J Comp Physiol B (1999) 169: 256±262
Ó Springer-Verlag 1999
ORIGINAL PAPER
J. T. Pearson á H. Tazawa
Ontogeny of heart rate in embryonic and nestling crows (Corvus corone and Corvus macrorhynchos )
Accepted: 3 March 1999
Abstract The developmental patterns of mean heart rate (MHR) and instantaneous heart rate (IHR) were investigated in embryos and chicks of altricial Corvus corone and Corvus macrorhynchos. The MHR of embryos increased linearly with time from 250 beats á min)1 at mid-incubation to 290 beats á min)1 in hatchlings. MHR during the pipping period was maximal, but only marginally higher than in hatchlings. MHR was stable at about 290±300 beats á min)1 during the 1st week after hatching. Spontaneous heart rate (HR) decelerations and accelerations were found in embryos and chicks, disturbing the baseline HR with increasing frequency during development. However, the IHR accelerations developed later and were less frequent than in precocial species. IHR and body temperature decreased during mild cold exposure (23±25 °C) and IHR accelerations were reduced in nestlings during the 1st week. We suggest that the development of parasympathetic control of HR in crows occurs at 60% of incubation, similar to precocial embryos, but sympathetic control may be delayed and suppressed in contrast to precocial embryos. Key words Corvus corone á Corvus macrorhynchos á Embryo á Chick á Heart rate Abbreviations ACG acoustocardiogram á ECG electrocardiogram á EP external pipping á HR heart rate á HRI heart rate irregularities á IHR instantaneous heart rate á IP internal pipping á MHR mean heart rate á Ta ambient temperature á Tb body temperature
J.T. Pearson (&) á H. Tazawa Department of Electrical and Electronic Engineering, Muroran Institute of Technology, Muroran 050-8585, Japan e-mail:
[email protected] Tel.: +81-143-46-5533; Fax: +81-143-46-5501
Introduction An increasing number of studies in the ®eld of avian physiology suggest that dierences in hatchling developmental state originate from dierences in the development rates of embryos. Embryonic mass and metabolism increase most rapidly during the second half of incubation, but increase later in altricial embryos than precocial embryos, re¯ecting the lower maintenance costs of more immature embryonic tissues in altricial embryos (Hoyt 1987; Vleck and Vleck 1987; Prinzinger and Dietz 1995; Prinzinger et al. 1995; Ricklefs and Starck 1998; Vleck and Bucher 1998). This led Vleck and Vleck (1987) to suggest that by hatching earlier in the developmental sequence, the incubation period of altricial embryos has been signi®cantly reduced in comparison to precocial species. In which case, does the shorter developmental time of altricial species aect the development of physiological functions? It is well known that altricial species hatch without any thermogenic abilities and are therefore poikilothermic (for review see Visser 1998). Altricial hatchlings have poorer developed sensory and motor capacities than other hatchling modes. Thus the low metabolic intensity of altricial embryos is paralleled by low metabolic intensity during the posthatching period, until growth is nearly completed (Bucher 1987; Visser 1998; Vleck and Bucher 1998). However, some altricial avian groups, such as the parrots, may be exceptional in their early development of endothermy (see Pearson 1998). Recently, Prinzinger and his colleagues concluded that the developmental patterns of embryonic metabolism do not dier signi®cantly in pattern, but plateau metabolism is reached later and is of shorter duration in altricial species than in precocial species (Prinzinger and Dietz 1995; Prinzinger et al. 1995). Based on many studies of precocial species we know that embryonic heart rate (HR) changes greatly over the incubation period (Tazawa et al. 1991; Pirow and Nichelmann 1994; Pearson et al. 1998; Tazawa et al. 1998; all studies
