Thermoregulation of weaned northern elephant seal (Mirounga angustirostris) pups in air and water

Fasting weaned northern elephant seal pups (Mirounga angustirostris) experience diverse environmental conditions on land and in water on a daily basis. Each environment undoubtedly induces distinct energetic costs that may vary for pups of differing body condition. To determine the energetic costs associated with different environmental conditions and whether costs vary between individuals, body mass, surface area, volume, body composition, resting metabolic rate, and core body temperature were determined for 17 weaned northern elephant seal pups from A?o Nuevo, California. Metabolic rate and body temperature were measured for pups resting in air (20.9 degrees +/-0.8 degrees C), cold water (3.8 degrees+/-0.4 degrees ;C), and warm water (14.5 degrees+/-0.2 degrees C). Resting metabolic rate increased with body mass (range: 62.0-108.0 kg) and was also correlated with lean mass and lipid mass. Metabolic rates ranged from 293.6 to 512.7 mL O(2) min(-1) and were lowest for pups resting in cold water. Thermal conductance, calculated from metabolic rate and core body temperature, ranged from 3.1 to 15.2 W degrees C(-1), with the highest values in air and the lowest values in cold water. Metabolic responses to the three environmental conditions did not differ with individual variation in body condition. For all elephant seal pups, a consequence of high lipid content is that thermoregulatory costs are greatest on land and lowest in cold water, a pattern that contrasts markedly with terrestrial mammals.     

Behavioral thermoregulation by hypoxic rats.

To address whether a shift in hypothalamic thermal setpoint might be a significant factor in induction of hypoxic hypothermia, behavioral thermoregulation was examined in 7 female Sprague-Dawley rats implanted with radiotelethermometers for deep body temperature (Tb) measurement in a thermocline during normoxia (PO2 = 125 torr) and hypoxia (PO2 = 60 torr). Normoxic rats (TNox) selected a mean ambient temperature of 19.7 +/- 1.4 (SE) degrees C and maintained Tb at 37.0 +/- 0.2 degrees C. Hypoxic rats selected a significantly higher ambient temperature (THox = 28.6 +/- 2.2 degrees C) but maintained Tb significantly lower at 35.5 +/- 0.3 degrees C. Without a thermal gradient (ambient temperature = 25 degrees C), Tb during hypoxia was 35.4 +/- 0.4 degrees C. The maintenance of a lower body temperature during hypoxia through behavioral thermoregulation despite having warmer temperatures available supports the hypothesis that the thermoregulatory setpoint of hypoxic rats is shifted to promote thermoregulation at a lower Tb, effectively reducing oxygen demand when oxygen supply is limited.     

Insulation and thermal balance of fasting harp and grey seal pups.

1. Harp and grey seal pups were examined during the post-weaning period to quantify their thermoregulatory abilities and thermal limits. 2. Deep body temperatures of harp seals (37.8 +/- 0.8 degrees C) were not significantly different from those of grey seals (38.9 +/- 0.4 degrees C). 3. As blubber depth declined during the fast, temperature gradients extended increasingly deeper into the muscle layer potentially decreasing heat loss. 4. Blubber conductivity (approximately 0.18 W/m/degrees C) did not vary regionally within an animal, or between animals or species. 5. Calculated lethal cold limits in air were between -85.4 degrees C and -116.1 degrees C, suggesting that fasting, weaned pups can easily cope with temperatures they would normally experience.     

Age-dependent core temperature responses of conscious rabbits to acute hypoxemia.

