m depth category. There was a significant interaction effect between call type and depth category for all of the 6-way ANOVAs which considered repetition, site, season, photoperiod, call type and depth category effects (all p-values < 0.001). Call types were therefore examined individually using a series of 5-way ANOVAs which considered repetition, site, season, photoperiod and depth category effects (Table 11).
Table 11. Summary of the multifactor mixed ANOVA results testing the mean frequency (kHz) of Weddell seal calls emitted within each depth category, and the mean frequency of calls of each call type within each depth category of the BROAD, ROUGH and SINGLE analyses. The data were log transformed for all analyses, and both F and p values are given. Depth category effects and all other significant interactions and main effects (p-values < 0.05) from each ANOVA are given. “Not significant” indicates that an effect is not significant for a particular analysis. Repetition (the 10 minute segments) and site were considered random factors and season, photoperiod and depth category were considered fixed factors.
A N O V A R esults ANOVA
Effect BROAD
RO UG H
SIN G LE
5-way ANOVA for overall frequency
Depth category
F 1 ,2 2 3 3 = 0.06 p = 0.800 Not significant
F2 , 1 2 3 3 = 0 . 1 0 p = 0.906 Not significant
F 1,1401 = 4.70 p = 0.030
5-way ANOVA for W D
Depth category
F 1,1217 = 4.09 p = 0.043
F2 ,6 9 4 = 3.94 p = 0.020
= 1-81 p = 0.180 Not significant
5-way ANOVA fo rC
Depth category
F 1,575 = 30.95 p < 0.001
F2 , 9 6 . 4 = 15.27 p < 0.001
F 1,395 = 5.24 p = 0.0.023
5-way ANOVA for W A
Depth category
= 28.28 p < 0.001
= 5.32 p = 0.006 Not significant
5-way ANOVA for OTHER
Depth category
F 1 ,2 7 5
= 1.14 p = 0.288 Not significant F i ,1 5 2
F i ,7 3 4
F 2 .1 8 6
F2 , 8 1 . 3 = 3.54 p = 0.034
F
i
, 1 2 5 = 11-05 p = 0.001
F 1 ,94.8 = 9 .7 7 p = 0.002
For the BROAD analysis, significant differences were present between the mean frequencies (kHz) of calls emitted within the two depth categories for calls of types WD, C and WA (Table 11). The mean frequency (kHz) of calls emitted within the < 35 m depth category was significantly higher than the mean frequency (kHz) of calls emitted in the > 35 m depth category for the Type C calls. The opposite trend (higher mean frequency (kHz) at the > 35 m depth category) occurred for WD and WA calls (Figure 26). There was a significant difference among the mean frequencies of call types WD, C and OTHER emitted at each depth category for the ROUGH analysis (Table 11). The Tukey’s post-hoc tests indicated that the mean frequency (kHz) of calls emitted at the > 35 m depth category was significantly higher than the mean frequency (kHz) of calls within the ~ 35 m depth category for Type WD calls, and the mean frequency (kHz) of calls within the ~ 35 m depth category was significantly higher than the mean frequency (kHz) of the > 35 m depth category for OTHER calls. For the Type C calls, the mean frequencies (kHz) at all three depth categories were significantly different from each other, with the highest mean frequency (kHz) occurring at the > 35 m depth category and lowest mean frequency (kHz) occurring at the ~ 35 m depth category. For the SINGLE analysis, the mean frequencies (kHz) emitted at each depth category were significantly different for call types C, WA and OTHER (Table 11). Frequency (kHz) of the calls was significantly higher at the < 35 m depth category for the C and OTHER calls, and significantly higher at the > 35 m depth category for the WA calls.
WD
WA
OTHER
Call Type Category
Figure 26. Mean start frequency (kHz) of Weddell seal calls within each call type category for the < 35 m (darker bars) and > 35 m (lighter bars) depth categories of calls analyzed as part of the BROAD analysis. Error bars represent ± 1SD (of the mean frequency (kHz) calculated for each call type at each depth category).