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of Gallus domesticus embryos summarised in Tazawa and Hou 1997). However, HR developmental patterns are very variable between precocial species, and in embryos of some species mean HR (MHR) decreases during the second half of incubation (Tazawa et al. 1991). Nevertheless, HR is an important contributor to cardiac output, which increases directly with embryo mass (Tazawa and Hou 1997). The results of the fewer studies of the development of MHR in altricial embryos suggest that MHR increases nearly continuously in relation to incubation time in 12 species of small egg mass, but is stable in the domestic pigeon Columba domestica (fresh egg mass 18 g) and decreases in the cockatiel Nymphicus hollandicus (5±6 g) during mid incubation (Burggren et al. 1994; Tazawa et al. 1994; Pearson and Tazawa 1999). Furthermore, MHR is stable during the last days of incubation before pipping in three species of seabird of larger egg mass (Tazawa et al. 1991; Tazawa and Whittow 1994). Until recently HR variability has never been examined on a beat-to-beat basis. Several studies of the development of instantaneous HR (IHR) in Gallus and Cairina moschata embryos have demonstrated that the stable IHR baseline is disturbed with increasing frequency by spontaneous, transient IHR decelerations and accelerations, collectively termed as HR irregularities (HRI), during the second half of incubation (Pirow 1995; Akiyama et al. 1997; HoÈchel et al. 1998). Administration of autonomic drugs to Gallus embryos con®rmed that rapid decelerations are mediated by the parasympathetic nervous system, and therefore in¯uence the control of HR from about 60% of incubation (HoÈchel et al. 1998). Rapid IHR accelerations occur later at about 70% of incubation, and possibly re¯ect the initiation of sympathetic mediation of HR. To date no study has investigated the development of IHR and occurrence of HRI in an altricial species. The ®rst aim of this study was to investigate the development pattern of MHR in the larger embryos of Corvus species. We then determined whether the MHR development pattern was similar to smaller passerine species, a continuously increasing MHR pattern, or more similar to that of Columba domestica of similar fresh egg mass, mid-incubation plateau MHR pattern. The second aim was to examine the ontogeny of HRI and HR ¯uctuations in Corvus and compare this data with precocial Gallus embryos (HoÈchel et al. 1998; Tazawa et al. 1999).
Materials and methods Egg collection and incubation Eggs from the carrion crow Corvus corone and the jungle crow Corvus macrorhynchos were collected during the spring of 1997 and 1998, from the surrounds of Muroran and Noboribetsu Cities (Japanese Department of Environment Permit). Nests were inspected from March onwards and eggs removed from completed clutches. Nests with only 1 or 2 eggs were subsequently inspected
every 2±3 days and eggs were numbered accordingly with a felt pen to determine egg-laying dates. Eggs were incubated in a commercial still air incubator at 36 °C (0.2 °C). Relative humidity was automatically controlled so that eggs achieved approximately 15% mass loss during incubation (55±65% relative humidity). All HR measurements were made in a smaller still-air incubator (Sakura IF-B3, Tokyo) at 36 °C (0.5 °C). Eggs were turned manually every 2 h between 08:00 hours and 20:00 hours each day until pipping. The age of the embryos was determined from records of egg-laying dates. Embryonic acoustocardiogram Embryonic HR prior to internal pipping (IP, determined by candling eggs) was determined using a modi®cation of the acoustocardiogram (ACG) method (Rahn et al. 1990; Wang et al. 1990), whereby a small condenser microphone was glued directly to the outer surface of the intact eggshell to record the pulsatile pressure waves (Akiyama et al. 1997). This non-invasive technique is routinely used in our laboratory in studies of precocial embryos (Akiyama et al. 1997; Tazawa et al. 1998). The output signal from the condenser microphone was ampli®ed by about 40 dB, bandpass ®ltered at 2±8 Hz, then stored on a personal computer after digitisation by a 12 bit A/D converter at a sampling frequency of 100 Hz. The signals were later interpolated on a computer using a wave restoring sinc function and the peak-to-peak time interval was converted to IHR (beats á min)1) (Akiyama et al. 1997). ACG signals were sometimes distorted in young embryos (when ACG was ®rst detectable) by unknown noises and during the pipping period by ventilatory movements, which prevented IHR determination. In such cases MHR determination was only possible by power spectral analysis and therefore we used this method for all ACG recordings. MHR was then calculated from restored ACG signals using a fast Fourier transform. Each 30-min recording was divided into 214 point series (therefore a remainder of 2.7 min was not included in analyses), power spectra were calculated and then moved half one time-series (213 points) using a rectangular window and power spectra calculated for the subsequent series. Finally, the cumulative average of all power spectra series for each recording was determined and examined for signi®cant spectral peaks