Experiments were carried out on chronically instrumented newborn and older rabbits to characterize their core temperature (T(c)) responses to acute hypoxemia and to differentiate "forced" vs. "regulated" thermoregulatory responses. Three age ranges of kits were studied: 4-6, 9-11, and 28-30 days of age. During an experiment, T(c), selected ambient temperature (T(a)), and oxygen consumption were measured from kits studied in a thermocline during a control period of normoxemia, an experimental period of normoxemia or hypoxemia (fraction of inspired oxygen 0.10), and a recovery period of normoxemia. We reasoned that no change or a decrease in T(a) while T(c) decreased during hypoxemia would indicate a regulated thermoregulatory response, whereas an increase in T(a) while T(c) decreased during hypoxemia would indicate a forced thermoregulatory response. T(c) decreased during acute hypoxemia in the older kits but not in the 4- to 6-day-old kits; the decrease in T(c) was accentuated on postnatal days 28-30 compared with postnatal days 9-11. T(a) decreased or stayed the same during exposure to acute hypoxemia. Our data provide evidence that postnatal maturation influences the T(c) response of rabbits to acute hypoxemia and that the decrease in T(c) during hypoxemia in the older kits results from a regulated thermoregulatory response.     

Cold-rearing normalizes capacity for norepinephrine-stimulated thermogenesis but not body temperature in 16-day-old fatty Zucker rats

The effects of a lowered rearing temperature on body weight, core temperature (Tc) and norepinephrine(NE)-stimulated thermogenesis were investigated in 16- to 17-day-old Zucker rat pups. 16-day-old fatty pups were significantly heavier (9%) than lean littermates in litters reared at 18 degrees C ("cold-reared") but not in litters reared at 25 degrees C ("normally-reared"). After 2 h isolation at 25 degrees C, Tc of lean pups was slightly higher (37.1 degrees C) in cold-reared litters than in normally-reared litters (36.4 degrees C), while fatty pups reared at either temperature were severely hypothermic (Tc = 33 - 34 degrees C). At an ambient temperature of 25 degrees C Tc in fatty and lean cold-reared pups increased to 39.5 degrees C after subcutaneous injection of 800 micrograms/kg NE. Normally-reared lean pups reached the same peak Tc after NE injection, while their fatty littermates reached a significantly lower peak Tc of 38.4 degrees C. The hypothermia associated with the onset of excess fat deposition in suckling fatty Zucker rats is not caused by a reduced capacity for NE-stimulated thermogenesis.     

Physical fitness and thermoregulatory reactions in a cold environment in men

The relationship between the physical fitness level (maximal O2 consumption, VO2max) and thermoregulatory reactions was studied in 17 adult males submitted to an acute cold exposure. Standard cold tests were performed in nude subjects, lying for 2 h in a climatic chamber at three ambient air temperatures (10, 5, and 1 degrees C). The level of physical fitness conditioned the intensity of thermoregulatory reactions to cold. For all subjects, there was a direct relationship between physical fitness and 1) metabolic heat production, 2) level of mean skin temperature (Tsk), 3) level of skin conductance, and 4) level of Tsk at the onset of shivering. The predominance of thermogenic or insulative reactions depended on the intensity of the cold stress: insulative reactions were preferential at 10 degrees C, or even at 5 degrees C, whereas colder ambient temperature (1 degree C) triggered metabolic heat production abilities, which were closely related to the subject's physical fitness level. Fit subjects have more efficient thermoregulatory abilities against cold stress than unfit subjects, certainly because of an improved sensitivity of the thermoregulatory system.     

Thermoregulatory responses of dystrophic hamsters to changes in ambient temperatures

The ability of dystrophic hamsters to maintain their body temperature despite abnormal muscle and brown adipose tissue, two organs involved in thermoregulation, was evaluated. Dystrophic hamsters (CHF 146) between the ages of 30 and 160 days kept at 21 degrees C had core (rectal) temperatures (TR) that were 0.5-1.5 degrees C lower than Golden Syrian controls. The reduced core temperatures of dystrophic hamsters were unlikely the result of an incapacity to generate heat since the dystrophic hamsters were able to maintain their TRs during 3 h of acute cold stress (4 degrees C) and to adapt to prolonged cold exposure. However, TRs of cold-acclimated dystrophic hamsters were still 1 degree C below TRs of cold-acclimated control animals. By contrast, increasing the ambient temperature raised TRs of both normal and dystrophic hamsters. When kept at 32 degrees C overnight, the TRs of dystrophic hamsters remained significantly below those of control animals. When heat-exposed dystrophic hamsters were returned to 21 degrees C, their TRs returned to values significantly lower than those of control hamsters. Thus, dystrophic hamsters showed a capacity to thermoregulate, like control hamsters, but appeared to do so at a lower temperature. The reduced core temperatures of dystrophic hamsters kept at 21 degrees C cannot be explained by a reduction in metabolic activity since newborns and 30- and 140-day-old dystrophic hamsters had rates of oxygen consumption (VO2) and carbon dioxide production (VCO2) that were similar to those of controls. These results suggest that the thermoregulatory set point may be altered in dystrophic hamsters.     

