Predicting military working dog core temperature during exertional heat strain: Validation of a Canine Thermal Model

Military working dogs (MWDs) operate under a wide range of conditions, including hot environments. Predicting how long a MWD can safely work without overheating is important for both health and performance. A Canine Thermal Model (CTM) was developed to predict core temperature (Tc) of MWDs. The CTM calculates heat storage from the balance of heat production from metabolism and heat exchange with the environment. Inputs to the CTM are: meteorological conditions (ambient temperature, relative humidity, solar radiation and wind speed), physical characteristics of the dog (mass, length), and metabolic activity (MET level, estimated from accelerometer data). The CTM was validated against Tc measured in 23 MWDs during training sessions (11.6 ± 5.0 min (mean ± standard deviation), range 4–26 min) in October (24 °C, 52% RH), March (14 °C, 74% RH), or August (28 °C, 64% RH), and 24 kennel MWDs during a standard exercise walk (11.4 ± 3.3 min, range 5.6–18 min) in July (26 °C, 77% RH). The CTM was considered acceptable if predicted Tc was within ±0.5 °C of measured Tc at the end of exercise. Compared to Tc at the end of training sessions (39.8 ± 0.6 °C, range 38.4–41.1 °C) and exercise walks (40.0 ± 0.7 °C, range 38.9–41.4 °C), the CTM-predicted Tc was within ±0.5 °C for 71 of 84 cases (85%) and 19 of 24 cases (79%), respectively. The mean difference between CTM-predicted and measured final Tc during training was -0.04 ± 0.43 °C, with 80 of 84 cases (95%) within the range of ±2 SD (Bland Altman comparison). During exercise walks the mean difference was -0.15 °C ± 0.57, with 23 of 24 cases (96%) within ±2 SD. These results support the use of the CTM to predict Tc of MWDs for the types of physical activities described above.     

Thermoregulatory model for immersion of humans in cold water

The mathematical models of thermoregulation of Stolwijk and Hardy, and Montgomery were used to develop a model suitable for the simulation of human physiological responses to cold-water immersion. Data were obtained from experiments where 13 healthy male volunteers were totally immersed under resting and nude conditions for 1 h in water temperatures of 20 and 28 degrees C. At these temperatures, the mean measured rectal temperature (Tre) fell by approximately 0.9 and 0.5 degrees C, respectively, yet mean measured metabolic rate (M) rose by approximately 275 and 90 W for the low body fat group (n = 7) and 195 and 45 W for the moderate body fat group (n = 6). To predict the observed Tre and M values, the present model 1) included thermal inputs for shivering from the skin independent of their inclusion with the central temperature to account for the observed initial rapid rise in M, 2) determined a thermally neutral body temperature profile such that the measured and predicted initial values of Tre and M were matched, 3) confined the initial shivering to the trunk region to avoid an overly large predicted initial rate of rectal cooling, and 4) calculated the steady-state convective heat loss by assuming a zero heat storage in the skin compartment to circumvent the acute sensitivity to the small skin-water temperature difference when using conventional methods. The last three modifications are unique to thermoregulatory modeling.     

Evaluation of two cold thermoregulatory models for prediction of core temperature during exercise in cold water.

Cold thermoregulatory models (CTM) have primarily been developed to predict core temperature (T(core)) responses during sedentary immersion. Few studies have examined their efficacy to predict T(core) during exercise cold exposure. The purpose of this study was to compare observed T(core) responses during exercise in cold water with the predicted T(core) from a three-cylinder (3-CTM) and a six-cylinder (6-CTM) model, adjusted to include heat production from exercise. A matrix of two metabolic rates (0.44 and 0.88 m/s walking), two water temperatures (10 and 15 degrees C), and two immersion depths (chest and waist) were used to elicit different rates of T(core) changes. Root mean square deviation (RMSD) and nonparametric Bland-Altman tests were used to test for acceptable model predictions. Using the RMSD criterion, the 3-CTM did not fit the observed data in any trial, whereas the 6-CTM fit the data (RMSD less than standard deviation) in four of eight trials. In general, the 3-CTM predicted a rapid decline in core temperature followed by a plateau. For the 6-CTM, the predicted T(core) appeared relatively tight during the early part of immersion, but was much lower during the latter portions of immersion, accounting for the nonagreement between RMSD and SD values. The 6-CTM was rerun with no adjustment for exercise metabolism, and core temperature and heat loss predictions were tighter. In summary, this study demonstrated that both thermoregulatory models designed for sedentary cold exposure, currently, cannot be extended for use during partial immersion exercise in cold water. Algorithms need to be developed to better predict heat loss during exercise in cold water.     