in the expected frequency ranges (3±6 Hz). Electrocardiogram of embryos and chicks Three copper or silver wires (0.2 mm diameter) 30 mm long were used as ECG leads for embryonic measurements as described in Pearson et al. (1998). Crow eggs were ®tted with electrocardiogram (ECG) electrodes on day 18 of incubation. Two of the electrodes were inserted across the mid-equator of each egg, across the embryo's back, and a third electrode closer to the sharp pole of the egg so as to form an equilateral triangle with sides 25 mm long. Electrodes were in contact with the embryo, but did not puncture or irritate the embryo's skin as the tip of electrode was bent and directed away from the body. Prepared eggs were rewarmed in the large incubator until the epoxy hardened (1 h) before transfer to the measurement incubator. The electrical signal was similarly ampli®ed, band-pass ®ltered (25±200 Hz) before being digitised at a sampling rate of 4000 Hz. IHR was calculated from the time interval between adjacent R waves of the QRS complex of the ECG signal that exceeded the user-determined threshold voltage set in the computer. The HR of hatchlings and a few older chicks (see below) was measured using Solid-gel disposable electrodes (Vitrode A-50, Nihon Kohden), commercially available for neonate ECG/respiration monitoring. The ¯exible sticky gel pads (2 cm diameter) were reduced to triangles of about a third the original size. Two electrode leads were attached to the naked skin of the thoracic wall below the wings, caudal to the humeral joint. A third electrode was attached to the abdomen caudal to the sternum and anterior to the cloaca. The ECG signal was ampli®ed, band pass-®ltered and recorded on computer as for embryonic ECG measurements.
258 Experimental plan and statistics Each crow embryo was measured at least once a day at 36 °C (0.5 °C) for 30 min to determine the daily pattern of change in MHR during the second half of incubation and in chicks. As breathing events from shortly before IP increasingly disturbed the ACG signal, it was often not possible to record a HR signal after IP from the ®xed microphones. In such cases, eggs were ®tted with ECG wire electrodes and measured until hatching. Hereafter we de®ne MHR as both the average HR determined by ACG (power spectrum) and the average of IHR over the ®rst 30-min interval of recordings determined by ECG (usually 2±4 h continuous measurements on days 19 and 20) for IP and externally pipped (EP) embryos and chicks (day 0 and days 5±6). Exceptionally, chick IHR during cooling trials was averaged over 10-s intervals to be comparable with body temperature (Tb) sampling frequency (see below), and is de®ned as MHR10. Further trends in baseline HR changes of IP and EP embryos were examined by averaging IHR over 2-s intervals (MHR2). After hatching, IHR was measured at 36 °C and 38 °C in a darkened incubator for six hatchlings (day 0, both species). IHR measurements were also made on day 5 or day 6 for three C. corone chicks that hatched from eggs in this study (not siblings) and were fostered into a single nest on campus (no other brood). Chicks were removed from the parental nest and prepared for ECG measurements immediately, before being returned to their nest after about 2.5±3.5 h until the next measurement day. First measurements were made at 36 °C and then during gradual cooling to room temperature (23±26 °C) by removing the chick within its porcelain dish from the incubator and allowing the chick to cool for a 30-min period. Finally, they were rewarmed in an incubator at 38 °C and recorded for 30 min after a 30-min equilibration period. Simultaneous to HR measurements, Tb was measured by a ¯exible thermocouple (accuracy 0.01 °C) connected to a digital thermometer (LT-8, Gram Corporation). A thermocouple was inserted at least 1 cm into the cloaca and taped to the body of the chick. Tb was then recorded at 10-s intervals concurrent to measurements of IHR at 36 °C and during gradual cooling. Cooling constants (k, min)1) for young chicks were determined as the slope of the linear relation between ln[Tb ) ambient temperature (Ta)] and time (min) for individual chicks. Relationships between variables were examined by linear regression analysis using the least squares method. Con®dence intervals (95%) of regression means and standard error of regression coecients (sb) were determined for signi®cant relations. Values are presented as mean SD, where n is the number of individuals measured. The statistical power of tests was determined according to Zar (1984).
