Changes in ambient temperature at the onset of thermoregulatory responses in exercise-trained rats

Spontaneous running in a wheel has emerged as a useful method of exercise in rodents. We investigated how exercise training with a running wheel affects ambient temperatures (T _a) at the onset of thermoregulatory responses in rats. Female rats were allowed to run freely in the wheel for 6 months. Sedentary control rats did not exercise during the same period. After the exercise training period, they were loosely restrained and T _a values at the onset of tail skin vasodilation and cold- induced thermogenesis were determined by raising or lowering T _a. Resting levels of core temperature and heat production of the exercise-trained rats were significantly higher than those of the controls. T _a values at the onset of tail skin vasodilation and cold-induced thermogenesis of the exercise-trained rats were higher than those of the controls. The results suggest that, in rats, exercise training with a running wheel elevates ambient temperatures for heat loss and heat production, which may then contribute to maintaining the core temperature at a high level. Received: 20 August 1999 / Revised: 22 November 1999 / Accepted: 24 November 1999     

Expired air temperature during prolonged exercise in cool- and hot-humid environments

The purpose of this investigation was to measure expired air temperature under cool- and hot-humid environmental conditions at rest and during prolonged exercise to: (1) establish if significant increases in body core temperature affected expired air temperature, and (2) to determine if the temperature setting for heating the pneumotachometer in an open-circuit system requires adjustment during prolonged exercise tests to account for changes in expired air temperature. Six male distance runners completed two tests in cool-humid [dry bulb temperature (T _db) 15.5 (SD 1.3)°C, wet bulb temperature (T _wb) 12.1 (SD 1.4)°C] and hot-humid [T _db 31.6 (SD 0.6)°C, T _wb 24.9 (SD 0.6)°C, black globe temperature (T _g) 34.3 (SD 0.3)°C] environments, running at a velocity corresponding to 65% [67.1 (SD 2.82)%] of their maximal oxygen uptake. Rectal temperature and expired air temperatures were compared at rest, and after 30 min and 60 min of exercise for each environment. The main finding of this investigation was a significant (P < 0.05) but small increase in expired air temperature between the 30-min and 60-min measures in the hot-humid environment. No significant differences in expired air temperature were found between the 30-min and 60-min measures in the cool-humid environment. These findings suggest that: (1) expired air temperature is influenced by elevations in body core temperature during prolonged exercise in hot-humid conditions, and (2) that the temperature setting for heating the head of the pneumotachometer (after determining the appropriate temperature through measuring expired air temperature for the set environmental condition) may require adjustment during prolonged exercise trials in hot-humid environmental conditions. Accepted: 27 February 1997     

Simultaneous telemetric monitoring of the circadian changes in core and BAT temperature in rats: Endogenous vasopressin may contribute to reduced BAT themogenesis and body temperature in the light phase of the circadian cycle

The purpose of the present study was to analyze simultaneously the temporal relationship between the changes of circadian rhythms of brown adipose tissue (BAT) thermogenesis and core temperature (T_c) by dual probe telemetric monitoring transmitters and to determine the role of endogenous arginine vasopressin (AVP) in the circadian rhythms of BAT temperature (T_BAT) and T_c in male rats. The key observations in this study are: (1) Increase in T_BAT commenced approximately 8 min before T_c increases at the start of transition from the light to dark phase. Whereas at the start of transition from the dark to light phase, decrease in T_BAT commenced approximately 3 min before T_c decreases. The data show that circadian changes of BAT thermogenesis do indeed play a significant role in the overall maintenance of the circadian rhythm of core temperature. (2) The plasma AVP level was significantly elevated when core temperature decreases during the light phase, suggesting that endogenous AVP is involved in thermoregulatory processes during the light phase. V1a receptor antagonist could elevate core and BAT temperature during the light period, suggesting that endogenous AVP, acting through V1a receptor, could be involved in tonic thermoregulatory processes.V1a receptor antagonist can increase the blood lipid metabolism, suggesting that the mechanism of endogenous AVP in tonic thermoregulatory processes during light period could involve the suppression of lipolysis in BAT and other peripheral tissues. In summary, this study demonstrated that endogenous vasopressin contributes to reduced BAT themogenesis and body temperature in the light phase of the circadian cycle.     