There was a significant interaction effect between call subtype and depth category for all of the 6-way ANOVAs which considered repetition, site, season, photoperiod, call subtype and depth category effects (all p-values < 0.001). Call subtypes were therefore examined individually using a series of 5-way ANOVAs which considered repetition, site, season, photoperiod and depth category effects (Table 12). For the BROAD analysis, significant differences were present between the mean frequencies (kHz) of calls emitted within the two depth categories for calls of subtypes WD1, C l and WA1, WA2 and OTHER (Table 12). The mean frequency (kHz) of calls emitted within the < 35 m depth category was significantly higher than the mean frequency (kHz) of calls emitted in the > 35 m depth category for calls of subtype C l and OTHER. The opposite trend (higher mean frequency (kHz) at the > 35 m depth category) occurred for the WD1, WA1 and WA2 calls. There was a significant difference amoung the mean frequencies (kHz) at each depth category of subtypes WD1, WD2, C l, WA2 and OTHER for the ROUGH analysis (Table 12). The Tukey’s post-hoc tests indicated that the mean frequency (kHz) of calls emitted at the > 35 m depth category was significantly higher than the mean frequency (kHz) of calls within the ~ 35 m depth category for WD1 calls. The mean frequency (kHz) of calls within the < 35 m depth category was significantly higher than the mean frequency (kHz) of calls emitted within the ~ 35 m and > 35 m depth categories (which were not significantly different from each other) for WD2 calls. The mean frequencies (kHz) of calls within the < 35 m and ~ 35 m depth categories were not different from each other but were both significantly higher than the mean frequency (kHz) of calls emitted within the > 35 m depth category for C l calls. For the WA2 calls, the mean
Table 12. Summary of the multifactor mixed ANOVA results testing the mean frequency (kHz) of Weddell seal calls of each call subtype within each depth category of the BROAD, ROUGH and SINGLE analyses. The data were log transformed for all analyses, and both F and p values are given. Depth category effects and all other significant interactions and main effects (p-values < 0.05) from each ANOVA are given. “Not significant” indicates that an effect is not significant for a particular analysis. Repetition (the 10 minute segments) and site were considered random factors and season, photoperiod and depth category were considered fixed factors.
A N O V A R esults ANOVA
Effect BROAD
RO UG H
S IN G LE
5-way ANOVA fo rW D I
Depth category
F 1,524 = 5.89 p = 0.016
F2 , 2 2 7 = 3.22 p = 0.042
F 1 ,3 4 0 = 0 .1 1 p = 0.745 Not significant
5-way ANOVA for W D2
Depth category
F1 ,6 6 3 = 0 . 1 2 p = 0.725 Not significant
F2,339 = 3.30 p = 0.038
F1,379 = 0.06 p = 0.800 Not significant
5-way ANOVA for C1
Depth category
= 7.38 p < 0.001
F1,393 = 1 -29 p = 0.257
5-way ANOVA for WA1
Depth category
F 1 ,96.9 = 7.92 p = 0.006
F2.55.9 = 1 -49 p = 0.234 Not significant
5-way ANOVA for W A2
Depth category
F 1,140 = 46.46 p < 0.001
F2,66 = 21.50 p < 0.001
F i ,84.i = 9.84 p = 0.002
5-way ANOVA for OTHER
Depth category
F 1,218 = 7.82 p = 0.006
F2,140 = 12.80 p < 0.001
F 1,113 = 3.03 p = 0.085 Not significant
= 21.84 p < 0.001
F l,5 4 8
F 2 .1 6 3
= 12.89 p = 0.002
F 1 ,1 9 .5
frequencies (kHz) at all three depth categories were significantly different from each other, with the highest mean frequency (kHz) occurring at the > 35 m depth category and lowest mean frequency (kHz) occurring at the < 35 m depth category. For the OTHER calls, the mean frequencies (kHz) at all three depth categories were significantly different from each other, with the highest mean frequency (kHz) occurring at the ~ 35 m depth category and lowest mean frequency (kHz) occurring at the > 35 m depth category. For the SINGLE analysis, the mean frequencies (kHz) emitted at each depth category were significantly different for call types C l, WA1, and WA2 (Table 12). Frequency (kHz) of the calls was significantly higher at the > 35 m depth category in all cases. The regression of frequency (kHz; log transformed) and depth using the point depth measurements of the CLEAR calls showed no significant relationship between frequency and depth (Fi;43 = 0.01, p = 0.909) and none of the variance was explained by the regression equation obtained (R2 = 0; Figure 27). When a regression of the point depth estimates of only a single call type (Type WD, the most numerous CLEAR call) and frequency (kHz) was performed, again, no significant relationship was found between frequency and depth (Fi ;43 = 0.01, p = 0.903) and none of the variance was explained by the regression equation obtained (R = 0; Figure 28).
