Analysis of changes in heart rate and temperature of the ground squirrel Citellus undulatus in various physiological states

Heart rate (HR) of ground squirrel C. undulatus was studied in dependence of season, level of activity, physiological state and air temperature (T). In summer HR varies from 110-130 beat/min in sleep up to 420 beat/min at flight from danger. During winter hibernation HP was minimal (3-5 beat/min) at T 1-4 degrees C, the increase in T induced the growth of HP in correspondence to the Arrenius van't Hoff law. The temperature of the body in hibernation exceeded T on 1.5-3 degrees C. The time of getting off the hibernation increased with the decrease in T (6-7 hours at -1 degree C and 2.5-3 hours at +18 degrees C). At phase of increased thermogenesis during arousal heart temperature exceeded rectal one on 10-12 degrees C and heart rate run up to 360-420 beat/min i.e. 2-3 time higher than in active state. The decrease in T stimulated the increase in HP up to 3.8 in winter and 5.3 beat/min/degree C in summer. The highest values of Q10 for HP were revealed at the beginning of hibernation (15-20) and at the beginning of arousal (6-7), in other periods Q10 was similar to the normal biological values (2-2.5). Thus, at the beginning of transitional periods changes of HP were determined mainly by endogenic mechanisms that inhibited myocardium at the beginning of hibernation and activated in arousal. Some mechanisms of coordination between activities of heart and other systems of organism are considered. The features of hit exchange providing the hibernation in wide range of T are discussed.     

Comparison of surface temperature in 13-lined ground squirrel (Spermophilus tridecimlineatus) and yellow-bellied marmot (Marmota flaviventris) during arousal from hibernation

Surface temperatures (Ts) of eight 13-lined ground squirrels and seven yellow-bellied marmots were measured during arousal from hibernation using infrared thermography (IRT) and recorded on videotape. Animals aroused normally in 5 degrees C cold rooms. Body temperatures were recorded during arousal using both cheek pouch and interscapular temperature probes. Warming rate in arousal was exponential. Mean mass specific warming rates show the squirrels warm faster (69.76 degrees C/h/kg) than the marmots (4.49 degrees C/h/kg). Surface temperatures (Ts) for 11 regions were measured every few minutes during arousal. The smaller ground squirrel shows the ability to perfuse distal regions without compromising rise in deep body temperature (Tb). All squirrel Ts's remained low as Tb rose to 18 degrees C, at which point, eyes opened, squirrels became more active and all Ts's rose parallel to Tb. Marmot Ts remained low as Tb rose initially. Each marmot showed a plateau phase where Tb remained constant (mean Tb 20.3+/-1.0 degrees C, duration 9.4+/-4.1 min) during which time all Ts's rose, and then remained relatively constant as Tb again began to rise. An anterior to posterior Ts gradient was evident in the ground squirrel, both body and feet. This gradient was only evident in the feet of the marmots.     

Torpor and hibernation in a basal placental mammal, the Lesser Hedgehog Tenrec Echinops telfairi

The patterns of heterothermy were measured in Lesser Hedgehog Tenrecs, Echinops telfairi, under semi-natural conditions in an outdoor enclosure during the austral mid-winter in southwestern Madagascar. The animals were implanted with miniaturized body temperature (Tb) loggers (iButtons) that measured body temperature every 42 min for 2 months (May and June). The tenrecs entered daily torpor on all 60 consecutive days of measurement, that is, on 100% of animal days, with body temperature closely tracking ambient temperature (Ta) during the ambient heating phase. The mean minimum daily Tb of the tenrecs was 18.44 +/- 0.50 degrees C (n = 174, N = 3), and never exceeded 25 degrees C whereas, apart from a few hibernation bouts in one animal, the mean maximum daily Tb was 30.73 +/- 0.15 degrees C (n = 167, N = 3). Thus during winter, tenrecs display the lowest normothermic Tb of all placental mammals. E. telfairi showed afternoon and early evening arousals, but entered torpor before midnight and remained in torpor for 12-18 h each day. One animal hibernated on two occasions for periods of 2-4 days. We consider E. telfairi to be a protoendotherm, and discuss the relevance and potential of these data for testing models on the evolution of endothermy.     

