J Comp Physiol B (2006) 176: 505–512 DOI 10.1007/s00360-006-0072-0
O R I GI N A L P A P E R
Carol E. Sparling Æ John R. Speakman Michael A. Fedak
Seasonal variation in the metabolic rate and body composition of female grey seals: fat conservation prior to high-cost reproduction in a capital breeder? Received: 4 October 2005 / Revised: 26 January 2006 / Accepted: 1 February 2006 / Published online: 28 February 2006 Springer-Verlag 2006
Abstract Many animals rely on stored energy through periods of high energy demand or low energy availability or both. A variety of mechanisms may be employed to attain and conserve energy for such periods. Wild grey seals demonstrate seasonal patterns of energy storage and foraging behaviour that appear to maximize the allocation of energy to reproduction—a period characterized by both high energy demand and low food availability. We examined seasonal patterns in resting rates of oxygen consumption as a proxy for metabolic rate (RMR) and body composition in female grey seals (four adults and six juveniles), testing the hypothesis that adults would show seasonal changes in RMR related to the reproductive cycle but that juveniles would not. There was significant seasonal variation in rates of resting oxygen consumption of adult females, with rates being highest in the spring and declining through the summer months into autumn. This variation was not related to changes in water temperature. Adults increased in total body mass and in fat content during the same spring to autumn period that RMR declined. RMR of juveniles showed no clear seasonal patterns, but did increase with increasing mass. These data support the hypothesis that seasonal variation in RMR in female grey seals is related to the high costs of breeding.
Communicated by G. Heldmaier C. E. Sparling (&) Æ M. A. Fedak Sea Mammal Research Unit, Gatty Marine Laboratory, University of St Andrews, St Andrews, KY16 8LB, Fife, UK E-mail:
[email protected] Tel.: +44-1334-462628 Fax: +44-1334-462632 J. R. Speakman Zoology Building, School of Biological Sciences, University of Aberdeen, Tillydrone Avenue, AB24 2TZ, Aberdeen, UK
Keywords Resting metabolic rate Æ Seasonal Æ Grey seal Æ Body composition
Introduction The capacity for energy storage allows for temporal separation between resource acquisition and resource utilization. Many species of animals accumulate body fat reserves prior to seasonal events characterized by either high energy demand or low food availability, or even a combination of both these factors. Fattening can be facilitated through either a reduction in metabolic rate or an increase in food intake or a combination of both (e.g. Dark and Zucker 1986; Speakman and Rowland 1999). Examples of these include fattening prior to the high energy demands of migration in birds (e.g. Totzke et al. 2000; Butler and Woakes 2001) and mammals storing fat prior to hibernation, when food supplies are very low (Kunz et al. 1998; Speakman and Rowland 1999; Carey et al. 2003). A variety of physiological, morphological and behavioural strategies are employed in order to allow accumulation of large fat stores. In mammals, most attention has been focused on fattening prior to hibernation. The seasonal increase in white adipose tissue by fat-storing hibernators can lead to a near doubling of body weight from spring emergence to fall immergence (Carey et al. 2003). For example, studies on grey mouse lemurs (Microcebus murinus) have shown that exposure to short day photoperiodic regimes results in increased body mass and decreased resting metabolic rate (RMR), this is thought to be adaptive with fattening occurring during natural conditions of short day length at the beginning of the dry season when resources are still available (Perret et al. 1998). With birds, pre-migratory fattening has been studied extensively. For example, barnacle geese (Branta leucopsis) have been shown to become mildly hypothermic in the period prior to autumn migration, a period when food is not scarce, but