IV. DISCUSSION 1.
Limitations in determining point calling depth using Cato’s (1998) calculations Although the method described in this study offers a more practical and less
expensive means of determining calling depth of animals than large complex hydrophone
LOG (FREQUENCY) = 3.46 + 0.00018 DEPTH Fi,44 = 0.01 p = 0.909 R2 = 0
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Point Depth Estim ate (m)
Figure 27. Point depth estimates and start frequency (kHz) of the first element within a call obtained from the Weddell seal CLEAR calls (N = 73 calls). Call types are indicated in the legend. Results of a simple linear regression (F, p and R2 values) used to examine the relationship between two variables is also given.
LOG (FREQUENCY) = 0.459 + 0.00019 DEPTH Fi,43 = 0.01 p = 0.903 R2 = 0
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Point Depth Estim ate (m )
Figure 28. Point depth estimates and start frequency (kHz) of the first element within calls of Type WD (whistle descending) obtained from the Weddell seal CLEAR calls (N = 45 calls). Results of a simple linear regression (F, p and R2 values) used to examine the relationship between the frequency (Hz) and depth for Type WD calls is also given.
arrays, it does have limitations. A major constraint of this method was finding calls to which the calculations could be applied. Accurate ATD and AD measures could only be obtained from calls that stood out very clearly against background noise on both hydrophone channels. These calls were not common, especially on recordings where background noise levels were high (which was the case for most of the recordings analyzed). From the large sample of harp and Weddell seal calls examined, point depth measurements could only be obtained from a small portion (less than 9% of harp seal calls and less than 2% of Weddell seal calls). Fewer point depth estimates were obtained from Weddell seal recordings probably as a result of fewer seals present at the sites where recordings were obtained. The harp seal herd consists of thousands of seals while the Weddell seal breeding groups probably did not exceed 30 individuals (Rouget 2004). Accurate ATD and AD measures could only be obtained from calls with a high signal to noise ratio and so CLEAR call results were biased towards loud calls typically made in close proximity to the hydrophone array. The underwater vocal behavior of the seals may vary depending on their surroundings. Seals vocalizing at or near breathing holes may behave differently than seals in other areas of the water column, such as under large ice floes or solid, unbroken surfaces of the land-fast ice (where less light would penetrate and different social interactions may occur). For example, it is known that Weddell seals actively defend breathing holes (Stirling 1969, Kooyman 1981), and some call types such as trills, pulses and chirps may have functions related to territorial defense and seals approaching a guarded breathing hole (Watkins and Schevill 1968; Thomas et al. 1983; Oetelaar et al. 2003; Rouget 2004). Calling behavior in close proximity to the breathing holes may be related to
aggressive and submissive interactions, while these types of behaviors are less likely to occur farther away from breathing holes. Because most CLEAR calls examined were made near the hydrophone array, the location of the array (at breathing holes through the middle of large ice floes or along a lead for harp seals, and at man-made holes at least 20 m away from the nearest breathing hole for Weddell seals) may potentially be influencing the results (Terhune et al. 2001). CLEAR call results were also biased towards certain call types and subtypes. It was difficult to determine the beginning and end, or particular points on certain calls, such as constant frequency (kHz) calls or calls that did not have abrupt beginnings or ends. For example, it was difficult to obtain accurate ATD and AD measures for tones and trills (harp seal call types 1 and 4 and Weddell seal call types O and T). Certain call types were therefore under-represented in the CLEAR analyses while other types were better suited for obtaining ATD and AD measures and occurred frequently in the CLEAR call data set (such as the harp seal Type 14 calls, or the Weddell seal WD calls). Another limitation of this method is its sensitivity to the accuracy in amplitude measures. Even small alterations in amplitude may greatly affect the depth calculated (Figure 6). This results in uncertainly about the depth estimates, especially as the distance of the seal from the hydrophone array increases or as the seal moves towards the midpoint of the two hydrophones (Figure 6). The higher estimated uncertainty levels and low number of suitable calls around the 35 m depth range (Figures 9 and 20) is probably a reflection of the decreasing accuracy of the depth calculations as the seal approaches the midpoint between the two hydrophones. One safeguard against highly inaccurate amplitude measures is that they result in inaccurate ATD and AD combinations and can
not be solved by Cato’s (1998) equations. The ATD and AD measures have to be realistic in order to provide a depth estimate. When using Cato’s (1998) calculations to determine depth of calling animals, it should be remembered that the method was not intended for localizing individuals; rather it was developed as a means of determining call source levels. The calculations do, however, offer a logistically simplistic means of acquiring information about a vocalizing animal’s position (depth in the case of a vertical hydrophone array). The uncertainty levels associated with the depth measures and low sample sizes obtained may limit the information that can be gained from the calls, but the point depth estimates were still valuable, especially when paired with data obtained from the rough depth approximations. The CLEAR call data helped confirm and give more detailed information on trends observed from the rough depth estimates.