Results MHR during incubation The change in MHR during the second half of incubation is shown in Fig. 1. Mean fresh mass for all eggs was 20.5 2.2 g (n = 15). ACG signals were only detectable in a few embryos younger than day 10. MHR ¯uctuated in individual embryos prior to IP relative to incubation age, but on average MHR increased from 241 beats á min)1 to 272 beats á min)1 according to a signi®cant linear correlation between days 7±18 (Fig. 1). MHR increased more during the pipping phase (days 18±20) than the period prior. Correlation coecients for both regressions of MHR on incubation age (days 7±18 and days 17±20) were low (as regression coecient were low), but signi®cantly dierent from zero (r = 0.484 and 0.701, both P < 0.0001). MHR was more variable
Fig. 1 The relation between mean heart rate (MHR) (beats á min)1) and incubation age (days) of individual Corvus embryos (n = 13) and hatchlings (n = 5) at 36 °C. Data were combined for Corvus corone (n = 11 embryos, 2 hatchlings) and Corvus macrorhynchos (n = 2 embryos, 3 hatchlings). Open circles pre-internal pipping embryos [acoustocardiogram (ACG) measurements], ®lled circles internally pipped (IP) embryos, crossed circles externally pipped (EP) embryos [ACG and electrocardiogram (ECG)], Asterisks hatchlings (ECG). Solid lines represent signi®cant regressions ®tted by the least squares method with 95% con®dence intervals of regression means (day 7±18: MHR = 222.08 + 2.78 Age, sb = 0.64 r2 = 0.234 F1,63 =18.93 P < 0.0001; day 17±20: MHR = )92.43 + 20.98 Age, sb = 4.10 r2 = 0.492 F1,63 =26.13 P < 0.0001)
among pipped embryos than pre-pipped embryos. With the exception of one embryo, which was IP on day 18 with a low MHR of 242 beats á min)1, MHR increased to more than 280 beats á min)1 in pipped embryos (mean of IP embryos = 306 34 beats á min)1 and EP embryos = 322 22 beats á min)1, n = 8). A one-sample t-test indicated that MHR was not signi®cantly dierent between IP and EP embryos (null hypothesis, H0: DMHR = 0, t1,7 = 1.535 n.s.; power of test, i.e. P of detecting a true dierence of at least 10 beats á min)1, 1 ) b = 0.91). The MHR of EP embryos, which were actively hatching, was higher on average than that of ``resting'' hatchlings (36 °C), but not signi®cantly so (t1,3 = )2.532 n.s.; power of test, 1 ) b = 0.96). IHR during incubation IHR was calculated for all ACG measurements; typical examples of short-term changes in IHR are illustrated in Fig. 2. Baseline IHR was stable in four embryos between day 10 and day 11 of incubation. Spontaneous decelerations according to the de®nition of HoÈchel et al. (1998), IHR decelerations of 10±30 beats á min)1 below the baseline HR, were ®rst exhibited in two of four embryos on day 12, in ®ve of seven embryos on day 13, and in all eight embryos on day 14. The frequency of occurrence of spontaneous decelerations increased in all embryos, after their ®rst appearance, during the remainder of incubation. Decelerations followed by restoration of near baseline HR occurred in repeated bursts in all embryos
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Fig. 3A±D Examples of 30-min intervals of instantaneous heart rate (IHR) (beats á min)1) during the pipping period determined by ECG. A and B illustrate IP (day 18) and EP (day 19) IHR respectively, for the same C. corone embryo. C and D illustrate IP (day 19) and EP (day 20) IHR respectively, for the same C. macrorhynchos embryo
Fig. 2A±F Examples of 10-min intervals of instantaneous heart rate (HR; beats á min)1) during incubation determined by ACG. Arrows indicate examples of artefacts that appear as extremely rapid spurious HR changes. Asterisks indicate examples of spontaneous HR decelerations of 10 beats á min)1 or more from the baseline HR. Dashed line illustrates an interval of repeated spontaneous decelerations. A and B day 12 embryos, C and D day 15 embryos, and E and F day 17 embryos