Human heat balance during postexercise recovery: separating metabolic and nonthermal effects

Previous studies report greater postexercise heat loss responses during active recovery relative to inactive recovery despite similar core temperatures between conditions. Differences have been ascribed to nonthermal factors influencing heat loss response control since elevations in metabolism during active recovery are assumed to be insufficient to change core temperature and modify heat loss responses. However, from a heat balance perspective, different rates of total heat loss with corresponding rates of metabolism are possible at any core temperature. Seven male volunteers cycled at 75% of Vo(2peak) in the Snellen whole body air calorimeter regulated at 25.0 degrees C, 30% relative humidity (RH), for 15 min followed by 30 min of active (AR) or inactive (IR) recovery. Relative to IR, a greater rate of metabolic heat production (M - W) during AR was paralleled by a greater rate of total heat loss (H(L)) and a greater local sweat rate, despite similar esophageal temperatures between conditions. At end-recovery, rate of body heat storage, that is, [(M - W) - H(L)] approached zero similarly in both conditions, with M - W and H(L) elevated during AR by 91 +/- 26 W and 93 +/- 25 W, respectively. Despite a higher M - W during AR, change in body heat content from calorimetry was similar between conditions due to a slower relative decrease in H(L) during AR, suggesting an influence of nonthermal factors. In conclusion, different levels of heat loss are possible at similar core temperatures during recovery modes of different metabolic rates. Evidence for nonthermal influences upon heat loss responses must therefore be sought after accounting for differences in heat production.     

Different thermoregulatory strategies in nearly weaned pup, yearling, and adult Weddell seals (Leptonychotes weddelli)

Mammals balance heat dissipation with heat production to maintain core body temperatures independent of their environment. Thermal balance is undoubtedly most challenging for mammals born in polar regions because small body size theoretically results in high surface-area-to-volume ratios (SA:V), which facilitate heat loss (HL). Thus, we examined the ontogeny of thermoregulatory characteristics of an ice-breeding seal (Weddell seal Leptonychotes weddelli). Morphology, blubber thickness, rectal temperature (T(r)), muscle temperature (T(m)), and skin temperatures on the trunk (T(s)) and flipper (T(f)) in 3-5-wk-old pups, yearlings, and adults were measured. Adults maintained the thickest blubber layers, while yearlings had the thinnest; T(r) and T(m) fell within a narrow range, yet T(r) and T(m) decreased significantly with body length. All seals maintained skin temperatures lower than T(r), our index of core body temperature. The T(s)'s were positively correlated with environmental temperatures; conversely, T(f)'s were not. Although pups had the greatest proportion of blubber, their greater SA:V and limited ability to minimize body-to-environment temperature gradients led to the greatest calculated mass-specific HL. This implies that pups relied on elevated metabolic heat production to counter HL. Heat production in pups and yearlings may have been aided by nonshivering thermogenesis in the skeletal muscle via the enhanced muscle mitochondrial densities that have been observed in these segments of this population.     

Prediction of human core body temperature using non-invasive measurement methods

The measurement of core body temperature is an efficient method for monitoring heat stress amongst workers in hot conditions. However, invasive measurement of core body temperature (e.g. rectal, intestinal, oesophageal temperature) is impractical for such applications. Therefore, the aim of this study was to define relevant non-invasive measures to predict core body temperature under various conditions. We conducted two human subject studies with different experimental protocols, different environmental temperatures (10 °C, 30 °C) and different subjects. In both studies the same non-invasive measurement methods (skin temperature, skin heat flux, heart rate) were applied. A principle component analysis was conducted to extract independent factors, which were then used in a linear regression model. We identified six parameters (three skin temperatures, two skin heat fluxes and heart rate), which were included for the calculation of two factors. The predictive value of these factors for core body temperature was evaluated by a multiple regression analysis. The calculated root mean square deviation (rmsd) was in the range from 0.28 °C to 0.34 °C for all environmental conditions. These errors are similar to previous models using non-invasive measures to predict core body temperature. The results from this study illustrate that multiple physiological parameters (e.g. skin temperature and skin heat fluxes) are needed to predict core body temperature. In addition, the physiological measurements chosen in this study and the algorithm defined in this work are potentially applicable as real-time core body temperature monitoring to assess health risk in broad range of working conditions.     