Determinants of bovine thermal response to heat and solar radiation exposures in a field environment

Continuous exposure of cattle to summer heat in the absence of shade results in significant hyperthermia and impairs growth and general health. Reliable predictors of heat strain are needed to identify this condition. A 12-day study was conducted during a moderate summer heat period using 12 Angus x Simmental (Bos taurus) steers (533 ± 12 kg average body weight) to identify animal and ambient determinations of core body temperature (T _core) and respiration rate (RR) responses to heat stress. Steers were provided standard diet and water ad libitum, and implanted intraperitoneally with telemetric transmitters to monitor T _core hourly. Visual count of flank movement at 0800 and 1500 hours was used for RR. Dataloggers recorded air temperature (T _a), and black globe temperatures (T _bg) hourly to assess radiant heat load. Analysis was across four periods and 2 consecutive days averaged within each period. Average T _a and T _bg increased progressively from 21.7 to 30.3°C and 25.3 to 34.0°C, respectively, from the first to fourth periods. A model utilizing a quadratic function of T _a explained the most variation in T _core (R ² = 0.56). A delay in response from 1 to 3 h did not significantly improve R ² for this relationship. Measurements at 0800 and 1500 hours alone are sufficient to predict heat strain. Daily minimum core body temperature and initial 2-h rise in T _a were predictors of maximum core temperature and RR. Further studies using continuous monitoring are needed to expand prediction of heat stress impact under different conditions.     

Ice slurry ingestion before and during exercise inhibit the increase in core and deep-forehead temperatures in the second half of the exercise in a hot environment

Many studies have reported that pre-exercise ice slurry ingestion improves exercise performance; however, it may increase the risk of developing heat stroke. Some studies have suggested that pre-exercise ice slurry ingestion accelerates the core temperature increase that occurs during exercise. Therefore, this study aimed to investigate whether the ingestion of ice slurry before and during exercise can inhibit this acceleration. Moreover, we measured the deep-forehead temperature (T_{deep head}) to determine whether ice slurry ingestion before and during exercise can maintain this reduction in brain temperature. Eleven male participants at room temperature (24 °C, 50% relative humidity [RH]) ingested 7.5 g/kg of ice slurry or a thermoneutral sports drink within 30 min. They then exercised for approximately 60 min at 50% of the maximal oxygen uptake in a hot environment (34 °C, 50% RH) while ingesting 1.25 g/kg of ice slurry or a thermoneutral sports drink every 10 min. Rectal temperature (T_re), T_{deep head}, forehead skin temperature, mean skin temperature, heart rate, nude body mass, and urine specific gravity were measured as physiological indices. The rating of perceived exertion, thermal sensation, and thermal comfort were measured at 5-min intervals throughout the experiment. The T_re and T_{deep head} during the second half of the exercise session were significantly reduced after ingestion of the ice slurry before and during exercise (p < 0.05). In addition, the rate of increase in T_re and T_{deep head} slowed during the second half of the exercise session after the ingestion of the ice slurry before and during exercise (p < 0.05). These results indicate that the increases in T_re and T_{deep head}, reflecting brain temperature in the second half of the exercise session, were significantly inhibited by ice slurry ingestion before and during exercise.     