(Fig. 2D). Deceleration bursts ®rst occurred in 4 of 8 embryos on day 15 and in all 11 embryos by day 18 (ACG and ECG measurements combined). The results of the ECG measurements con®rmed that several patterns of HRI found prior to IP by the ACG method were also present in pipped embryos of both species (n = 5). Spontaneous decelerations occurred intermittently and in repeated bursts in both IP and EP embryos (Fig. 3). However, small spontaneous accelerations (20±40 beats á min)1) were found for the ®rst time in EP embryos, and occurred both as intermittent events (Fig. 3D) and in repeated bursts (Fig. 3B). Furthermore, ¯uctuations in the baseline IHR oscillated over a 20±25 beats á min)1 range. MHR increased between IP and hatching from 300 beats á min)1 to 360 beats á min)1 in several embryos (Fig. 1). Baseline HR increased throughout the IP period (days 18±19), but was occasionally disturbed by sustained HR decelerations that lasted up to 5±6 min. Sometimes these bradycardias developed as a result of spontaneous decelerations (10± 15 s) and a lowering of the baseline HR (Fig. 3C), but in other embryos spontaneous decelerations were fewer and the baseline HR decreased continuously for 2±3 min before being restored to the former stable baseline level (Fig. 3A). In all cases a minimum level of 50±100 beats á min)1 lower than the stable baseline was reached
during decelerations. MHR2 for IP and EP embryos indicated that the baseline HR increased to a maximal level during incubation over a period of half a day, and that sustained bradycardia occurred repeatedly with a frequency of 30±40 min until about the time EP was con®rmed. After EP, the baseline HR permanently decreased again in one embryo (Fig. 4).
Fig. 4 Examples of continuous MHR measurements (averaged over 2-s intervals, beats á min)1) conducted during the IP and EP periods to examine ¯uctuations in baseline HR (same C. macrorhynchos embryo illustrated in Fig. 3C±D). Upper panel (2 h) and 2 mid-panels (4 h) indicate consecutive measurements on day 19 (IP), and lower panel a 2-h measurement on day 20 (EP), immediately prior to hatching
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Fig. 5 Example of IHR (beats á min)1) changes over a 60-min period at 36 °C followed by a 30-min period of gradual cooling to an ambient temperature (Ta) of 23 °C in a C. macrorhynchos hatchling. Lower panel indicates body temperature (Tb) (solid line) and Ta (dashed line) during the measurement
Chicks and HR during cooling tests Mean hatchling mass of both species was 15.8 1.46 g (n = 10), and was estimated to be 73.8 4.0% of fresh egg mass (range 65.2±72.3%). Internal yolk reserves that were removed from four hatchlings were on average
10.7 2.7% of total hatchling mass. The MHR of hatchlings (day 0) at 36 °C was 289 13 beats á min)1 (n = 5) and 293 21 beats á min)1 at 38 °C (n = 7). The baseline HR of crow hatchlings was generally stable at 270±340 beats á min)1 (variable between individuals), but was frequently disturbed by small spontaneous tachycardia, when IHR increased by 20±30 beats á min)1 above the previous baseline level within the interval of 2±3 beats (Fig. 5). Such spontaneous accelerations occurred intermittently throughout measurement periods (1 h, n = 5 hatchlings) at 36 °C. However, spontaneous decelerations persisted after hatching and were the predominant HRI pattern on day 0. At the onset of cooling to 23±26 °C, the HR baseline of ®ve hatchlings of both species decreased exponentially within 30 min to 180±210 beats á min)1 (Fig. 5). Tb was measured in three of the ®ve hatchlings during cooling tests (C. macrorhynchos). During cooling Tb decreased exponentially to a minimum of 29±30 °C, which was 4± 6 °C above Ta (Tb measurements, n = 3). In contrast to HRI recorded at 36 °C, only intermittent decelerations (20±50 beats á min)1) were found during cooling. MHR10 during cooling periods fell linearly with Tb below 37± 38 °C (Fig. 6). The Q10 of hatchling MHR10 at a Tb of 38 °C was 1.83, 1.90 and 1.93 and increased with decreasing Tb to 2.75, 3.13 and 3.21 at Tbs of 30.0±30.5 °C. The MHR10 of day 5 and day 6 nestlings (n = 3) similarly decreased linearly with Tb below 38 °C (Fig. 6). Cooling constants were not signi®cantly dierent between day 0 and day 5 nestlings (overall mean k = 0.025 0.004 min)1, n = 6; two-sample t-test t1,2 = 1.748 n.s.), despite an increase in body mass of two- to three-fold. The Q10 of MHR10 at a Tb of 38 °C was 1.3±1.6 in day 5±6 nestlings and was little changed in two nestlings, but increased to 1.9 in the third nestling as Tb decreased to 32.0±32.5 °C.