Reproduction of rock pigeon exposed to extreme ambient temperatures

1. The breeding biology of rock pigeon (Columba livia) exposed to ambient temperatures (Ta) between 50 and 60 degrees C was investigated. 2. Four families accomplished three complete life cycles after long term daily exposure to extreme Ta, with about 100% success. 3. The steady state temperatures in the nest were 60, 58, 53 and 44.6 degrees C in the air, substrate surface, underwing, and in the egg's microenvironment, respectively. 4. At thermal conditions between 30 and 60 degrees C, egg temperature (Tegg) was regulated between 36.8 +/- 0.8 (S.D.) and 41.7 +/- 0.4 (S.D.). Tegg increases by 0.163 degrees C/1 degree C rise in Ta. 5. Mean Tb of the nonincubating parent exposed to 30-60 degrees C is 41.6 +/- 0.6 degrees C (S.D.). Under the same conditions the incubating parent regulated a significantly (P less than 0.01) lower Tb (38.8 degrees C) at 45 degrees C Ta and about 1 degree C lower Tb at 30 and 60 degrees C Ta, respectively. 6. By comparing the differences between fast (5 min) cooling of hot egg (44.8 degrees C) to slow heating (60-90 min), we could demonstrate the high sensitivity of the incubating parent to the danger of embryo overheating. 7. The significance of the adaptive behavioral and physiological mechanisms in breeding under extreme thermal conditions are discussed.     

The Van't Hoff rule in studying various phases of hibernation of the squirrel Citellus undulatus Pallas. Physico-chemical route to cold adaptation

Taking into account the van't Hoff's law the rates of relative erythrocyte hemolysis in ground squirrel Citellus undulatus Pallas during different phases of hibernation were first studied by the method of acid erythrograms with some modifications. The temperature of the erythrogram registration (8 degrees C) models the body temperature in hibernation, and temperature of 35 degrees C, corresponds to the body temperature of the awakened ground squirrel. The positions of the erythrogram maxima for the ground squirrel during short-term arousal and during hibernation coincide for each temperature studied: 8 or 35 degrees C. Therefore, the increase in HCl concentration at 8 degrees C reflects an increase in the stability of erythrocyte membranes or a decrease in the rate of relative hemolysis for the ground squirrel during hibernation. Thus, the adaptive response of erythrocytes in hibernating ground squirrel is revealed. The correlation of the rates of physicochemical processes in vitro with the rates of physiological ones in vivo is shown using the ratio of the rates mentioned above for the short-term arousal and for hibernation. First the physicochemical way of cold adaptation due to the van't Hoff's law is proved.     

Inhibition of the preoptic area and anterior hypothalamus by tetrodotoxin alters thermoregulatory functions in exercising rats.

We have previously demonstrated a functional role of the preoptic area and anterior hypothalamus (PO/AH) in thermoregulation in freely moving rats at various temperature conditions by using microdialysis and biotelemetry methods. In the present study, we perfused tetrodotoxin (TTX) solution into the PO/AH to investigate whether this manipulation can modify thermoregulation in exercising rats. Male Wistar rats were trained for 3 wk by treadmill running. Body core temperature (Tb), heart rate (HR), and tail skin temperature (Ttail) were measured. Rats ran for 120 min at speed of 10 m/min, with TTX (5 microM) perfused into the left PO/AH during the last 60 min of exercise through a microdialysis probe (control, n=12; TTX, n=12). Tb, HR, and Ttail increased during the first 20 min of exercise. Thereafter, Tb, HR, and Ttail were stable in both groups. Perfusion of TTX into the PO/AH evoked an additional rise in Tb (control: 38.2 +/- 0.1 degrees C, TTX: 39.3 +/- 0.2 degrees C; P <0.001) with a significant decrease in Ttail (control: 31.2 +/- 0.5 degrees C, TTX: 28.3 +/- 0.7 degrees C; P <0.01) and a significant increase in HR (control: 425.2 +/- 12 beats/min, TTX: 502.1 +/- 13 beats/min; P <0.01). These results suggest that the TTX-induced hyperthermia was the result of both an impairment of heat loss and an elevation of heat production during exercise. We therefore propose the PO/AH as an important thermoregulatory site in the brain during exercise.     