The relationship between rate of oxygen consumption, heart rate and thermal conductance of the dik-dik antelope (Rhynchotragus kirkii) at various ambient temperatures

1. The extent of cardiovascular adjustments to heat and cold were investigated between ambient temperatures of 5 and 45 degrees C by measuring conductance and the rates of oxygen consumption and heart beats. 2. Minimum heart rate was observed at 25 degrees C (114 +/- 9 beats/min). In the heat at 45 degrees C heart rate was observed to increase only slightly (127 +/- 12 beats/min) but in the cold -5 degrees C heart rate nearly doubled that at 25 degrees C. 3. Thermal conductance was on average 0.031 mlO2 (g. hr. degrees C)-1 below 25 degrees C but increased by more than 20 times at 40 degrees C. 4. A positive correlation between heart rate and rate of oxygen consumption was demonstrated below 25 degrees C and the relation may be of practical use.     

Effects of cold and capsaicin desensitization on prostaglandin E hypothermia in rats

Intraperitoneal injection of prostaglandin E1 (PGE) produces a transient hypothermia in rats that lasts 1-2 h. Rats exposed to an ambient temperature (Ta) of 26 degrees C displayed a decrease in rectal temperature (Tre) of 0.95 +/- 0.12 degrees C (SE) after injection with PGE (100 micrograms/kg ip). Hypothermia was produced mainly by heat losses, as indicated by increases in tail blood flow. At Ta of 4 degrees C, PGE produced a comparable fall in Tre of 1.00 +/- 0.14 degrees C. However, in the cold the hypothermia was caused solely by decreases in heat production. These results indicate that the PGE-induced hypothermia is not the result of a peripheral vasodilation induced by the direct action of PGE on the tail vascular smooth muscle but is a central nervous system-mediated response of the thermoregulatory system induced by PGE within the peritoneal cavity. Capsaicin injected subcutaneously induces a transient hypothermia in rats because of stimulation of the warm receptors. If administered peripherally in sufficient amounts, it is reputed to impair peripheral warm receptors so that they become desensitized to the hypothermic effects of capsaicin. We measured PGE-induced hypothermias in rats both before and after capsaicin desensitization at Ta of 26 degrees C. Before desensitization the hypothermia was -1.14 +/- 0.12 degrees C, whereas after capsaicin treatment the PGE-induced hypothermia was -0.34 +/- 0.17 degrees C. The biological effects of capsaicin are diverse; however, based on current thinking about the thermoregulatory effects of capsaicin desensitization, our results indicate that peripheral warm receptor pathways are in some manner implicated in the hypothermia induced by intraperitoneal PGE.     

The influence of ambient temperature on metabolism and body temperature of newborn and growing reindeer calves (Rangifer tarandus tarandus L.)