Discussion Development of MHR
Fig. 6 Examples of MHR over 10-s intervals (beats á min)1) during gradual cooling tests at Ta = 23 °C and Tb = 24 °C in Corvus chicks (day 0 and day 5) in relation to Tb (°C). The regression line indicates a signi®cant correlation (day 0: MHR = )438.2 + 20.7 Tb, r2 = 0.966 F1,188 = 5309.8 P < 0.001; day 5: MHR = )242.1 + 14.2 Tb, r2 = 0.831 F1,178 = 875.9 P < 0.001). Inset: Q10 curve for day 0 Corvus macrorhynchos (n = 3, dashed lines) and day 5±6 C. corone (n = 3, solid lines) calculated from the regression lines of MHR10 on Tb over 0.5 °C intervals
Embryonic MHR is already a large fraction of the hatchling MHR (mean 289 beats á min)1) at mid-incubation and increases approximately linearly with time during the second half of incubation, until the pipping period (embryonic MHR 86±101% of hatchling MHR between day 10 and day 18; Fig. 1). The rate of increase in MHR increases ten-fold after day 17 during the pipping period and MHR of pipped embryos exceeds the average hatchling MHR by about 26% at EP. The timing of such MHR increases is variable between day 18 and day 20, but undoubtedly is associated with increased embryonic activity during pipping and preparations to hatch. Thus the development pattern of Corvus MHR is more like that of the pigeon Columba domestica, and less like the continuously increasing MHR patterns of smaller altricial passerines, since Corvus MHR changes by only 5% over the period
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50±85% of normalised incubation (Tazawa et al. 1994; Pearson and Tazawa 1999). Nevertheless, the regression slope of Corvus MHR on incubation age is positive and signi®cantly dierent from zero prior to pipping (P < 0.0001, Fig. 1). The lower rate of increase in prepipping MHR in Corvus during incubation may be attributed to the larger fresh egg mass in comparison to smaller passerine species of Pearson and Tazawa (1999). In common to all altricial species investigated to date (J.T. Pearson, Y. Noma and H. Tazawa, 1999), Corvus MHR does not decrease signi®cantly during late incubation in contrast to precocial embryos. Furthermore MHR increases more during pipping than during the pre-pipping period in all the altricial species examined, whereas MHR does not increase signi®cantly in some precocial species during the pipping period (Tazawa et al. 1991). HR variability and irregularities HoÈchel et al. (1998) ®rst characterised the patterns of HRI, both rapid decelerations (lengthened beat-to-beat intervals) and accelerations (shortened interbeat intervals), in the development of precocial Gallus domesticus embryos. Following on from that work, we are able to show in this study that some of the same patterns of HRI also occur during the development of HR regulation in altricial Corvus embryos. However, there are some noteworthy dierences, which may relate to differences between these species in their degree of maturity in autonomic control of HR, that are related to hatchling developmental mode. In common with Gallus embryos, the HR baseline of Corvus embryos was stable up until day 16, after which it ¯uctuated and was constantly perturbed by HRI (Fig. 2). Signi®cant rapid decelerations were con®rmed by inspection of the ACG signals. These rapid decelerations ®rst developed intermittently on days 12±13 (60% of incubation) and increased in frequency of occurrence thereafter in Corvus, similar to that of Gallus embryos (Akiyama et al. 1997; HoÈchel et al. 1998). Rapid accelerations identi®ed in Gallus embryos at 70% of incubation, however, were ®rst found on days 18±19 or 90± 95% of incubation, in internally pipped Corvus embryos (Fig. 3). Since the parasympathetic nervous system is known to mediate the transient decelerations of Gallus, we suggest that the development of parasympathetic control of HR in Corvus is similar to that of Gallus. Evidence for the suggested role of the sympathetic nervous system in the mediation of transient HR accelerations of Gallus was not conclusive, but nevertheless, accelerations developed signi®cantly later in Corvus. The dierence between Gallus and Corvus increases further during pipping and after hatching. The studies of Tazawa et al. (1999) and Moriya et al. (1999) continue from that of HoÈchel et al. (1998) by describing the HR ¯uctuations and HRI of Gallus pipped embryos and hatchlings. The main ®nding was that the rapid accel-