Comparative thermoregulatory adaptations of field mice of the genus Apodemus to habitat challenges

Thermoregulatory abilities, which may play a role in physiological adaptations, were compared between two field mouse species (Apodemus mystacinus and A. hermonensis) from Mount Hermon. While A. hermonensis is common at altitudes above 2100 m, A. mystacinus is common at 1650 m. The following variables were compared in mice acclimated to an ambient temperature of 24°C with a photoperiod of 12L:12D, body temperature during exposure to 4°C for 6 h, O₂ consumption and body temperature at various ambient temperature, non-shivering thermogenesis measured as a response to a noradrenaline injection, and the daily rhythm of body temperature. Both species could regulate their body temperature at ambient temperatures between 6 and 34°C. The thermoneutral zone for A. mystacinus lies between 28 and 32°C, while for A. hermonensis a thermoneutral point is noted at 28°C. Both species increased O₂ consumption and body temperature as a response to noradrenalin. However, maximal VO ₂ consumption as an response to noradrenaline and non-shivering thermogenesis capacity were higher in A. mystacinus, even though A. hermonensis is half the size of A. mystacinus. The body temperature rhythm in A. hermonensis has a clear daily pattern, while A. mystacinus can be considered arhythmic. The results suggest that A. hermonensis is adapted to its environment by an increase in resting metabolic rate but also depends on behavioural thermoregulation. A. mystacinus depends more on an increased non-shivering thermogenesis capacity.Abbreviations C thermal conductance - NA noradrenaline - NST non-shivering thermogenesis - OTC overall thermal conductance - RMR resting metabolic rate - STPD standard temperature and pressure dry - T _a ambient temperature - T _b body temperature - I _b Min minimal T _b , measured before NA iniection - T _b NA maximal - T _b as a response to NA injection - T _lc lower critical point - TNP thermoneutral point - TNZ thermoneutral zone - VO₂ O₂ consumption - VO₂ Min minimal VO₂ measured before NA injection - VO₂NA maximal VO₂, as a response to NA injection     

Comparison of heat dissipation response between Malaysian and Japanese males during exercise in humid heat stress

This study investigated the differences in heat dissipation response to intense heat stress during exercise in hot and humid environments between tropical and temperate indigenes with matched physical characteristics. Ten Japanese (JP) and ten Malaysian (MY) males participated in this study. Subjects performed exercise for 60 min at 55% peak oxygen uptake in 32°C air with 70% relative humidity, followed by 30 min recovery. The increase in rectal temperature (T _re) was smaller in MY during exercise compared to JP. The local sweat rate and total body mass loss were similar in both groups. Both skin blood flow and mean skin temperature was lower in MY compared to JP. A significantly greater increase in hand skin temperature was observed in MY during exercise, which is attributable to heat loss due to the greater surface area to mass ratio and large number of arteriovenous anastomoses. Also, the smaller increase in T _re in MY may be explained by the presence of a significantly greater core–skin temperature gradient in MY than JP. The thermal gradient is also a major factor in increasing the convective heat transfer from core to skin as well as skin blood flow. It is concluded that the greater core–skin temperature gradient observed in MY is responsible for the smaller increase in T _re.     

Diet-induced obesity reduces core body temperature across the estrous cycle and pregnancy in the rat

Obesity during pregnancy causes adverse maternal and fetal health outcomes and programs offspring for adult-onset diseases, including cardiovascular disease. Obesity also disrupts core body temperature (T_c) regulation in nonpregnant rodents; however, it is unknown whether obesity alters normal maternal T_c adaptations to pregnancy. Since T_c is influenced by the circadian system, and both obesity and pregnancy alter circadian biology, it was hypothesized that obesity disrupts the normal rhythmic patterns of T_c before and during gestation. Obesity was induced by cafeteria (CAF) feeding in female Wistar rats for 8 weeks prior to and during gestation, whereas control (CON) animals had free access to chow. Intraperitoneal temperature loggers measured daily T_c profiles throughout the study, while maternal body composition and leptin levels were assessed near term. Daily temperature profiles were examined for rhythmic features (mesor, amplitude and acrophase) by cosine regression analysis. CAF animals exhibited increased fat mass (93%) and associated hyperleptinemia (3.2-fold increase) compared to CON animals. CAF consumption reduced the average T_c (by up to 0.29°C) across the estrous cycle and most of pregnancy; however, T_c for CAF and CON animals converged toward the end of gestation. Obesity reduced the amplitude of T_c rhythms at estrus and proestrus and on day 8 of pregnancy, but increased the amplitude at day 20 of pregnancy. Photoperiod analysis revealed that obesity reduced T_c exclusively in the light period during pre-pregnancy but only during the dark period in late gestation. In conclusion, obesity alters rhythmic T_c profiles and reduces the magnitude of the T_c decline late in rat gestation, which may have implications for maternal health and fetal development.     