Effects of passive heat adaptation and moderate sweatless conditioning on responses to cold and heat

Two series of experiments were performed in physically untrained subjects. In series A (heat adaptation, HA), seven male subjects were adapted to dry heat (five consecutive days at 55 degrees C ambient air temperature (Ta) for 1 h X day-1) under resting conditions. Before and after HA, the subjects' shivering responses were determined in a cold test (Ta + 10 to 0 degrees C). In series B, eight male subjects underwent mild exercise training (five consecutive days at a heart rate, HR, of 120 b X min-1) under Ta conditions individually adjusted (Ta + 15 to +5 degrees C) to prevent both sweating and cold sensations. Before and after "sweatless training", the subjects were subjected to a combined cold and heat test. During HA the thresholds for shivering, cutaneous vasodilatation (thumb and forearm) and sweating were shifted significantly (p less than 0.05) towards lower mean body temperatures (Tb). The mean decrease in threshold Tb was 0.36 degrees C. "Sweatless training" resulted in a mean increase in work rate (at HR 120 b X min-1) and oxygen pulse of 13 and 8%, respectively. However, "sweatless training" did not change the threshold Tb for shivering or sweating. Neither HA nor "sweatless training" changed the slopes of the relationships of shivering and sweating to Tb. It is concluded that the previously reported lowering of shivering and sweating threshold Tb in long-distance runners is not due to an increased fitness level, but is essentially identical with HA. The decreased shivering threshold following HA is interpreted as "cross adaptation" produced by the stressors cold and heat.     

Electrophysiological characteristics of cardiac function and the intensity of protein synthesis in cardiomyocytes during the arousal of susliks from hibernation

It has been demonstrated that during winter hibernation (body temperature 2-4 degrees C), the heart rate in ground squirrels is equal to 100 10-12 beats/min. At the initial stage of the arousal, while body temperature remains still low (9-10 degrees C), the heart rate may increase up to 160-200 beats/min. At this stage, practically all electrophysiological parameters of the heart correspond to those in active animals. These results may indicate the ability of "cold" heart in arousing ground squirrels to operate as a normothermic organ and reveal certain role of the heart in body warming. Significant increase of the intensity of protein synthesis in cardiomyocytes together with periodic changes in protein composition of their membranes were found during arousal which may account for regulation of the level of metabolism in cells and for adaptation of the latter to different temperatures.     

Effects of hypoxia and cold acclimation on thermoregulation in the rat.

The effects of hypoxia (inspired O2 fraction = 0.12) on thermoregulation and on the different sources of thermogenesis were studied in rats before and after periods of 1-4 wk of cold acclimation. Measurements of metabolic rate (VO2) and body temperature (Tb) were made at 5-min intervals, and shivering activity was recorded continuously in groups of rats subjected to three protocols. In protocol 1, rats were exposed to normoxia to an ambient temperature (Ta) of 5 degrees C for 2 h. In protocol 2, at Ta of 5 degrees C, rats were exposed for 30 min to normoxia, then for 45 min to hypoxia, and finally for 30 min to normoxia. In protocol 3, in the non-cold-acclimated (NCA) rats, Ta was decreased from 30 to 5 degrees C in steps of 5 degrees C and of 30-min duration while in cold-acclimated (CA) rats at 5 degrees C for 4-wk, Ta was increased from 5 to 30 degrees C in steps of 5 degrees C and of 30-min duration. Recordings were made in normoxia and in hypoxia on different days in the same animals. The results showed that 1) in NCA rats, cold exposure in normoxia induced increases in VO2 and shivering that were proportional to the decrease in Ta; 2) in CA rats in normoxia, for a given Ta, VO2 and Tb were higher than in NCA rats, whereas shivering was generally lower; and 3) in both NCA and CA rats, hypoxia induced a transient decrease in shivering and a sustained decrease in nonshivering thermogenesis associated with a marked decrease in Tb that was about the same in NCA and CA rats. We speculate that hypoxia acts on Tb control to produce a general inhibition of thermogenesis. Nonshivering thermogenesis is markedly sensitive to hypoxia, especially demonstrable in CA rats; a recovery or even an increase in shivering can compensate for the decrease in nonshivering thermogenesis.     