erations of Gallus increased in amplitude and dominance from late incubation to the extent that IHR increased by 100 beats á min)1 or more within several beats, and spontaneous decelerations in HR were rarely found during the ®rst week posthatching. In contrast, the IHR of Corvus during the ®rst week after hatching are dominated by both decelerations and accelerations, identical to those of pipped Corvus embryos. Whether altricial Corvus nestlings dier from precocial species in their sympathovagal balance remains to be investigated. Other studies suggest that ectothermy in chicks of altricial species, and in Corvus during their 1st week after hatching, is correlated immaturity of thermogenic and neural control systems (see Visser 1998). HR regulation The hatchling state of crows is typical of altricial birds. On the other hand, the snipe Gallinago media hatchling, of similar body mass to Corvus, is active and has some endothermic powers. In this semi-precocial species the regulation of metabolism appears to dier from that of regulation of HR (Steen et al. 1991). Gallinago hatchlings show small thermogenic responses to maintain Tb of about 30±35 °C during mild-exposure at Ta of 23 °C, and metabolism only decreases signi®cantly with Tb during more severe cold exposure. However, Gallinago HR is directly related to Tb over the whole Ta range. The metabolic response of Corvus hatchlings during cooling was not determined in this study, but their HR between day 0 and day 5 also appears to be regulated according to Tb (Fig. 6). In contrast to Gallinago, at a Ta of 23 °C Corvus chicks (day 0 and day 5) are ectothermic and cool rapidly when exposed to the same Ta. In Corvus chicks of both ages, HR decreased exponentially with time from 300 beats á min)1 at a Tb of 38 °C to 180±210 beats á min)1 at a Tb of 30 °C, and HR was directly and linearly related to falling Tb. The Q10 of HR increased progressively from about 2.0 at a Tb of 38 °C to 3.0±3.2 at a Tb of 30 °C, and therefore HR regulation can be explained entirely by the physical eects of temperature. It is probable that the lower temperature of returning blood circulation from the cooling peripheral tissues act directly on the heart, and/or the decreasing metabolic demands of those peripheral tissues act indirectly on the heart to reduce HR proportionately. The poikilothermic responses of crow are similar to that of smaller house martin Delichon urbica and house wren Troglodytes aedon chicks (Odum 1941; Steen et al. 1989). All these species tolerate mild hypothermia, down to a Tb of at least 30 °C without any loss of coordination. Tolerance of low Tb would undoubtedly conserve energy in Corvus and altricial chicks in general, which appear to remain ectothermic for some period after hatching, and allow more energy to be allocated to growth. In this respect it is noteworthy that accelerative HRI disappear completely during cooling (Fig. 5). Neural mediation of IHR is a balance of parasympathetic and sympathetic activities in endothermic
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animals. Thermosensory feedback during cooling in altricial hatchlings might inhibit the sympathetic tone that would normally act inappropriately to increase HR and therefore possibly increase heat loss as well, and as a result accelerative HRI are greatly suppressed. In conclusion we found that the development of MHR in altricial Corvus embryos is similar to that of Columba livia (Tazawa et al. 1994), increasing slowly throughout much of incubation. The MHR of pipped embryos is temporarily elevated over that of hatchlings, re¯ecting higher activity levels associated with preparations for hatching. Similar patterns of HRI, spontaneous decelerations and accelerations, were found in the crow as previously described for precocial Gallus embryos, with the exception that the amplitudes of accelerative HRI were very reduced in Corvus. The timing of decelerations is similar in both species, but the ontogeny of accelerations appears to be delayed and suppressed in Corvus embryos, occurring infrequently in late embryos and young ectothermic nestlings (day 0 and day 5). Thus further investigation is required to determine if neural control of HR in this altricial species diers signi®cantly from that of precocial species, which hatch with a higher degree of hatchling maturity. Acknowledgements The authors are grateful to Ryuichi Akiyama, Kenji Moriya and Yasuhiro Noma whose programming skills were of great assistance in this study. An earlier draft of this manuscript was improved by comments from two anonymous referees.
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