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when there is a necessity to store fat for their long migratory flight south (Butler and Woakes 2001). Other species have been demonstrated to use torpor as a mechanism to conserve energy stores for migration (Heibert 1993; Carpenter and Hixon 1988). Relatively little attention has been given to prebreeding fattening strategies of mammals which rely entirely on stored energy to provision the cost of lactation as well as maintenance metabolic costs during this period (capital breeders). Phocid seals undergo dramatic seasonal changes in body mass and composition, accumulating energy reserves prior to reproduction (Schusterman and Gentry 1971; Fedak and Anderson 1987; Chabot and Stenson 2002) and have been documented to show seasonal changes in metabolic rates (Renouf and Gales 1994; Rosen and Renouf 1998; Boily and Lavigne 1997). Grey seal (Halichoerus grypus) females come ashore once a year to give birth to and suckle a single offspring. Lactation lasts around 17 days (Fedak and Anderson 1982) and during this time the female remains continuously on shore, fasting. Maternal energy storage is extremely important to offspring survival and female fitness (Mellish et al. 1999; Pomeroy et al. 1999; Hall et al. 2001). Wild grey seals show seasonal and sex-related changes in patterns of energy storage (Beck et al. 2003a) and foraging behaviour (Beck et al. 2003b). Females accumulate fat stores during 7 months of foraging at sea between the moulting and breeding period, and the amount stored is roughly equivalent to the amount lost during breeding (Pomeroy et al. 1999; Beck et al. 2003a). Females also exhibit clear seasonal patterns of diving behaviour; they increase dive effort, both in terms of accumulated bottom time (h/day), and in the occurrence of dives classified as foraging dives, in the 2 months prior to the breeding season (Beck et al. 2003b). This pattern of increasing foraging effort towards the breeding season can be seen as a strategy to maximise energy allocation to reproduction. A question arises however as to the extent to which these foraging strategies are augmented by seasonal modifications in resting energy expenditure. To test this hypothesis we measured the RMRs of wild female grey seals, temporarily taken in to captivity. We predicted that adult females would reduce their RMR during the period of pre-breeding fat deposition. We would not expect to observe these patterns in juveniles if these changes were related to preparation for reproduction.
Materials and methods Animals Ten female grey seals were used in this study, four adults of unknown age and six pups in their first year of life. All animals were captured from the wild and transported to the captive facility by boat. All were from the local
population of grey seals. They were either captured from Abertay sands (5625.59¢N–245.59¢W), an intertidal haul-out 10 km north of St Andrews, or the Isle of May (5610.59¢N–233.59¢W), a breeding colony located 22 km south of St Andrews in the mouth of the Firth of Forth. Names, age classes, origins and masses are shown in Table 1. Seals from Abertay were caught using either a rush and grab method or beach seining while they rested on exposed sand banks at low tide. Rush and grab involved a fast landing by boat right in front of the haulout; animals were then entangled in hoop nets on the sandbank. Beach seining involved rapidly deploying nets adjacent to the shore where animals are hauled out. The seals got tangled in the net as they tried to escape underwater. Seals at the Isle of May were caught on shore on the breeding colony. All seals were restrained in pole-nets and were transported back to the captive facility by boat. The two unweaned pups were weaned within 3 weeks of being brought into captivity. Blood samples were taken from adult females in early summer to confirm pregnancy, these were sent for progesterone assay to Vetlab Services Horsham, UK. These indicated that all four adults were pregnant, although one (L) aborted in June 2002. The seals were fed mainly on herring (Clupea harengus), (Lunar Freezing, Peterhead, Scotland) which were obtained in bulk and stored at – 20C. Prior to use, the fish was thawed overnight in a sink of cold tap water. To ensure that changes in metabolism were not confounded by changes in voluntary food intake animals were not fed ad libitum; the amount offered daily depended on each individual’s age and size. Adults were given 4–6 kg/day and pups 2– 2.5 kg/day. Seals were housed in the outdoor captive facility of the Sea Mammal Research Unit at the University of St Andrews; therefore, environmental conditions experienced by the seals were very similar to conditions experienced by their wild counterparts in the North Sea. All seals were released back to the wild after veterinary inspection after periods of between 7 and 13 months. All procedures were carried out in accordance with guidelines set out under The Animals (Scientific Procedures) Act 1986, under project licence #60/2589. Respirometry Resting oxygen consumption rates were obtained longitudinally from each animal over the entire period they were in captivity, in tandem with other studies (Sparling 2003; Sparling and Fedak 2004). Resting oxygen consumption in water was measured using the open-flow respirometry system described in detail previously (Sparling and Fedak 2004). Briefly, the experimental setup consisted of an outdoor seawater pool (42·6·2.5 m), which was covered with aluminium mesh to prevent the seals from surfacing anywhere other than a Perspex respirometry hood set in one corner of the pool. Air was drawn through this hood into the laboratory where the