Rectal and esophageal temperatures during upper- and lower-body exercise

This study investigated the question: is core temperature measurement influenced by whether exercise involves predominantly upper- or lower-body musculature? Healthy men were allocated to three groups: treadmill ergometry (T) n=4, cycle ergometry (C) n=6 and arm crank ergometry (AC) n=5. Subjects underwent an incremental exercise test to exhaustion on an exercise-specific ergometer to determine maximum/peak oxygen consumption (V˙O_2max). One week later subjects exercised for 36 min on the same ergometer at approximately 65% V˙O_2max while temperatures at the rectum (T _re) and esophagus (T _es) were simultaneously measured. The V˙O_2max (l · min⁻¹) for groups T [4.76 (0.50)] and C [4.35 (0.30)] was significantly higher than that for the AC group [2.61 (0.24)]. At rest, T _re was significantly higher than T _es in all groups (P<0.05). At the end of submaximal exercise in the C group, T _re [38.32 (0.11)°C] was significantly higher than T _es [38.02 (0.12)°C, P<0.05]. No significant differences between T _re and T _es at the end of exercise were noted for AC and T groups. The temperature difference (T _diff) between T _re and T _es was dissimilar at rest in the three groups; however, by the end of exercise T _diff was approximately 0.2°C for each of the groups, suggesting that at the end of steady-state exercise T _re can validly be used to estimate core temperature. Accepted: 3 November 1997     

Hypothalamic Temperature of Rats Subjected to Treadmill Running in a Cold Environment

Different strategies for cooling the body prior to or during physical exercise have been shown to improve prolonged performance. Because of ethical and methodological issues, no studies conducted in humans have evaluated the changes in brain temperature promoted by cooling strategies. Therefore, our first aim sought to measure the hypothalamic temperature (T_hyp) of rats subjected to treadmill running in a cold environment. Moreover, evidence suggests that T_hyp and abdominal temperature (T_abd) are regulated by different physiological mechanisms. Thus, this study also investigated the dynamics of exercise-induced changes in T_hyp and T_abd at two ambient temperatures: 25°C (temperate environment) and 12°C (cold). Adult male Wistar rats were used in these experiments. The rats were implanted with a guide cannula in the hypothalamus and a temperature sensor in the abdominal cavity. After recovery from this surgery, the rats were familiarized with running on a treadmill and were then subjected to the two experimental trials: constant-speed running (20 m/min) at 12°C and 25°C. Both T_hyp and T_abd increased during exercise at 25°C. In contrast, T_hyp and T_abd remained unchanged during fatiguing exercise at 12°C. The temperature differential (i.e., T_hyp - T_abd) increased during the initial min of running at 25°C and thereafter decreased toward pre-exercise values. Interestingly, external cooling prevented this early increase in the temperature differential from the 2^nd to the 8^th min of running. In addition, the time until volitional fatigue was higher during the constant exercise at 12°C compared with 25°C. Together, our results indicate that T_hyp and T_abd are regulated by different mechanisms in running rats and that external cooling affected the relationship between both temperature indexes observed during exercise without environmental thermal stress. Our data also suggest that attenuated hypothalamic hyperthermia may contribute to improved performance in cold environments.     