2. Types of depth measures obtained and analyses types The number of ROUGH, SINGLE and CLEAR calls obtained from the harp and Weddell seal recordings varied between the two species (Figure 7). Most of the harp seal rough depth estimates were made from calls that were relatively loud and occurred on both hydrophone channels (ROUGH calls), while very few calls occurred on only a single hydrophone (SINGLE calls; Figure 7). In comparison, the Weddell seal calls tended to be quieter and frequently occurred on only one hydrophone channel. The number of ROUGH and SINGLE depth estimates obtained from the Weddell seal calls were very similar to each other (Figure 7). For the Weddell seals, the ROUGH calls also commonly occurred first on one hydrophone but louder on the other, a phenomenon that
was relatively rare for the harp seal calls. These differences between the calls recorded at the harp and Weddell breeding sites may be a result of physical or behavioral differences between the two species and/or physical differences between the pack-ice and land-fast ice environments. Schevill and Watkins (1971) present evidence that Weddell seals produce calls with a directional sound beam projected forward and downward from the throat. The amplitude of vocalizations were found to suddenly change when a seal turned towards or away the hydrophones while vocalizing, and the received amplitude levels appeared to be dependant on the orientation of the seal. The authors also report that calls frequently occur first on one hydrophone of an array but louder on another, and suggest that this phenomenon may be explained by directionality of the sound beam (Schevill and Watkins 1971). The extent to which harp seals direct their calls has not been investigated, although these results suggest that the harp seal calls are not as highly directed as the Weddell seal calls.
3. Trends in call types, subtypes, NOE, rhythm patterns and total duration with depth 3a. Harp seals All harp seal call types, subtypes, NOE categories and rhythm patterns considered in this study occurred in the < 35 m, ~ 35 m and > 35 m depth categories (Figures 1013). Although calls of each attribute occurred in all of the depth categories examined, more calls of each call type category, subtype, NOE and rhythm pattern tended to be made at the shallower depth categories within the BROAD, ROUGH, CLEAR 1 and CLEAR2 analyses (Figures 10-13). No call types or subtypes were made predominately
at the deeper depth categories, nor were calls having a specific NOE or rhythm pattern emitted more often at a deeper depth. Although the proportion of call types and subtypes within each depth category of the BROAD and ROUGH analyses were significantly different, they were relatively consistent across depth categories (Figures 10 and 11). There was no significant difference in the percentage of calls having a specific NOE emitted at each depth category (Figure 12), or the mean total call duration among depth categories (Table 3; Figure 14). These results suggest that the functions of the different types of calls emitted by the harp seals are not depth dependent and that the seals do not change their vocalization behavior with depth. Because the frequency of occurrence of calls of each call type, subtype and NOE did not vary greatly with depth, the higher number of calls emitted at the shallower depth categories may be due to more seals vocalizing in the shallower depth range and may not necessarily be a result of specific call functions. Lydersen and Kovacs (1993) studied diving behavior of four lactating female harp seals within the Gulf herd using time-depth recorders. The females spent the majority of their time in the water, either sitting at the surface (30-40% of their time) or diving (35-50% of their time). The majority of the dives (61%) made by the four females had a maximum depth of < 30 m. Staying in the water probably offers a greater thermal advantage than remaining on the ice (Lydersen and Kovacs 1993), however, it would be advantageous for females in the water to stay in close vicinity of the breathing hole through which they entered in order to easily relocate their pup (Terhune et al. 1979). The Lydersen and Kovacs (1993) study indicates that the females spent the majority of their time in the water at shallow depths (< 30 m).