507 Table 1 Details of the seals used in this study, the field sites they were caught at, along with ages, dates, mass at capture and release and mean mass specific rates of resting oxygen consumption (mean ± SD) Seal ID
Age class
Origin
Capture date
Mass at capture (kg)
Release date
Mass at release (kg)
Mean mass specific oxygen consumption O2 ml/min per kg
A B C F H K L N W X
Pup Pup Adult Adult Adult Pup Adult Pup Pup Pup
Abertay Abertay Abertay Abertay Abertay Isle of May Isle of May Isle of May Abertay Abertay
31 Jan 2000 7 Feb 2000 21 Mar2000 15 Feb 2001 8 Mar 2001 12 Dec 2001 12 Dec 2001 12 Dec 2001 15 Jan 2004 13 Feb 2004
30 25 96 95 118 33 168 27 26 22
7 Feb2001 7 Feb 2001 3 April 2001 11 Oct 2001 11 Oct 2001 15 Nov 2002 25 Oct 2002 15 Nov 2002 5 Oct 2004 14 Oct 2004
51 45 126 143 148 35 172 36 41.4 45.6
7.82±0.65 6.95±0.70 6.67±1.15 5.51±1.09 5.04±1.22 8.04±1.43 6.29±0.89 8.87±0.89 9.40±1.26 7.72±0.80
oxygen concentration of a sub sample of this air stream (after CO2 was absorbed using soda lime) was determined. The respirometry system was calibrated at the beginning of every run using the nitrogen dilution technique described by Fedak et al. (1981). Oxygen consumption of the seal was calculated using the following equation from Fedak et al. (1981): _ _VO2 ¼ 0:2094VN2 DC DC 0:8 Where DC and DC* refer to the deflection of the oxygen analyser during measurement and calibration, respectively, and V_N2 was the flow rate of nitrogen used during the calibration. The output from the oxygen analyser was monitored continuously in a laboratory inside the building so that seals were not aware of any human presence throughout experimental trials. Oxygen consumption measurements were made in tandem with another study on diving energetics (Sparling and Fedak 2004), in which animals were allowed to dive freely in the large pool. Seals were not forcibly confined to the breathing chamber at any time so resting rates were obtained opportunistically. Rest periods could be identified from a combination of activity records from attached time–depth recorders (TDR Mk 8, Wildlife Computers), visual observations and from the pattern of deflections on the trace of the oxygen analyser. The length of resting periods was completely under the control of the animals so that errors associated with confinement or physical restraint were avoided. Durations of resting periods were therefore variable and ranged from 10 min to 1 h. Because previous studies have shown that both apnoea and passive submergence can have a significant effect on estimates of RMR (Hurley and Costa 2001; Sparling 2003) we only included measurements made where animals were continuously at the surface (for more than 10 min) and breathing regularly. All measurements were made between the hours of 9:00 am and 4:00 pm, with the seals at least 15-h postprandial, but less than 24 h since their last feed. (Boily
and Lavigne 1995 demonstrated a change in RQ after 24 h of food deprivation.) In order to compare our measurements with predicted values of resting energy expenditure we converted V_O2 to energy using the conversion factor of 4.7 kcal/l of oxygen consumed, based on the assumption that postabsorptive seals have a fat-based metabolism (Boily and Lavigne 1995). We could then compare this to the predicted value for similarly sized terrestrial adults from Kleiber’s (1975) equation: Pmet=70·M0.75 where Pmet is metabolic power in kcal/day and M is body mass in kg. Body composition Body composition was measured using the isotope dilution method. The seals were immobilised during the procedures described below using either intravenous or intramuscular injection of a tiletamine/zolazepam mixture (Zoletil 100, Virbac, France) (Baker et al. 1990). After blood sampling for determination of background 2H levels in body fluids, seals were injected intravenously with weighed doses of deuterated water (0.12 ml/kg of 