Thermal insulation and body temperature wearing a thermal swimsuit during water immersion

This study evaluated the effects of a thermal swimsuit on body temperatures, thermoregulatory responses and thermal insulation during 60 min water immersion at rest. Ten healthy male subjects wearing either thermal swimsuits or normal swimsuits were immersed in water (26 degrees C or 29 degrees C). Esophageal temperature, skin temperatures and oxygen consumption were measured during the experiments. Metabolic heat production was calculated from oxygen consumption. Heat loss from skin to the water was calculated from the metabolic heat production and the change in mean body temperature during water immersion. Total insulation and tissue insulation were estimated by dividing the temperature difference between the esophagus and the water or the esophagus and the skin with heat loss from the skin. Esophageal temperature with a thermal swimsuit was higher than that with a normal swimsuit at the end of immersion in both water temperature conditions (p<0.05). Oxygen consumption, metabolic heat production and heat loss from the skin were less with the thermal swimsuit than with a normal swimsuit in both water temperatures (p<0.05). Total insulation with the thermal swimsuit was higher than that with a normal swimsuit due to insulation of the suit at both water temperatures (p<0.05). Tissue insulation was similar in all four conditions, but significantly higher with the thermal swimsuit in both water temperature conditions (p<0.05), perhaps due to of the attenuation of shivering during immersion with a thermal swimsuit. A thermal swimsuit can increase total insulation and reduce heat loss from the skin. Therefore, subjects with thermal swimsuits can maintain higher body temperatures than with a normal swimsuit and reduce shivering thermo-genesis.     

Finite-element solution of thermal conductivity of muscle during cold water immersion

The in vivo or effective thermal conductivity (keff) of muscle tissue of the human forearm was determined through a finite-element (FE) model solution of the bioheat equation. Data were obtained from steady-state temperatures measured in the forearm after 3 h of immersion in water at temperatures (Tw) of 15 (n = 6), 20 (n = 5), and 30 degrees C (n = 5). Temperatures were measured every 0.5 cm from the longitudinal axis of the forearm to the skin approximately 9 cm distal from the elbow. Heat flux was measured at two sites on the skin adjacent to the temperature probe. The FE model is comprised of concentric annular compartments with boundaries defined by the location of temperature measurements. Through this approach, it was possible to include both the metabolic heat production and the convective heat transfer between blood and tissue at two levels of blood flow, one perfusing the compartment and the other passing through the compartment. Without heat exchange at the passing blood flow level, the arterial blood temperature would be assumed to have a constant value everywhere in the forearm muscles, leading to a solution of the bioheat equation that greatly underpredicts keff. The extent of convective heat exchange at the passing blood flow level is estimated to be approximately 60% of the total heat exchange between blood and tissue. Concurrent with this heat exchange is a decrease in the temperature of the arterial blood as it flows radially from the axis to the skin of the forearm, and this decrease is enhanced with a lowered Tw.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential heating of trunk and extremities. Effects on thermoregulation mechanisms

Experiments in which the whole human body was heated or cooled are compared with others in which one extremity (arm or leg) was simultaneously cooled or heated. With a warm load on the rest of the body resulting in general sweating, a cold load on one extremity did not evoke local shivering; with general body cooling, heating one limb did not stop the shivering. Skin temperatures of the other parts of the body were not influenced by warming or cooling one extremity. Evaporative heat loss was influenced by local, mean skin and core temperature, whereas shivering did not depend on local temperature, and vasomotor control seemed to be controlled predominantly by central temperatures. A cold load on an extremity during whole body heating in most cases induced an oscillatory behaviour of core temperature and of the evaporative heat loss from the body and the extremity. It is assumed that local, mean skin and core temperatures influence the three autonomous effector systems to very different degree.     