3b. Weddell seals Similar to the harp seal results, the Weddell seal call types, subtypes, NOE categories and rhythm patterns considered in this study occurred in the < 35 m, ~ 35 m and > 35 m depth categories (Figures 21-24). More calls of each tended to be made at shallower depths (Figures 21-24). Season and photoperiod generally did not have a significant effect on the number of calls of each call type, subtype, NOE or rhythm pattern occurring at each of the depths. The trends displayed by these call attributes were consistent between the pre pupping and pupping seasons, as well as between light and dark hours. Rouget (2004) investigated changes in the Weddell seal calling behavior in the Holme Bay region (with respect to call types emitted and calling rates) over the pre-pupping and pupping season, as well as over 24 hour periods. Daytime calling rates were found to increase during the pupping season, however, similar call types were emitted during both seasons. Calling rates also tended to increase at night although call types did not vary over a 24 hour period (Rouget 2004). The mean number of calls of each call type, subtype, NOE or rhythm pattern tended to greater at the < 35 m depth category for the BROAD and SINGLE analyses, while calls frequently occurred in both the < 35 m and ~ 35 m depth categories of the ROUGH analysis (there was generally no difference in the number of calls emitted within these two depth categories for any of the call attributes). This result is supported by the CLEAR1 and CLEAR2 analyses (Figure 19). None of the common call types or subtypes were made predominately at the > 35 m depth category, nor were calls of a
specific NOE or rhythm pattern emitted more frequently at the deeper depth (Figures 2124).
Unlike the harp seal calls, the frequency of occurrence of Weddell seal call types and subtypes were significantly different between depth categories for all analyses examined, and the proportional usage of calls of each type and subtype across depth categories was not consistent (Figures 21 and 22). Chugs (Type C) and descending whistles (Type WD) constituted the majority of the calls emitted at the shallower depths, while ascending whistles (Type WA) and OTHER calls were made more frequently at the > 35 m depth than at the < 35 m depth (Figure 21 and 22). Chugs are typically viewed as an aggressive call (Thomas et al. 1983) and thus it makes sense that they occur more frequently at shallower depth ranges where the seals are more likely to face conflicts with conspecifics and display territorial defense behaviors. Trills are also related to territorial defense and aggressive interactions between males (Thomas et al. 1983; Pahl et al. 1997; Thomas et al. 1988). Only a few depth estimates were obtained for trills (N = 45; Table 5), most of which were emitted in the shallower depth range (Tables 7 and 9; Figure 27). The function of the whistles is unknown. There was a significant difference in the percentage of calls having a specific NOE at each depth category for the BROAD and SINGLE analyses, however, the proportion of calls of each NOE category made at the different depths appeared to be relatively consistent (Figure 23), and no significant difference in the frequency of occurrence of each NOE category was observed in the ROUGH calls. Mean total duration of the calls also did not vary with depth (Table 10; Figure 25).
Because male Weddell seals actively defend underwater territories near breathing holes (Stirling 1969; Kooyman 1981; Rouget 2004), it is highly likely that most social interactions between the seals occur at shallower depths in close proximity to the males’ territories. Due to limited light levels underwater (especially in the sea-ice environment), sound is the most effective means of communication underwater and most underwater social interactions between the seals probably involve acoustic communication. The majority of the seal vocalizations would therefore be expected to occur at shallower depths where the seals are actively defending territories and displaying aggressive and submissive behaviors. Schevill and Watkins (1971) observed that Weddell seals vocalizations occurred frequently on the 10 m hydrophone channel of a three-element hydrophone array (with hydrophones at 10,150 and 300 m depth), while fewer calls were heard on the deeper hydrophone channels. Evans et al. (2004) found that the vocalizations emitted by Weddell seals were associated with approaching a breathing hole and looking at the undersurface of the ice, and occurred most frequently at shallower depths during the ascent portion of a dive.