Thermoregulatory capacities of 51 reindeer calves (Rangifer tarandus tarandus L.) aged 1-35 days were studied at -26.5 to +35.0 degrees C ambient temperatures at Kaamanen reindeer research station, Finland (69 degrees 10' N) during calving periods in May 1981 and May-July 1982. The newborn calves aged 1-4 days maintained a high body temperature (Tre) (mean +40.2 degrees C) even at the lowest experimental temperature of -22.5 degrees C by increasing their metabolic rate five-fold above the level at +11.0 degrees C. Heat production of the new-born calves was largely based on the metabolism of brown adipose tissue, stimulated by cold-induced discharge of the sympathetic nervous transmitter, noradrenaline (NA). Sensitivity of the calves to exogenous NA disappeared during the first 3-4 weeks of life. Thermal conductance of the calves was low at low ambient temperatures, but rose strongly as Ta increased above +10 degrees C. The extensive peripheral cooling, especially in the feet, was demonstrated in the calves aged 1-10 days. The lowest foot temperature (+10.5 degrees C) was measured in a 4-day-old calf at -14.5 degrees C. Slight shivering thermogenesis was recorded in the calves aged 1-4 days and occasionally in the older calves at low values of Ta. Shivering appears to be a reserve mechanism against severe cold. At about +20 degrees C and above the calves increased their Tre (approximately 1 degree C), oxygen consumption and heart rate. In the newborn calves oxygen consumption rose four- to five-fold and in 1-month-old calves about two-fold. Fast growing calves (maximum 400 g/day) appear to be more stressed by heat than by cold exposure.     

Effect of creatine supplementation on cycle ergometer exercise in a hyperthermic environment

In order to examine thermoregulatory response to creatine (CR) supplementation, competitive male cyclists and triathletes (n = 7, VO2max = 50.6 +/- 0.8 ml x kg(-1) x min(-1)) completed three 1-hour hyperthermic (ambient temperature = 38.7 +/- 1.0 degrees C, relative humidity = 33 +/- 4%) exercise sessions at 181 +/- 12 W (50% of Wmax, approximately 66% of VO2max). Subjects completed a baseline (BL) session, then 2 sessions following 5 days of CR (20 g x d(-1)) and placebo (PL, 20 g x d(-1)) administered in a double-blind counterbalanced crossover manner with > or = 28-day washout. Pre-exercise BL, CR, and PL body mass were unchanged, with similar decreases in postexercise mass among the three conditions. Tympanic temperature, heart rate, systolic blood pressure, perceived exertion, and lactate, cortisol, and aldosterone concentrations increased similarly during BL, CR, and PL exercise. A greater (p = 0.013) estimated decrease in plasma volume occurred following BL (-16.5 +/- 2.0%) and PL (-17.6 +/- 1.7%) exercise compared to CR (-13.5 +/- 2.1%). Creatine supplementation reduces plasma volume loss during 1 hour of hyperthermic exercise but does not appear to otherwise change thermoregulatory response to hyperthermic exercise.     

Neural and hormonal control of arterial pressure during cold exposure in unanesthetized week-old rats

Infant rats respond to cold exposure with increased heat production by brown adipose tissue (BAT). BAT thermogenesis increases steadily with increasing cold exposure, but a point occurs at which thermogenesis can increase no further, resulting in cold-induced bradycardia. Previous work has shown that mean arterial pressure (MAP) is maintained even when cardiac rate decreases as much as 50% from baseline values. We examined the neural and hormonal contributions to peripheral resistance during cold exposure after pups were injected subcutaneously with vehicle, an alpha1-adrenoceptor antagonist (prazosin; 0.5 mg/kg), an ANG II receptor antagonist (losartan; 1 mg/kg), a vasopressin receptor antagonist (Manning compound; 0.5 mg/kg), or simultaneous administration of all three antagonists (triple block). Interscapular temperature, oxygen consumption, cardiac rate, and arterial pressure were monitored as air temperature was sequentially decreased from thermoneutral (i.e., 35 degrees C) to 29, 23, and 17 degrees C. Only pups in the triple block condition exhibited significant decreases in MAP with cooling, even though all pups exhibited substantial decreases in cardiac rate. A followup study suggested that blockade of all three systems was more effective than blockade of any two systems. Finally, at 17 degrees C, ultrasonic vocalizations were accompanied by significant increases in MAP, replicating a previous finding and supporting the hypothesis that the vocalization is the acoustic by-product of the abdominal compression reaction, a maneuver that helps to maintain venous return during cardiovascular challenge.     