Prediction of mean skin temperature in warm environments

The data collected by the authors in four experimental series have been analysed together with data from the literature, to study the relationship between mean skin temperature and climatic parameters, subject metabolic rate and clothing insulation. The subjects involved in the various studies were young male subjects, unacclimatized to heat. The range of conditions examined involved mean skin temperatures between 33 degrees C and 38 degrees C, air temperatures (Ta) between 23 degrees C and 50 degrees C, ambient water vapour pressures (Pa) between 1 and 4.8 kPa, air velocities (Va) between 0.2 and 0.9 m.s-1, metabolic rates (M) between 50 and 270 W.m-2, and Clo values between 0.1 and 0.6. In 95% of the data, mean radiant temperature was within +/- 3 degrees C of air temperature. Based on 190 data averaged over individual values, the following equation was derived by a multiple linear regression technique: Tsk = 30.0 + 0.138 Ta + 0.254 Pa-0.57 Va + 1.28.10(-3) M-0.553 Clo. This equation was used to predict mean skin temperature from 629 individual data. The difference between observed and predicted values was within +/- 0.6 degrees C in 70% of the cases and within +/- 1 degrees C in 90% of the cases. It is concluded that the proposed formula may be used to predict mean skin temperature with satisfactory accuracy in nude to lightly clad subjects exposed to warm ambient conditions with no significant radiant heat load.     

Core temperature circdian rhythm during 35 days of horizontal bed rest

The present study evaluated whether the previously reported alterations in core temperature circadian rhythm associated with bed rest might be attributable to increased heat loss from the skin. Infra-red thermograms were obtained at weekly intervals during 5 weeks of bed rest and after 4 weeks of active recovery. Tympanic temperature (Tty) was measured at hourly intervals from 0800 to 2300 hrs on similar occasions during bed rest. There were no significant changes in mean tympanic temperature or amplitude of Tty circadian rhythm during the 5 week bed rest period. Skin temperature decreased progressively during bed rest (P<0.005), with distal regions being the most affected.     

Heat balance precedes stabilization of body temperatures during cold water immersion.

Certain previous studies suggest, as hypothesized herein, that heat balance (i.e., when heat loss is matched by heat production) is attained before stabilization of body temperatures during cold exposure. This phenomenon is explained through a theoretical analysis of heat distribution in the body applied to an experiment involving cold water immersion. Six healthy and fit men (mean +/- SD of age = 37.5 +/- 6.5 yr, height = 1.79 +/- 0.07 m, mass = 81.8 +/- 9.5 kg, body fat = 17.3 +/- 4.2%, maximal O2 uptake = 46.9 +/- 5.5 l/min) were immersed in water ranging from 16.4 to 24.1 degrees C for up to 10 h. Core temperature (Tco) underwent an insignificant transient rise during the first hour of immersion, then declined steadily for several hours, although no subject's Tco reached 35 degrees C. Despite the continued decrease in Tco, shivering had reached a steady state of approximately 2 x resting metabolism. Heat debt peaked at 932 +/- 334 kJ after 2 h of immersion, indicating the attainment of heat balance, but unexpectedly proceeded to decline at approximately 48 kJ/h, indicating a recovery of mean body temperature. These observations were rationalized by introducing a third compartment of the body, comprising fat, connective tissue, muscle, and bone, between the core (viscera and vessels) and skin. Temperature change in this "mid region" can account for the incongruity between the body's heat debt and the changes in only the core and skin temperatures. The mid region temperature decreased by 3.7 +/- 1.1 degrees C at maximal heat debt and increased slowly thereafter. The reversal in heat debt might help explain why shivering drive failed to respond to a continued decrease in Tco, as shivering drive might be modulated by changes in body heat content.     