季节、环境温度与黄鼠冬眠的关系

观察了达乌尔黄鼠在实验室内冬眠的一般情况。常温黄鼠的体温有着规律性的年周期,与环境温度的年周期变动不完全呈依从关系。出眠初期(4月下旬),动物体温高而稳定。4月至6月常温黄鼠的平均体温(皮温)为35.6℃,波动菹围32—37.5℃。随着体重达到顶峰,体温逐渐降低。8月份部分黄鼠出现低于32℃的低常体温,表明部分黄鼠自8月盛夏开始冬眠。但就整个种群而言,北京地区实验室内黄鼠冬眠季自9月下半月开始。3月底止,共6.5个月。秋季室温下降,动物入眠趋势增长。浅低体温(31.9—15℃)的比数逐渐升高。9月至12月,低体温(低于31.9℃)的百分比从47％增至84.8％,反映了动物从浅冬眠向深冬眠过渡。1月至2月份,低体温占85％以上,深低体温(低于15℃)占绝对优势。标志着动物种群的深眠月份。秋季动物从常温期向冬眠期转化的界线是不清的,而春季从冬眠期向常温相转化的界限却比较明显。     

Exercise delays the hypoxic thermal response in rats.

Exercise exacerbates acute mountain sickness. In infants and small mammals, hypoxia elicits a decrease in body temperature (Tb) [hypoxic thermal response (HTR)], which may protect against hypoxic tissue damage. We postulated that exercise would counteract the HTR and promote hypoxic tissue damage. Tb was measured by telemetry in rats (n = 28) exercising or sedentary in either normoxia or hypoxia (10% O2, 24 h) at 25 degrees C ambient temperature (Ta). After 24 h of normoxia, rats walked at 10 m/min on a treadmill (30 min exercise, 30 min rest) for 6 h followed by 18 h of rest in either hypoxia or normoxia. Exercising normoxic rats increased Tb ( degrees C) vs. baseline (39.68 +/- 0.99 vs. 38.90 +/- 0.95, mean +/- SD, P < 0.05) and vs. sedentary normoxic rats (38.0 +/- 0.09, P < 0.05). Sedentary hypoxic rats decreased Tb (36.15 +/- 0.97 vs. 38.0 +/- 0.36, P < 0.05) whereas Tb was maintained in the exercising hypoxic rats during the initial 6 h of exercise (37.61 +/- 0.55 vs. 37.72 +/- 1.25, not significant). After exercise, Tb in hypoxic rats reached a nadir similar to that in sedentary hypoxic rats (35.05 +/- 1.69 vs. 35.03 +/- 1.32, respectively). Tb reached its nadir significantly later in exercising hypoxic vs. sedentary hypoxic rats (10.51 +/- 1.61 vs. 5.36 +/- 1.83 h, respectively; P = 0.002). Significantly greater histopathological damage and water contents were observed in brain and lungs in the exercising hypoxic vs. sedentary hypoxic and normoxic rats. Thus exercise early in hypoxia delays but does not prevent the HTR. Counteracting the HTR early in hypoxia by exercise exacerbates brain and lung damage and edema in the absence of ischemia.     