Seasonal changes in daily metabolic patterns of Lacerta viridis

Summary Lacerta viridis maintained under natural photoperiodic conditions show daily and seasonal changes in metabolic rates and body temperature (T _b) as well as seasonal differences in sensitivity to temperature change. At all times of the year lizards have a daily fluctuation in oxygen consumption, with higher metabolic rates during the light phase of the day when tested at a constant ambient temperature (T _a) of 30°C. Rhythmicity of metabolic rate persists under constant darkness, but there is a decrease in the amplitude of the rhythm.Oxygen consumption measured at various T_as shows significant seasonal differences at T _as above 20°C. Expressed as the Arrhenius activation energy, metabolic sensitivity of Lacerta viridis shows temperature dependence in autumn, which changes to metabolic temperature independence in spring at T _as above 20°C. The results indicate a synergic relationship between changing photoperiod and body temperature selection, resulting in seasonal metabolic adjustment and seasonal adaptation.Abbreviations ANOVA analysis of variance - LD long day (16 h light) - SD short day (8 h light) - T _a ambient temperature - T _b body temperature     

ASSOCIATION OF THE ANTIDIABETIC EFFECTS OF BROMOCRIPTINE WITH A SHIFT IN THE DAILY RHYTHM OF MONOAMINE METABOLISM WITHIN THE SUPRACHIASMATIC NUCLEI OF THE SYRIAN HAMSTER

Bromocriptine, a dopamine D₂ agonist, inhibits seasonal fattening and improves seasonal insulin resistance in Syrian hamsters. Alterations in daily rhythms of neuroendocrine activities are involved in the regulation of seasonal metabolic changes. Changes in circadian neuroendocrine activities that regulate metabolism are believed to be modulated by central circadian oscillators within the hypothalamic suprachiasmatic nuclei (SCN) of seasonal animals. We examined the association of metabolic responses to bromocriptine with its effects on the daily rhythms of metabolic hormones and daily monoamine profiles within the SCN, a primary circadian pacemaker known to regulate metabolism, in Syrian hamsters. Obese glucose-intolerant male Syrian hamsters (body weight [BW] 185 ± 10 g) held on 14h daily photoperiods were treated at light onset with bromocriptine (800 μg/animal/day, ip) or vehicle for 2 weeks. Animals were then subjected to a glucose tolerance test (GTT) (3 g/kg BW, ip). Different subsets of animals (n = 6) from each treatment group were sacrificed at 0h/24h, 5h, 10h, 15h, or 20h after light onset for analyses of SCN monoamines, plasma insulin, prolactin, cortisol, thyroxin (T₄), triiodothyronine (T₃), glucose, and free fatty acids (FFAs). Compared with control values, bromocriptine treatment significantly reduced weight gain (14.9 vs. ?2.9 g, p <. 01) and the areas under the GTT glucose and insulin curves by 29% and 48%, respectively (p <. 05). Basal plasma insulin concentration was markedly reduced throughout the day in bromocriptine-treated animals without influencing plasma glucose levels. Bromocriptine reduced the daily peak in FFA by 26% during the late light span(p <. 05). Bromocriptine significantly shifted the daily plasma cortisol peak from the early dark to the light period of the day, reduced the plasma prolactin (mean 1.8 vs. 39.4 ng/dL) and T₄ throughout the day (mean 1.6 vs. 3.8 μg/dL), and selectively reduced T₃ during the dark period of the day (p <. 01). Concurrently, bromocriptine treatment significantly reduced SCN dopamine turnover during the light period and shifted daily peaks of SCN serotonin and 5-hydroxy-indoleacetic acid (5-HIAA) content by 12h from the light to the dark period of the day (p <. 05). This was confirmed by a further in vivo microdialysis study in which bromocriptine increased SCN extracellular 5-HIAA of glucose-intolerant hamsters during the dark phase (47% increase, p <. 05) toward levels observed in normal glucose-tolerant hamsters. Thus, bromocriptine-induced resetting of daily patterns of SCN neurotransmitter metabolism is associated with the effects of bromocriptine on attenuation of the obese insulin-resistant and glucose-intolerant condition. A large body of corroborating evidence suggests that such bromocriptine-induced changes in SCN monoamine metabolism may be functional in its effects on metabolism. (Chronobiology International, 17(2), 155–172, 2000)     