3c. Implications for vocal behavior None of the call types or subtypes emitted by the harp or Weddell seals appeared to be limited to a certain depth range. Therefore, the types of vocalizations produced by the seals were not affected by physical restrictions imposed on the seal due to increasing water pressure. Call type, subtype, NOE, rhythm pattern and the total call duration were not physically constrained by depth.
The function of the harp and Weddell seal calls appears to be independent of depth, and calls made at deeper depths were not different than the calls emitted at shallower depths (with regards to the call attributes studied). This means that listening conspecifics would not be able to determine from what depth range a call was emitted based on these attributes alone. For both species, the majority of calls of each call type, subtype, NOE and rhythm pattern occurred at shallower depths (of < 35 m and ~ 35 m). This infers that the social behaviors of both species occur predominately near the surface (during both the pre pupping period and pupping period, as well as during dark and light hours for the Weddell seals).
4. Trends in frequency (kHz) over depth 4a. Harp seals Frequency (kHz) of the harp seal calls did not increase with increasing depth for the majority of the analyses. Although a slight positive relationship between depth and frequency (kHz) was evident in the regressions using the CLEAR call data, the relationship between frequency (kHz) and depth was not strong when overall frequencies were examined or when frequencies of each call type and subtype were examined separately (Figures 16 and 17). No consistent relationship between depth and frequency (kHz) was evident overall or when frequencies of each call type and subtype were examined separately for the BROAD, ROUGH and SINGLE analyses (Table 4; Figure 15). Although the overall mean frequencies (kHz) of the ROUGH calls were significantly different among depth categories, when the calls were analyzed by call type
or subtype frequency (kHz) differences were not observed over depth (Table 4). There were also significant differences in the overall mean frequency (kHz) among depth categories for the CLEAR1 and CLEAR2 analyses (Table 4). This difference remained when the calls were analyzed by call type or subtype (Table 4), however, there was no consistent trend in the mean frequency (kHz) over depth, and calls made at the deeper depth category were not necessarily the calls with the highest mean frequency.
4b. Weddell seals Frequency (kHz) of the Weddell seal calls also did not increase with increasing depth. No strong relationship between frequency (kHz) and depth was evident overall or when frequencies (kHz) of each call type and subtype were examined separately (Table 11; Figures 26-28). The overall mean frequency (kHz) of calls emitted at each depth category of BROAD and ROUGH analyses were not significantly different from one another (Table 11). Although the overall mean frequencies (kHz) of the SINGLE calls were significantly different among depth categories (Table 11), calls of the < 35 m depth category were found to be significantly higher. There were significant differences in the mean frequencies (kHz) among depth categories for the different call types and subtypes analyzed as part of the BROAD, ROUGH and SINGLE analyses (Tables 11 and 12), however, there was no consistent trend in the mean frequency (kHz) over depth. Calls made at the deeper depth category were not necessarily the calls with the highest mean frequency (kHz).
4c. Implications for how the seals produce underwater vocalizations Frequency (kHz) of the harp and Weddell seal vocalizations was not altered when the seals produced calls in deeper ranges of the water column. There was no consistent trend in the mean frequency (kHz) over depth observed for either species. The pressure imposed on diving seals increases by 1 ATM (one atmosphere of pressure) for every 10 m increase in depth; at depths over 60 m the seals must deal with pressure exceeding 7 ATM. Pressures of this magnitude cause a reduction in the size of non-rigid air spaces in the body of the seal. A considerable reduction in the size of the respiratory alveoli of Weddell seals was observed at pressures of less than 4 ATM and the trachea began to compress at pressures of less than 6 ATM (Kooyman et al. 1970). Because of their similar anatomy, it is likely that the harp seals experience the same physiological changes as Weddell seals when diving. Theoretically, once a resonance chamber becomes compressed to half its original size, the frequency (kHz) of the calls being emitted would be twice as high as their original frequency (kHz). For example, the fundamental frequency (Fo; Hz) resonated by a closed cylinder (a shape similar to the air space found lamyx of the seals; Pierard 1969) can be calculated using the equation Fo = Cair/ 4L (where Ca;r = the speed of sound in air at a particular temperature and L = length of the cylinder).The body temperature of a seal is approximately 36-38oC(Pabst et a l 1999), therefore, within the body cavities of the seal Cair~ 354 m/s. In the simplest case scenario, assuming a cylinder 20 mm in length resonates the sound produced by the vocal cords (a reasonable approximation of the larynx cavity length; Pierard 1969), Fo = 4426 Hz. If the cylinder was only reduced to % its original length (L = 15 mm), then Fo = 5901 Hz. If the cylinder was reduced to V2
its original length (L = 10 mm) then Fo = 8852 Hz. The frequency (kHz) changes over depth are expected to be much greater than the frequency changes exhibited by the regression results of the harp seal CLEAR calls (Figures 16 and 17) if they were due to compression of air chambers within the seals body. Frequency (kHz) of the harp and Weddell seal calls appears to be independent of depth and changes in the size of the non-rigid body cavities. This suggests that the seals control their vocalizations via their vocal cords and not through the size of resonance chambers or other internal air spaces. It is highly probable that the seals vocalize by means of a predominately source-driven system.