Control of total peripheral resistance during hyperthermia in rats

To elucidate the effect of blood volume on the circulatory adjustment to heat stress, we studied alpha-chloralose-anesthetized rats at three levels of blood volume: normovolemia (NBV), hypervolemia (HBV; +32% plasma volume by isotonic albumin solution infusion), and hypovolemia (LBV; -16% plasma volume by furosemide administration). Body surface heating was performed with an infrared lamp to raise arterial blood temperature (Tb) at the rate of approximately 0.1 degree C/min. Before heating, central venous pressure (CVP) was significantly higher in HBV (0.41 +/- 0.25 mmHg) and lower in LBV (-1.44 +/- 0.22 mmHg) than in NBV (-0.41 +/- 0.10 mmHg). The Tb at which CVP started to decrease was approximately 40 degrees C in HBV, approximately 41 degrees C in NBV, and approximately 42 degrees C in LBV, and it decreased by 1.53 +/- 0.14, 1.92 +/- 0.24, and 0.62 +/- 0.14 mmHg from 37 to 43 degrees C of Tb in HBV, NBV, and LBV, respectively. Stroke volume was closely correlated with CVP, and this relationship was not affected by Tb. Heart rate responses to the raised Tb were similar among the three groups. Mean arterial pressure (MAP) was not affected by blood volume modification or CVP and was maintained at preheating (Tb 37 degrees C) level until Tb rose to 40 degrees C. Above this Tb, MAP increased until Tb reached 43 degrees C (+30-40 mmHg) for all three groups. Total peripheral resistance (TPR) was inversely correlated with CVP, and the slope of the linear relationship between TPR and CVP in LBV was three- to fourfold steeper than in NBV or HBV.(ABSTRACT TRUNCATED AT 250 WORDS)     

Time-course of blood acid-base state during arousal from hibernation in the European hamster

1. Arterial blood was sampled at 15 min-intervals in European hamsters Cricetus cricetus fitted with indwelling catheters, from deep hibernation to full arousal. Temperature-corrected pH and PCO2, respectively pH* and P*CO2, were directly measured at 37 degrees C. 2. Deep hibernation corresponded to a respiratory acidosis: pH* = 7.01 +/- 0.01 (mean +/- SE), P*CO2 = 160 +/- 4 Torr (n = 9 animals). 3. Three periods could be distinguished in the arousal: (i) a period of hyperventilation (28 +/- 5 min), in which P*CO2 was reduced to 79 +/- 4 Torr, while cheek pouch temperature increased only by 0.9 +/- 0.2 degrees C; (ii) a period of metabolic acidification by lactate accumulation (84 +/- 6 min), corresponding to the period of peak thermogenesis; (iii) a progressive return to euthermic conditions (104 +/- 10 min), by simultaneous respiratory and metabolic alkalinization. 4. Over 60% of the blood CO2 stores accumulated at the beginning of the hibernation bout were released by hyperventilation during the first period, prior to the full development of thermogenesis. This is in agreement with the hypothesis of an inhibitory role of the respiratory acidosis in hibernation.     

Myoglobin content in skeletal muscles of hibernating ground squirrels rises in autumn and winter

The content of myoglobin (Mb) in skeletal muscles of Arctic Yakutian ground squirrel (Citellus undulatus Pallas) was measured in the active euthermic summer and prehibernating autumn animals as well as in hibernating and awake animals in winter. The myoglobin content in winter, irrespective of the state of the animal, was found to be about three times higher than in summer. The content of myoglobin in autumn was also two-fold increased compared to summer, suggesting that high myoglobin level is necessary for hibernation. Analysis of biochemical data available suggests that the increase in myoglobin content in winter is probably related to a high oxygen demand of muscles at the first stage of arousal (non-shivering thermogenesis) when rectal temperature rises from 0 to 10-12 degrees C. At this stage, the oxygen-dependent processes in muscles proceed under the conditions when peripheral blood flow is blocked and anaerobic glycolysis is switched off.     