Daily rhythmicity and hibernation in the Anatolian ground squirrel under natural and laboratory conditions

We studied daily rhythmicity of body temperature (T _b) before and during hibernation in Anatolian ground squirrels (Spermophilus xanthoprymnus) under natural and laboratory conditions using surgically implanted temperature loggers. Under both conditions, robust daily T _b rhythmicity with parameters comparable to those of other ground squirrel species was observed before but not during hibernation. Euthermic animals had robust daily T _b rhythms with a mean of 37.0°C and a range of excursion of approximately 4°C. No T _b rhythm was detected during torpor bouts, either because T _b rhythmicity was absent or because the daily range of excursion was smaller than 0.2°C. The general patterns of hibernation that we observed in Anatolian ground squirrels were similar to those previously observed by other investigators in other species of ground squirrels.     

How photoperiod influences body temperature selection in Lacerta viridis

Summary European green lizards, Lacerta viridis, show a distinct annual cycle in their day and nighttime selected body temperature (T _b) levels when monitored under natural photoperiod. The amplitude between daily photophase and scotophase temperatures varies throughout the year. Highest body temperatures with smallest day/night variation are selected from May through July. Throughout fall, the difference between day and nighttime selected T _b levels increases. Lizards inevitably enter a state of winter dormancy which terminates daily rhythmicity patterns. Under natural photoperiodic conditions, cessation of dormancy occurs spontaneously by mid-March, regardless whether high temperatures are available or not. Lacerta viridis respond to an artificial long photoperiod (16 h light, 8 h dark) at all times of the year with modifications in both diel patterns and levels of selected T _b to summer-like conditions. When, however, the natural photoperiod at different phases in the annual cycle is held constant for six to eight weeks, T _b selection of Lacerta viridis also remains stable at the level corresponding to the prevailing photoperiod. These results implicate that the photoperiod is a more prominent Zeitgeber for seasonal cueing of temperature selection than has been surmised in the past. Further, we suggest that the large variations recorded in daily T _b cycles do not imply that this lizard is an imprecise thermoregulator, but rather indicates an important integral process necessary for seasonal acclimatization.     

Daily Rhythms of Metabolic Rate and Body Temperature of Two Murids from Extremely Different Habitats

Daily circadian rhythms of body temperature (T_b) and oxygen consumption (VO₂) were measured in two murid species, which occupy extremely different habitats in Israel. The golden spiny mouse (Acomys mssalus) is a diurnal murid distributed in arid and hot parts of the great Syrio-African Rift Valley, while the broad-toothed field mouse (Apodeinns mystacinus) is a nocturnal species that inhabits the Mediterranean woodlands. In both species, the daily rhythms of T_b and VO₂ are entrained by the photoperiod. Under laboratory experimental conditions (ambient temperature T_a = 33^oC and photoperiod regime of 12L: 12D), Acomys russatus exhibits a tendency towards a nocturnal activity pattern, compared to the diurnal activity displayed by this species under natural conditions. Under the same photoperiod regime and at T_a = 28^oC, Apodemus mystacinus displays nocturnal activity, as observed under natural conditions. The maximal values of T_b were recorded in Acomys russatus at midnight (23:50 h), while the maximal values of VO₂ were recorded at the beginning of the dark period (18:20 h). In Apodemus mystacinus, the maximal values of T_b and VO₂ were recorded at 23:40 and 20:00 h, respectively. The ecophysiological significance of these results is discussed further.     