Response to cold in the blue fox and raccoon dog as evaluated by metabolism, heart rate and muscular shivering: a re-evaluation

Oxygen consumption (ml kg-0.75/min) in relation to ambient air temperature at or below the lower critical temperature (Tlc) of the winter-furred raccoon dog (+10 degrees C) and the blue fox (-6 degrees C) is described by the equations y = 14.8-0.28x and y = 7.5-0.20x, respectively. Muscular shivering activity (integrated EMG) of both species increased below thermoneutrality parallel with increasing oxygen uptake and heart rate. Seasonal changes in measured metabolic parameters were evident for both species. The results suggest that the overall body insulation or the metabolic response to cold are not essentially worse in the raccoon dog as compared with the blue fox. It is concluded that earlier speculations of surprisingly wide thermoneutral zone and very low Tlc of the Arctic fox are not evident for the blue fox.     

Regulation of body temperature and energy requirements of hibernating alpine marmots (Marmota marmota)

Body temperature and metabolic rate were recorded continuously in two groups of marmots either exposed to seasonally decreasing ambient temperature (15 to 0 degrees C) over the entire hibernation season or to short-duration temperature changes during midwinter. Hibernation bouts were characterized by an initial 95% reduction of metabolic rate facilitating the drop in body temperature and by rhythmic fluctuations during continued hibernation. During midwinter, we observed a constant minimal metabolic rate of 13.6 ml O(2) x kg(-1) x h(-1) between 5 and 15 degrees C ambient temperature, although body temperature increased from 7.8 to 17.6 degrees C, and a proportional increase of metabolic rate below 5 degrees C ambient temperature. This apparent lack of a Q(10) effect shows that energy expenditure is actively downregulated and controlled at a minimum level despite changes in body temperature. However, thermal conductance stayed minimal (7.65 +/- 1.95 ml O(2) x kg(-1) x h(-1) x degrees C(-1)) at all temperatures, thus slowing down cooling velocity when entering hibernation. Basal metabolic rate of summer-active marmots was double that of winter-fasting marmots (370 vs. 190 ml O(2) x kg(-1) x h(-1)). In summary, we provide strong evidence that hibernation is not only a voluntary but a well-regulated strategy to counter food shortage and increased energy demands during winter.     

Behavioral thermoregulation in obese and lean Zucker rats in a thermal gradient

Genetically obese Zucker (Z) rats have been reported to display a body core temperature (Tb) that is consistently below that of their lean littermates. We asked the question whether the lower Tb was a result of deficits in thermoregulation or a downward resetting of the set point for Tb. For a period of 45 consecutive hours, lean and obese Z rats were free to move within a thermal gradient with an ambient temperature (T(a)) range of 15-35 degrees C, while subjected to a 12:12-h light-dark cycle. Tb was measured using a miniature radio transmitter implanted within the peritoneal cavity. Oxygen consumption (VO2) was measured using an open flow technique. Movements and most frequently occupied position in the gradient (preferred T(a)) were recorded using a series of infrared phototransmitters. Obese Z rats were compared with lean Z rats matched for either age (A) or body mass (M). Our results show that obese Z rats have a lower Tb [37.1 +/- 0.1 degrees C (SD) vs. 37.3 +/- 0.1 degrees C, P < 0.001] and a lower VO2 (25.3 +/- 1.9 ml x kg(-1) x h(-1)) than lean controls [33.1 +/- 3.7 (A) and 33.9 +/- 3.9 (M) ml x kg(-1) x h(-1), P < 0.001]. Also, the obese Z rats consistently chose to occupy a cooler T(a) [20.9 +/- 0.6 degrees C vs. 22.7 +/- 0.6 degrees C (A) and 22.5 +/- 0.7 degrees C (M), P < 0.001] in the thermal gradient. This suggests a lower set point for Tb in the obese Z rat, as they refused the option to select a warmer T(a) that might allow them to counteract any thermoregulatory deficiency that could lead to a low Tb. Although all rats followed a definite circadian rhythm for both Tb and VO2, there was no discernible circadian pattern for preferred T(a) in either obese or lean rats. Obese Z rats tended to show a far less definite light-dark activity cycle compared with lean rats.     