达乌尔黄鼠入眠准备期的体温、代谢率及能量特征

贮脂类动物在冬眠前大量积累脂肪来准备冬眠,并在入眠时迅速降低体温和代谢率。为探究入眠准备期达乌尔黄鼠体温、代谢率、呼吸商及能量代谢的变化,将其入眠准备期分为育肥期、体重高峰期、育肥后期和冬眠前的试降期,使用植入式半导体温度记录元件iButton、开放式代谢仪和改进的代谢笼,监测其体温、代谢率及呼吸商和能量摄入的变化。结果显示:(1)达乌尔黄鼠体温在冬眠前13 - 34 d 开始下降,远早于冬眠但晚于体重高峰期;体重高峰期体温有降低的趋势,持续时间为1 - 3 d;育肥后期体温显著下降,体温日波动幅度增加。(2)体重高峰期的静止代谢率高于育肥期,育肥后期有降低的趋势,试降期最低。(3)呼吸商在体重高峰期先升高,之后迅速衰减;入眠准备期的能量摄入在体重达高峰期前达到最大值。结果表明,达乌尔黄鼠在入眠准备期,其体温和代谢率已开始降低,能源物质已开始转变;体重高峰期可能是达乌尔黄鼠入眠的一个转折点或启动入眠的开关。     

Periodic arousal from hibernation is necessary for initiation of immune responses in ground squirrels

Golden-mantled ground squirrels (Spermophilus lateralis) undergo seasonal hibernation during which core body temperature (T(b)) values are maintained 1-2 degrees C above ambient temperature. Hibernation is not continuous. Squirrels arouse at approximately 7-day intervals, during which T(b) increases to 37 degrees C for approximately 16 h; thereafter, they return to hibernation and sustain low T(b)s until the next arousal. Over the course of the hibernation season, arousals consume 60-80% of a squirrel's winter energy budget, but their functional significance is unknown and disputed. Host-defense mechanisms appear to be downregulated during the hibernation season and preclude normal immune responses. These experiments assessed immune function during hibernation and subsequent periodic arousals. The acute-phase response to bacterial lipopolysaccharide (LPS) was arrested during hibernation and fully restored on arousal to normothermia. LPS injection (ip) resulted in a 1-1.5 degrees C fever in normothermic animals that was sustained for > 8 h. LPS was without effect in hibernating squirrels, neither inducing fever nor provoking arousal, but a fever did develop several days later, when squirrels next aroused from hibernation; the duration of this arousal was increased sixfold above baseline values. Intracerebroventricular infusions of prostaglandin E(2) provoked arousal from hibernation and induced fever, suggesting that neural signaling pathways that mediate febrile responses are functional during hibernation. Periodic arousals may activate a dormant immune system, which can then combat pathogens that may have been introduced immediately before or during hibernation.     

Translational initiation is uncoupled from elongation at 18 degrees C during mammalian hibernation

Cellular and organismal homeostasis must be maintained across a body temperature (Tb) range of 0 to 37 degrees C during mammalian hibernation. Hibernators depress biosynthetic activities including protein synthesis, concordant with limited energy availability and temperature effects on reaction rates. We used polysome analysis to show that initiation of protein synthesis ceases during entrance into torpor in golden-mantled ground squirrels (Spermophilus lateralis) when Tb reaches 18 degrees C. Elongation of preinitiated polypeptides continues slowly throughout the torpor bout. As Tb begins to rise, initiation resumes even at temperatures below 18 degrees C, although the euthermic polysome pattern is not reestablished. At precisely 18 degrees C, there is a large increase in initiation events and a complete restoration of euthermic polysome distribution patterns. These data indicate a role for both passive and active depression of translation during torpor and are consistent with a requirement for new protein biosynthesis during each interbout arousal.     

An infrared thermographic study of surface temperature in relation to external thermal stress in the Mongolian gerbil, Meriones unguiculatus