Influence of circadian disruption on neurotransmitter levels,physiological indexes,and behaviour in rats

Brain monoamines – such as noradrenaline (NA), dopamine (DA) and serotonin (5-HT) – regulate several important physiological functions, including the circadian rhythm. The purpose of this study was to examine changes in NA, DA and 5-HT levels in various brain regions and their effect on core body temperature (T_c), heart rate (HR) and locomotor activity (Act) in rats following exposure to an artificial light/dark (LD) cycle. For this, male Wistar rats were housed at an ambient temperature (T_a) of 23?°C and 50% relative humidity with free access to food and water. Rats were exposed to either natural (12?h:12?h) or artificial (6?h:6?h) LD cycles for 1 month, after which each brain region was immediately extracted and homogenized to quantify the amounts of NA, DA and 5-HT by high-performance liquid chromatography. Behavioural changes were also monitored by the ambulatory activity test (AAT). Notably, we found that artificial LD cycles disrupted the physiological circadian rhythms of T_c, HR and Act. Although the 5-HT levels of rats with a disrupted circadian rhythm decreased in cell bodies (dorsal and median raphe nuclei) and projection areas (frontal cortex, caudate putamen, preoptic area and suprachiasmatic nucleus) relative to the control group, NA levels increased both in the cell body (locus coeruleus) and projection area (paraventricular hypothalamus). No significant changes were found with respect to DA. Moreover, circadian rhythm-disrupted rats also showed anxious behaviours in AAT. Collectively, the results of this study suggest that the serotonergic and noradrenergic systems, but not the dopaminergic system, are affected by artificial LD cycles in brain regions that control several neural and physiological functions, including the regulation of physiological circadian rhythms, stress responses and behaviour.     

Integumental buffering in an alpine grasshopper

In most freeze tolerant insects, the tolerance of the formation of internal body ice is arrived at by a two‐step process: (S‐1) a period of supercooling of the body fluids that is followed by (S‐2) the freezing event. To date, the necessity of S‐1 remains to be questioned seriously. The present study reports evidence that S‐1 may be almost completely substituted or superseded in large‐bodied insects by integumental buffering. In the New Zealand alpine grasshopper Sigaus australis Hutton, there is a substantial difference between external and body core temperatures at the moment when internal ice nucleation is registered. Using the invagination of the pleural suture as a nondetrimental proxy for the core and the sclerotized postnotum as a measure of surface temperature, comparisons of the temperature of crystallization (T_c) show a highly significant difference (P < 0.001; Kolmogorov–Smirnov test). Proxy core T_c values are in the range from ?0.11 to ?4.78 °C compared with the range of ?4.1 to ?14.2 °C in external proxy T_c values. Although a thermal lag may sometimes be quietly assumed in measurements of T_c, a temperature differential of this size (approximately 6 °C), which is equivalent to the entire supercooling potential of many freeze tolerant insects, is of particular note. These findings have wider application to other large‐bodied insects with similarly well‐developed integumental protection.     

Temperature regulation and microhabitat choice by free-ranging Galapagos fur seal pups (Arctocephalus galapagoensis)

Summary Behavioural activity, and core and surface temperatures of 4 unrestrained Galapagos fur seals were recorded in the natural habitat during their first weeks of life. Climatic variables were registered simultaneously. Pup behaviours were divided into bouts of resting (55% of total time), sucking (23%) and other activities (22%). Pups maintained a constant body temperature from their first day. Core temperature (T _e ) was 37.7° C±0.3° C (x ± SD) over 39 pup-days and 8 pup-nights. Skin temperature was correlated with T _c , but flipper temperature was not. No daily T _c rhythm was detected. Microclimate data were used to calculate operative temperature T _e . Environmental temperatures can be very high, with T _e above T _c 6–9 h a day for animals exposed to the sun, but below it in the shade. T _c is about 22° C at night. Pups avoid overheating mainly by withdrawing into the shade and reducing activity to a minimum during the hot hours of the day. Sun-exposed pups could be active at any time during the day if they had access to water, which was usually around high tide.     