99.9% 2H2O, Sigma-Aldrich Chemicals, UK). Blood samples were taken 3–4-h post-injection, after equilibration had occurred (Reilly and Fedak 1990). Serum samples were flame sealed into 50 ll capillary tubes. Distilled water was obtained from these serum samples by the method described by Speakman (1997). Samples were analysed in duplicate and where duplicate samples had a coefficient of variation of greater than 2%, the samples were re-distilled and re-run. Isotopic enrichment of the injection solution was verified by diluting the original source with a known amount of water, mimicking an injection of the dose into an organism’s body water pool. These dilution samples were analysed alongside the experimental samples and the exact enrichment of the original solution back-calculated. 2 H dilution space was calculated as in Speakman et al. (2001). Total body water (TBW), total body fat (TBF) were calculated using the following equations developed for grey seals by Reilly and Fedak (1990):
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Oxygen consumption data for adults and pups were examined separately. Seasonal, individual, temperaturerelated and mass effects were examined using a linear mixed effects model using R software (R version 2.1.1). Seal id was modelled as a random effect. Because month is a cyclic variable, (for instance month 1 is closer to month 12 than to month 6) it was transformed to the cosine and sine of month [cos((360/11)·month) and sin((360/11)·month)]. The maximum difference in cosine of month occurs between summer and winter, whereas the difference in sine of month is greatest between autumn and spring. Cosine and sine of month thus provide a measure of seasonality for use in linear modelling (Fisher 1993). Fixed effects included in the model were sine and cosine of month, mass and water temperature. Slope value ± standard error, t and P were reported for each significant effect. Unfortunately our small sample of directly corresponding, independent body composition and oxygen consumption measurements preclude a direct assessment of the effect of body composition on rates of oxygen consumption, therefore for seasonal changes in body composition a separate analysis was carried out where body composition measurements were pooled into three 2-month blocks, effectively splitting the time between moult and the breeding season into three sections. Repeated measures ANOVA was carried out to assess differences between ‘treatments’ (time period) with animals as blocks. All results were reported as significant at P>0.05. Means are presented ±1 standard deviation.
Body composition All adults increased in fat content in the period leading up to the breeding season in autumn (repeated measures ANOVA F2,11=34.13, P=0.003). Over this period mean fat mass approximately doubled. Total body mass did not increase over the first period but increased significantly between the second and third time period (Fig. 2, F2,11=13.34, P=0.001). Lean body mass remained constant over all three periods.
Adults
1 0.9
3
0.8
Pups 2
3
2
4
0.7
4
4 4
1
3
0.6 0.5 4
0.4
4
4
4
6
6
6
6
4
3
0.3 0.2 0.1 0 Ju ly Au g us Se t pt em be r O ct ob N ov er em b D ec er em be r
Statistical analysis
mean resting VO2 (litres per min)
Lean body mass was calculated as total body mass minus total body fat mass.
indicates that the largest differences in metabolic rates occurred between spring and autumn. The spring elevation and subsequent decline in metabolic rates in all four adults can be seen in Fig. 1. Rates are higher than predicted BMR’s for adult mammals of similar mass at all times of year.
Fe br ua ry M ar ch Ap ril M ay Ju ne
TBW (kg) = 0.382 + 0.965 2 H dilution space (kg) % TBF = 105.1 - 1.47 (% TBW)
Fig. 1 Seasonal variation in oxygen consumption in grey seals. Closed symbols are adult seals and open symbols are pups. Each point is the mean across all individuals ± standard errors. Numbers above each point represents the number of individuals measured in that month
Results
Resting rates of oxygen consumption Throughout the 2.5 years of the study absolute resting rates of oxygen consumption ranged from 0.485 to 0.999 l/min. These correspond to 1.07–2.83 (overall mean 1.95±0.37) times the predicted BMR of similarly sized adult terrestrial mammals (Kleiber 1975). Although there was considerable variation between and within animals, both between and within months, analysis on data from all four adults indicated that RMR (l/ min) varied significantly with sine of month (LME: slope=0.11±0.022, t=5.798, P