Heat debt as an index for cold adaptation in men

Several types of cold adaptation in men have been described in the literature (metabolic, insulative, hypothermic). The aim of this study is to show that the decrease of heat debt can be considered as a new index for cold adaptation. Ten male subjects were acclimated by water immersions (temperature 10-15 degrees C, 4 immersions/wk over 2 mo). Thermoregulatory responses before and after acclimation were tested by a standard cold test in a climatic chamber for 2 h at rest [dry bulb temperature (Tdb): 10 degrees C; relative humidity (rh): 25%]. After adaptation, four thermoregulatory modifications were observed: an increase in the delay for the onset of shivering (32.7 +/- 7.99 instead of 14.1 +/- 5.25 min); a decrease of body temperature levels for the onset of shivering [rectal temperature (Tre): 37.06 +/- 0.08 instead of 37.31 +/- 0.06 degrees C; mean skin temperature (Tsk): 24.83 +/- 0.56 instead of 26.86 +/- 0.46 degrees C; mean body temperature (Tb): 33.03 +/- 0.20 instead of 34.16 +/- 0.37 degrees C); a lower level of body temperatures in thermoneutrality (Tre = 37.16 +/- 0.08 instead of 37.39 +/- 0.06 degrees C; Tsk = 31.29 +/- 0.21 instead of 32.01 +/- 0.22 degrees C; Tb = 35.92 +/- 0.08 instead of 36.22 +/- 0.05 degrees C); a decrease of heat debt calculated from the difference between heat gains and heat losses (5.66 +/- 0.08 instead of 8.33 +/- 0.38 kJ/kg). The different types of cold adaptation observed are related to the physical characteristics of the subjects (percent body fat content) and the level of physical fitness (VO2max).(ABSTRACT TRUNCATED AT 250 WORDS)     

Immune changes in humans during cold exposure: effects of prior heating and exercise.

This study examined the immunological responses to cold exposure together with the effects of pretreatment with either passive heating or exercise (with and without a thermal clamp). On four separate occasions, seven healthy men [mean age 24.0 +/- 1.9 (SE) yr, peak oxygen consumption = 45.7 +/- 2.0 ml. kg(-1). min(-1)] sat for 2 h in a climatic chamber maintained at 5 degrees C. Before exposure, subjects participated in one of four pretreatment conditions. For the thermoneutral control condition, subjects remained seated for 1 h in a water bath at 35 degrees C. In another pretreatment, subjects were passively heated in a warm (38 degrees C) water bath for 1 h. In two other pretreatments, subjects exercised for 1 h at 55% peak oxygen consumption (once immersed in 18 degrees C water and once in 35 degrees C water). Core temperature rose by 1 degrees C during passive heating and during exercise in 35 degrees C water and remained stable during exercise in 18 degrees C water (thermal clamping). Subsequent cold exposure induced a leukocytosis and granulocytosis, an increase in natural killer cell count and activity, and a rise in circulating levels of interleukin-6. Pretreatment with exercise in 18 degrees C water augmented the leukocyte, granulocyte, and monocyte response. These results indicate that acute cold exposure has immunostimulating effects and that, with thermal clamping, pretreatment with physical exercise can enhance this response. Increases in levels of circulating norepinephrine may account for the changes observed during cold exposure and their modification by changes in initial status.     