Effects of metabolic rate on thermal responses at different air velocities in −10°C

The effects of exercise intensity on thermoregulatory responses in cold (−10°C) in a 0.2 (still air, NoWi), 1.0 (Wi1), and 5.0 (Wi5) m s⁻¹ wind were studied. Eight young and healthy men, preconditioned in thermoneutral (+20°C) environment for 60 min, walked for 60 min on the treadmill at 2.8 km/h with different combinations of wind and exercise intensity. Exercise level was adjusted by changing the inclination of the treadmill between 0° (lower exercise intensity, metabolic rate 124 W m⁻², LE) and 6° (higher exercise intensity, metabolic rate 195 W m⁻², HE). Due to exercise increased heat production and circulatory adjustments, the rectal temperature (T_re), mean skin temperature (T_sk) and mean body temperature (T_b) were significantly higher at the end of HE in comparison to LE in NoWi and Wi1, and T_re and T_b also in Wi5. T_sk and T_b were significantly decreased by 5.0 m s⁻¹ wind in comparison to NoWi and Wi1. The higher exercise intensity was intense enough to diminish peripheral vasoconstriction and consequently the finger skin temperature was significantly higher at the end of HE in comparison to LE in NoWi and Wi1. Mean heat flux from the skin was unaffected by the exercise intensity. At LE oxygen consumption (V ₂) was significantly higher in Wi5 than NoWi and Wi1. Heart rate was unaffected by the wind speed. The results suggest that, with studied exercise intensities, produced without changes in walking speed, the metabolic rate is not so important that it should be taken into consideration in the calculation of wind chill index.     

Relevance of individual characteristics for human heat stress response is dependent on exercise intensity and climate type

Multiple heterogeneous groups of subjects (both sexes and a wide range of maximal oxygen uptake V˙O_{2 max} , body mass, body surface area (A _D),% body fat, and A _D/mass coefficient) exercised on a cycle ergometer at a relative (%V˙O_2max, REL) or an absolute (60 W) exercise intensity in a cool (CO 21°C, 50% relative humidity), warm humid (WH 35°C, 80%) and a hot dry (HD 45°C, 20%) environment. Rectal temperature (T _re) responses were analysed for the influence of the individual's characteristics, environment and exercise intensity. Exposures consisted of 30-min rest, followed by 60-min exercise. The T _re was negatively correlated with mass in all conditions. Body mass acted as a passive heat sink in all the conditions tested. While negatively correlated with V˙O_{2 max} and V˙O_{2 max} per kilogram body mass in most climates, T _re was positively correlated with V˙O_{2 max} and V˙O_{2 max} per kilogram body mass in the WH/REL condition. Thus, when evaporative heat loss was limited as in WH, the higher heat production of the fitter subjects in the REL trials determined T _re and not the greater efficiency for heat loss associated with high V˙O_{2 max} . Body fatness significantly affected T _re only in the CO condition, where, with low skin blood flows (measured as increases in forearm blood flow), the insulative effect of fat was pronounced. In the warmer environments, high skin blood flows offset the resistance offered by peripheral adipose tissue. Contrary to other studies, T _re was positively correlated with A _D/mass coefficient for all conditions tested. For both exercise types used, being big (a high heat loss area and heat capacity) was apparently more beneficial from a heat strain standpoint than having a favourable A _D/mass coefficient (high in small subjects). The total amount of variance in T _re responses which could be attributed to individual characteristics was dependent on the climate and the type of exercise. Though substantial for absolute exercise intensities (52%–58%) the variance explained in T _re differed markedly for relative intensities: 72% for the WH climate with its limited evaporative capacity, and only 10%–26% for the HD and CO climates. The results showed that individual characteristics play a significant role in determining the responses of body core temperature in all conditions tested, but their contribution was low for relative exercise intensities when evaporative heat loss was not restricted. This study demonstrated that effects of individual characteristics on human responses to heat stress cannot be interpreted without taking into consideration both the heat transfer properties of the environment and the metabolic heat production resulting from the exercise type and intensity chosen. Their impact varies substantially among conditions. Accepted: 4 July 1997