Diurnal and menstrual cycles in body temperature are regulated differently: A 28-day ambulatory study in healthy women with thermal discomfort of cold extremities and controls

Diurnal cycle variations in body-heat loss and heat production, and their resulting core body temperature (CBT), are relatively well investigated; however, little is known about their variations across the menstrual cycle under ambulatory conditions. The main purpose of this study was to determine whether menstrual cycle variations in distal and proximal skin temperatures exhibit similar patterns to those of diurnal variations, with lower internal heat conductance when CBT is high, i.e. during the luteal phase. Furthermore, we tested these relationships in two groups of women, with and without thermal discomfort of cold extremities (TDCE). In total, 19 healthy eumenorrheic women with regular menstrual cycles (28–32 days), 9 with habitual TDCE (ages 29?±?1.5 year; BMI 20.1?±?0.4) and 10 controls without these symptoms (CON: aged 27?±?0.8 year; BMI 22.7?±?0.6; p?<?0.004 different to TDCE) took part in the study. Twenty-eight days continuous ambulatory skin temperature measurements of distal (mean of hands and feet) and proximal (mean of sternum and infraclavicular regions) skin regions, thighs, and calves were carried out under real-life, ambulatory conditions (i-Buttons® skin probes, sampling rate: 2.5?min). The distal minus proximal skin temperature gradient (DPG) provided a valuable measure for heat redistribution from the core to the shell, and, hence, for internal heat conduction. Additionally, basal body temperature was measured sublingually directly after waking up in bed. Mean diurnal amplitudes in skin temperatures increased from proximal to distal skin regions and the 24-h mean values were inversely related. TDCE compared to CON showed significantly lower hand skin temperatures and DPG during daytime. However, menstrual cycle phase did not modify these diurnal patterns, indicating that menstrual and diurnal cycle variations in skin temperatures reveal additive effects. Most striking was the finding that all measured skin temperatures, together with basal body temperature, revealed a similar menstrual cycle variation (independent of BMI), with highest and lowest values during the luteal and follicular phases, respectively. These findings lead to the conclusion that in contrast to diurnal cycle, variations in CBT variation across the menstrual cycle cannot be explained by changes in internal heat conduction under ambulatory conditions. Although no measurements of metabolic heat production were carried out increased metabolic heat generation during the luteal phase seems to be the most plausible explanation for similar body temperature increases.     

How should body heat storage be determined in humans: by thermometry or calorimetry?

The aim of this study was to determine whether in humans there are differences in the heat storage calculated by partitional calorimetry (S, the balance of heat gains and heat losses) compared to the heat storage obtained by conventional methods (thermometry) via either core temperature or mean body temperatures (Tb = 0.8Tc + 0.2Tsk, where Tc is core temperature and Tsk is mean skin temperature) when two different sites are used as an index of Tc [rectal (T(re)) and auditory canal (T(ac)) temperatures]. Since women respond to the heat differently than men, both sexes were studied. After a stabilisation period at thermal neutrality, six men and seven women were exposed to a globe temperature of 50 degrees C, relative humidity of 17% and wind speed of 0.8-1.0 m.s-1 for 90 min semi-nude at rest, where T(re), T(ac), Tsk, metabolic rate, dry (radiant + convective heat exchange) and evaporative heat losses, S, heat storage by Tc (STc) and heat storage by Tb (STb) were assessed every minute. In the mean, S was equal to 350.8(SEM 49.6) kJ whereas STc amounted to only 114.6(SEM 16.2) and 196.7(SEM 32.3) kJ for T(re) and T(ac), respectively (P less than 0.05). Final STb(re) underestimated S by 49% [177.7(SEM 23.0) kJ; P less than 0.05] whereas STb(ac) was not significantly different than S [255.7(SEM 37.9) kJ]. In the women, S corresponded to a total of 294.3(SEM 23.2) kJ, a value that was very similar to the STb(ac) [262.6(SEM 31.0) kJ], whereas STb(re) under-predicated S by 35% [190.4(SEM 26.3) kJ; P less than 0.05].(ABSTRACT TRUNCATED AT 250 WORDS)     

Temperature regulation in a hypometabolic primate, the slow loris (Nycticebus coucang).