Pre-exercise hyperhydration delays dehydration and improves endurance capacity during 2 h of cycling in a temperate climate

Whether the use of pre-exercise hyperhydration could improve the performance of athletes who do not hydrate sufficiently during prolonged exercise is still unknown. We therefore compared the effects of pre-exercise hyperhydration and pre-exercise euhydration on endurance capacity, peak power output and selected components of the cardiovascular and thermoregulatory systems during prolonged cycling. Using a randomized, crossover experimental design, 6 endurance-trained subjects underwent a pre-exercise hyperhydration (26 ml of water x kg body mass(-1) with 1.2 g glycerol x kg body mass(-1)) or pre-exercise euhydration period of 80 min, followed by 2 h of cycling at 65% maximal oxygen consumption (VO(.)2max) (26-27 degrees C) that were interspersed by 5, 2-min intervals performed at 80% V(.)O2max. Following the 2 h cycling exercise, subjects underwent an incremental cycling test to exhaustion. Pre-exercise hyperhydration increased body water by 16.1+/-2.2 ml.kg body mass(-1). During exercise, subjects received 12.5 ml of sports drink x kg body mass(-1). With pre-exercise hyperhydration and pre-exercise euhydration, respectively, fluid ingestion during exercise replaced 31.0+/-2.9% and 37.1+/-6.8% of sweat losses (p>0.05). Body mass loss at the end of exercise reached 1.7+/-0.3% with pre-exercise hyperhydration and 3.3+/-0.4% with pre-exercise euhydration (p<0.05). During the 2 h of cycling, pre-exercise hyperhydration significantly decreased heart rate and perceived thirst, but rectal temperature, sweat rate, perceived exertion and perceived heat-stress did not differ between conditions. Pre-exercise hyperhydration significantly increased time to exhaustion and peak power output, compared with pre-exercise euhydration. We conclude that pre-exercise hyperhydration improves endurance capacity and peak power output and decreases heart rate and thirst sensation, but does not reduce rectal temperature during 2 h of moderate to intense cycling in a moderate environment when fluid consumption is 33% of sweat losses.     

Effects of fasting on thermoregulatory processes and the daily oscillations in rats

To investigate the mechanism involved in the reduction of body core temperature (T(core)) during fasting in rats, which is selective in the light phase, we measured T(core), surface temperature, and oxygen consumption rate in fed control animals and in fasted animals on day 3 of fasting and day 4 of recovery at an ambient temperature (T(a)) of 23 degrees C by biotelemetry, infrared thermography, and indirect calorimetry, respectively. On the fasting day, 1) T(core) in the light phase decreased (P < 0.05) from the control; however, T(core) in the dark phase was unchanged, 2) tail temperature fell from the control (P < 0.05, from 30.7 +/- 0.1 to 23.9 +/- 0.1 degrees C in the dark phase and from 29.4 +/- 0.1 to 25.2 +/- 0.2 degrees C in the light phase), 3) oxygen consumption rate decreased from the control (P < 0.05, from 24.37 +/- 1.06 to 16.24 +/- 0.69 ml. min(-1). kg body wt(-0.75) in the dark phase and from 18.91 +/- 0.64 to 14.00 +/- 0.41 ml. min(-1). kg body wt(-0.75) in the light phase). All these values returned to the control levels on the recovery day. The results suggest that, in the fasting condition, T(core) in the dark phase was maintained by suppression of the heat loss mechanism, despite the reduction of metabolic heat production. In contrast, the response was weakened in the light phase, decreasing T(core) greatly. Moreover, the change in the regulation of tail blood flow was a likely mechanism to suppress heat loss.     

The entrainment of breathing rhythm to stride frequency in the dik-dik antelope (Rhynchotragus kirki) with observations on the thermoregulatory consequences

1. Three adult dik-dik antelopes with an average weight of 4.5 kg were used to investigate the rate of oxygen consumption (VO2) during exercise and the entrainment of respiratory rate with stride frequencies at running velocities between 2-11 km/hr. 2. The results of VO2 and the metabolic cost of horizontal locomotion were found to agree with what would be expected of an animal of this size. 3. Respiratory frequencies recorded during the most strenuous exercise were approximately 50% below the value observed when dik-diks are exposed to an ambient temperature (Ta) of 42 degrees C. 4. Respiratory evaporative heat loss was estimated to account for only 4% of the total heat production during exercise. 5. Respiratory frequencies were found to be entrained quite strongly to stride frequencies. The thermoregulatory consequences of this entrainment is discussed.