5. Overall calling depth trends 5a. Harp seals Overall, significantly more of the harp seal calls examined were made within the shallower depth range (< 35 m). This trend was supported by the BROAD, ROUGH and CLEAR analyses (Figures 8 and 9), and occurred even when calls of particular types, subtypes, NOE categories or rhythm patterns were analyzed separately. The point depth estimates suggest that the harp seals are not calling predominately from directly below the water surface, but rather are diving well below the ice to emit the majority of their vocalizations. Although most of the calls were emitted at < 35 m, the seals called more frequently at depths of > 10 m (Figure 9).
5b. Weddell seal Significantly more of the Weddell seal calls were made at the shallower depth categories (< 35 m and ~ 35 m). This trend was supported by the BROAD, ROUGH, SINGLE and CLEAR analyses (Figures 18-20), and generally occurred even when calls of particular types, subtypes, NOE categories or rhythm patterns were analyzed individually. The calling depth of the seals did not change with season (pupping or pre pupping) or photoperiod, although both factors did influence the seals’ vocalization behavior in previous studies (Rouget 2004). Rouget (2004) reported that Weddell seal vocalization rates at these sites were higher during the pupping season and at night. The more point estimates obtained from the Weddell seal calls also suggests that the seals dive well below the undersurface of the ice to emit most of their vocalizations, as the majority of the CLEAR calls occurred between 10 and 40 m depth (although this observation is based on a low sample size; Figure 20).
5c. Comparisons of calling depths of the two species Both the harp and Weddell seals call predominately from shallower depths, inferring that the majority of the underwater social interactions of each species occur at shallower depth ranges. Light would likely penetrate to shallow depths of the water column during daylight hours through holes or cracks in the ice (especially true in the pack-ice environment), and thus would offer vocalizing seals the advantage of being able to direct calls towards specific individuals that they can see. The situation at night, when the calling rates of harp and Weddell seals are higher (Terhune and Ronald 1976; Rouget
2004), is uncertain with respect to the possible link between calling depth and visual identification of possible recipients. Although the majority of the vocalizations of both species occur at shallower depths, the seals dive well below the undersurface of the ice to vocalize and therefore appear to be avoiding sea-ice interference to some extent. Calling at depths well below the ice surface would increase the transmission range of the calls by avoiding attenuation at the sea/ice interface. In this study, most ice noises occurred on the 10 m hydrophone channel for both the Weddell and harp seal recordings. In the pack-ice environment, it has been observed that background noise events generally decrease with depth (Greenings and Zakarauskes 1994). Providing that the caller and listener are at the same depth and assuming that ice noises are acting as point sources at the water surface, at 20 m depth (directly below a source of ice noise), the sounds produced by the ice will be 26 dB lower than at 1 m below the ice surface (201og(20) = 26 dB). Thus, seals calling to each other at 20 m below the ice surface will experience signal to noise ratios that are 26 dB higher than a pair of seals at 1 m depth. This advantage would not be present under areas of the sea ice where little noise is generated (such as near the center of large floes or solid ice fields). Calling at depths well below the ice surface or above the ocean floor would theoretically help maximize transmission range of vocalizations. Although the seals are avoiding sea-ice interference to some extent (by vocalizing at more than 10 m below the ice), the majority of the calls examined occurred at depths of < 35 m. This is the depth range where most social interactions between the seals would be expected to occur, because they are within visual light areas of the water column and because these are the
area’s that males actively defend (in the case of the territorial Weddell seals). This suggests that there may be a possible link (for some calls) between vocalizations and visual detection.