Six slow loris were exposed to air temperatures between 10 degrees C and 40 degrees C. Rectal temperature was stable (mean, 34.8 degrees C) at air temperatures between 17 degrees C and 31 degrees C; at higher air temperatures, the animals became hyperthermic. Oxygen consumption was minimal at air temperatures of 31.4-36.6 degrees C; the mean value (0.250 ml O2 g-1 h-1) was only 36% of the expected level for a eutherian Mammal. The slow loris increased its heat production at lower air temperatures. Thermal polypnea occurred in response to heat, and some of the animals were able to dissipate their entire metabolic heat production at lower air temperatures. Thermal polypnea occurred in response to heat, and some of the animals were able the combined thermal conductance of the tissues and haircoat was 73% of the predicted values. It was concluded that, in spite of its low metabolic rate, the slow loris had effective responses to moderate cold, and that, in addition, it was well adapted to a hot climate.     

A three-compartment thermometry model for the improved estimation of changes in body heat content

The aim of this study was to use whole body calorimetry to directly measure the change in body heat content (DeltaH(b)) during steady-state exercise and compare these values with those estimated using thermometry. The thermometry models tested were the traditional two-compartment model of "core" and "shell" temperatures, and a three-compartment model of "core," "muscle," and "shell" temperatures; with individual compartments within each model weighted for their relative influence upon DeltaH(b) by coefficients subject to a nonnegative and a sum-to-one constraint. Fifty-two participants performed 90 min of moderate-intensity exercise (40% of Vo(2 peak)) on a cycle ergometer in the Snellen air calorimeter, at regulated air temperatures of 24 degrees C or 30 degrees C and a relative humidity of either 30% or 60%. The "core" compartment was represented by temperatures measured in the esophagus (T(es)), rectum (T(re)), and aural canal (T(au)), while the "muscle" compartment was represented by regional muscle temperature measured in the vastus lateralis (T(vl)), triceps brachii (T(tb)), and upper trapezius (T(ut)). The "shell" compartment was represented by the weighted mean of 12 skin temperatures (T(sk)). The whole body calorimetry data were used to derive optimally fitting two- and three-compartment thermometry models. The traditional two-compartment model was found to be statistically biased, systematically underestimating DeltaH(b) by 15.5% (SD 31.3) at 24 degrees C and by 35.5% (SD 21.9) at 30 degrees C. The three-compartment model showed no such bias, yielding a more precise estimate of DeltaH(b) as evidenced by a mean estimation error of 1.1% (SD 29.5) at 24 degrees C and 5.4% (SD 30.0) at 30 degrees C with an adjusted R(2) of 0.48 and 0.51, respectively. It is concluded that a major source of error in the estimation of DeltaH(b) using the traditional two-compartment thermometry model is the lack of an expression independently representing the heat storage in muscle during exercise.     

Prediction of the thermoregulatory response for clothed immersion in cold water

A multi-compartmental thermoregulatory model was applied to data of ten resting clothed males immersed for 3 h in water at 10 and 15 degrees C. Clothing consisted of a dry suit and either a light or heavy undergarment, representing a total insulation of 0.15 (0.95) or 0.20 m2 degrees CW-1 (1.28 clo), respectively. Data were grouped according to low (less than 14%) and high (14 to 24%) body fat individuals. Mean decreases in rectal temperature ranged from 0.79 to 1.38 degrees C, mean decreases in the mean weighted skin temperature ranged from 6.3 to 10.2 degrees C, and mean increases in the metabolic rate ranged from 33.9 to 80.8 W. The model consists of eight segments, each representing a specific region of the body. Each segment is comprised of compartments representing the core, muscle, fat, skin, and clothing. Each compartment is assigned thermophysical values of heat conduction and heat capacitance, and with the exception of clothing, physiological values of blood flow and metabolic heat production. During cold exposure, responses are directed towards increased heat production in the form of shivering and heat conservation in the form of vasoconstriction and convective heat exchange at the vascular level. Agreement between the model predictions and the experimental observations was obtained by adjusting the parameters governing these responses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Temperature regulation of marine mammals