1. Temperatures of different body surface regions and deep body temperature (Tb) of unrestrained adult Mongolia gerbils exposed to ambient temperatures (Ta) of -10-35 degrees C were measured using infrared (i.r.) thermography and a thermocouple. 2. A strong positive linear relationship between the surface temperature and Ta was found. For Ta range -4-35 degrees C, the slope was lowest for the areas around the eyes and dorsal head, and steepest for the body extremities. At -10 degrees C, surface temperatures of the areas around the eyes and dorsal head were significantly lower then predicted. 3. Tb was lowest at Ta of 25 and 30 degrees C, increased at all temperatures above and up to Ta of -4 degrees C below this range, and began decline at -10 degrees C. 4. The thermoneutral zone (TNZ) is probably between 28 and 32 degrees C, and the absolute lower critical temperature (Tabsl) is probably -4 degrees C. 5. The Mongolian gerbil shows little control of surface temperature and in contrast to larger mammals it has not developed any special thermoregulatory surface areas to regulate heat exchange with its environment. At temperatures below -4 degrees C, this species is unable to maintain the surface temperature of body extremities above the freezing point. 6. It is suggested that the Mongolian gerbil uses mainly behavioral and ecological adaptive strategies to attenuate the stressful effects of its habitat.     

Endothermic heart rate response in broiler and White Leghorn chicks (Gallus gallus domesticus) during the first two days of post-hatch life

The embryonic modal value of heart rate (MHR) differs between broiler and White Leghorn chickens, but the initial development of cholinergic chronotropic control of embryonic heart rate (HR) does not. Thus, we hypothesized that hatchling MHR should also differ between broiler and White Leghorn strains, while the development of a physiological regulation, such as the endothermic HR response, should not be different between hatchlings of the two strains. To test this, we measured the response of HR and cloaca temperature (Tb) to alteration of ambient temperature (Ta); i.e., 35 degrees C-25 degrees C-35 degrees C, in four groups of hatchlings on Days 0 and 1 post-hatch. Fertile eggs of both strains with similar mass were incubated simultaneously in the same incubator. Eggs of broiler chickens hatched approximately 7 h earlier than White Leghorn chicken eggs. Chick mass at hatching was identical in both strains, but diverged during 2 days after hatching. Tb measured at the initial Ta of 35 degrees C was identical in both strains. MHR at the same Ta was approximately 30 bpm lower in broiler chicks than in White Leghorn chicks, but the difference was reversed to that observed in the embryos. The endothermic HR response was advanced by approximately 1 day in broiler chicks compared with White Leghorn chicks. As a result, eggs of similar mass in both strains produced chicks with similar mass and Tb at hatching, but during 2 days of post-hatch life their masses diverged and regulation of the endothermic HR response developed earlier in broiler than in White Leghorn hatchlings. This physiological heterochrony between strains is most likely due to genetic selection for fast growth in broiler chickens.     

Maturation of the homeothermic response of heart rate to altered ambient temperature in developing chick hatchlings (Gallus gallus domesticus)

On the basis of evidence showing that instantaneous heart rate (IHR) of chick hatchlings responds to exposure to altered ambient temperature (Ta; Tazawa H, Moriya K, Tamura A, and Akiyama R. Comp Biochem Physiol A 131A: 797-803, 2002), we elucidate here the developmental timeline for the homeothermic response of HR in newly hatched chicks (days 0-7) maintained at room temperature ( approximately 24-27 degrees C). Hatchlings were exposed to Ta of 25, 35, and 25 degrees C for 1-h periods, respectively, and IHR was measured together with skin temperature (Ts) during this warming and cooling bout. Early 0-day-old (0 day) chicks responded to warming and cooling exposures with various changes in HR baseline. In newly hatched chicks (0-7 h old), HR baseline was elevated during warming (Delta126 beats/min, n = 13) and declined during cooling (-Delta94 beats/min). With progress of development on day 0, the elevation of HR baseline during warming decreased and advanced 0-day chicks tended to decrease HR baseline during warming rather than increase HR. The more developed 1- to 7-day-old chicks exhibited the expected homeothermic decrease in HR during warming. The diurnal variations of HR responses during warming and cooling on the first day of post-egg life indicate that pronounced development of thermoregulatory competence occurs during the day of hatching (day 0). The response of IHR fluctuations to altered Ta was observed in the form of low- and high-frequency oscillations. High-frequency oscillations corresponding to respiratory sinus arrhythmia developed as the hatchlings aged. There was a significant increase in the number of chicks exhibiting both low- and high-frequency oscillations that depended on age and the development of thermoregulatory competence of hatchlings.