6. Conclusions Addressing the original objectives of this study, the main goal of this research was to determine the depth at which harp and Weddell seals vocalize in the water column. Calling depth of the seals was determined through rough and point depth estimates, and both species were found to vocalize throughout the water column, although they called primarily from shallower depths (generally < 35 m), and did avoid sea-ice interference to some extent by diving well below the ice surface to vocalize. The vocalizations generally did not change over depth (with respect to call types, subtypes, NOE, rhythm, patterns or total call duration), or with season and photoperiod (for the Weddell seals). Because of the consistency of the vocal repertoire over depth, call functions could not be inferred from the data obtained. Finally, frequency (kHz) of the calls did not increase with depth, indicating that the seals vocalize by means of a primarily source-driven system, likely by the vocal cords of the larynx.
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APPENDIX 1 Determining accuracy of point depth measures using Cato’s (1998) calculations:
The effect that the accuracy in the time and amplitude measures had on the depth estimates obtained when using Cato’s (1998) equations was determined. Several theoretical situations for a two-element hydrophone array (consisting of hydrophones HI and H2 placed at 10 and 60 m depths) with the sound source (calls produced by a vocalizing seal) originating from a known position P were examined. If a seal is calling from a known depth in the water column (D) and from a known distance away from the hydrophone array (d), (in other words, if the position P is known), then the distance of the seal from the two hydrophones (Ri and R 2) can be calculated. For example, if D = 20 m and d = 40 m, then Ri = 41.23 m and R 2 = 56.57 m. Knowing how far the signal has to travel in order to reach each hydrophone (Ri and R 2), the speed of sound in the water (C0 = 1439 m/sec), and assuming that the sound is spreading spherically, the amount of time it takes the sound to reach each hydrophone and the
amplitude loss of the signal as it travels to each hydrophone can be determined. For example, if D= 20 m and d = 40 m, it would take the call 28.65 msec to reach HI and 56.57 msec to reach H2. The amplitude of the call would also have decreased by 32.30 dB when it reached HI and by 35.05 dB when it reached H2. From these values, the exact arrival time difference (ATD) and amplitude difference (AD) of the signal between the two hydrophone channels can be obtained. In order to determine how accuracy of time measures affected the depth estimate, the exact ATD value was altered by ± 1 msec. The altered ATD value was used in Cato’s (1998) calculations with the exact AD value in order to obtain an estimate of ri (distance of the seal from the closest hydrophone) and 0 (angle of the seal from the axis of the hydrophone array) (Figure 1). The ri and 0 values were used to determine the point depth of the calling seal. The difference between the known depth (or actual depth) of the calling seal and the estimated point depth when the ATD values were altered could then be examined (Figure 5). This procedure was repeated for the amplitude measures, and AD values were varied by ± 1 dB, and the exact ATD values were used (Figure 6).
VITA Candidate’s Full Name:
Hilary Bernice Moors
Universities Attended:
University of New Brunswick, Saint John (1999-2003), B.Sc.
Publications: Moors, H.B. and Terhune, J.M. 2003. Repetition patterns within harp seal (Pagophilus groenlandicus) underwater calls. Aquat. Mamm. 29 (2): 278-288
Moors, H.B. and Terhune, J.M. 2004. Repetition patterns in Weddell seal (Leptonychotes weddellii) underwater multiple element calls. J. Acoust. Soc. Am. 116 (2): 12611270
Conference Presentations: Moors, H.B. 2003. Seals with rhythm: patterning in Weddell seal (Leptonychotes weddellii) multiple element underwater calls. XV Biennial Conference on the Biology of Marine Mammals, Greensboro, North Carolina.
Moors, H.B. 2004. Determining calling depth of harp seal (Pagophilus groenlandicus) underwater calls. Canadian Society of Zoologists Annual Meeting, Wolfville, Nova Scotia.