A mathematical model of heat loss from an aquatic animal to the surrounding water is presented. Heat is generated in metabolically active tissues and distributed by circulating blood and by conduction. The time dependent radial temperature profile of the animal is numerically solved from heat transfer equations by a computer. The model is applied to large whales, porpoises, and seals. For the whales, blood circulation to the dermal layer below appendage and body skin surfaces proved to be essential for sufficient heat dissipation. When decreasing the blood flow below a certain value (dependent on sea temperature and whale activity) the large whales would overheat. Blubber thickness was found to be of minor importance in whale thermoregulation, because the blubber coat can be bypassed by blood circulation. On the other hand, it is in general not possible for small porpoises and seals to stay warm in the coldest waters using normal mammalian resting metabolic rates, even if the peripheral circulation is shut off (or artery-vein heat exchangers used). Heat loss can be reduced if the outermost tissue layers are allowed to cool. This is achieved by minimizing convective radial heat flow via the circulation. (For large whales even minute radial blood flow raises the muscle temperatures to the core temperature level.) Seasonal acclimatization of harbour seals is explained by changes in their effective insulation thickness. Differences in whale activity induce changes in the temperature profile mainly within the first few centimeters from the skin surface. These superficial temperatures, if known, could be used to estimate whale metabolic rates. Since they drop close to the sea water temperature within minutes after whale death, the measurements should be done of live whales.     

Effects of exercise in the heat on thermoregulation of Japanese and Malaysian males

The effect of low-intensity exercise in the heat on thermoregulation and certain biochemical changes in temperate and tropical subjects under poorly and well-hydrated states was examined. Two VO2max matched groups of subjects consisting of 8 Japanese (JS) and 8 Malaysians (MS) participated in this study under two conditions: poorly-hydrated (no water was given) and well-hydrated (3 mL x Kg(-1) body weight of water was provided at onset of exercise, and the 15th, 35th and 55th min of exercise). The experimental room in both countries was adjusted to a constant level (Ta: 31.6+/-0.03 degrees C, rh: 72.3+/-0.13%). Subjects spent an initial 10 min rest, 60 min of cycling at 40% VO2max and then 40 min recovery in the experimental room. Rectal temperatures (Tre) skin temperatures (Tsk), heart rate (HR), heat-activated sweat glands density (HASG), local sweat rate (M sw-back) and percent dehydration were recorded during the test. Blood samples were analysed for plasma glucose and lactate levels.The extent of dehydration was significantly higher in the combined groups of JS (1.43+/-0.08%) compared to MS (1.15+/-0.05%). During exercise M sw-back was significantly higher in JS compared to MS in the well-hydrated condition. The HASG was significantly more in JS compared to MS at rest and recovery. Tre was higher in MS during the test. Tsk was significantly higher starting at the 5th min of exercise until the end of the recovery period in MS compared to JS.In conclusion, tropical natives have lower M sw-back associated with higher Tsk and Tre during the rest, exercise and recovery periods. However, temperate natives have higher M sw-back and lower Tsk and Tre during experiments in a hot environment. This phenomenon occurs in both poorly-hydrated and well-hydrated states with low intensity exercise. The differences in M sw-back, Tsk and Tre are probably due to a setting of the core temperature at a higher level and enhancement of dry heat loss, which occurred during passive heat exposure.     

Critical thermal maxima and acclimation rate of the tropical guppy Poecilla reticulata

Tropical guppies, Poecilia reticulata, collected from the canal of La Laguna Los Patos were acclimated over a four-week period at local water temperatures of 24–33 °C to determine their critical thermal maxima (CTM) and death points (DP), as criteria of thermal tolerance. In addition, individual thermal tolerance times at a lethal temperature of 38.5 °C were measured over 12 days for upward acclimation from 24 to 30 °C and over 16 days for downward acclimation from 30 to 24 °C to determine acclimation rate just before and after changing the acclimation temperatures. The CTM ranged from 38.95 to 40.61 °C and the average DP varied from 41.22 to 42.86 °C. Positive relationships were apparent between thermal tolerance and acclimation temperatures, and thus heat tolerance criteria (CTM and DP) were significantly different among acclimation temperatures. Individual heat tolerance times increased most rapidly during the first 6 hours of upward acclimation after transfer from 24 to 30 °C, continued to increase another 5 days and fluctuated after initial acclimation was completed. The heat tolerance times of fish transferred from 30 to 24 °C declined steadily over times, reaching a minimum at 14–16 days after transfer.