The effect of dynamic exercise on resting cold thermoregulatory responses measured during water immersion

The purpose of this study was to evaluate the effect of exercise on the subsequent post-exercise thresholds for vasoconstriction and shivering measured during water immersion. On 2 separate days, seven subjects (six males and one female) were immersed in water (37.5 degrees C) that was subsequently cooled at a constant rate of approximately 6.5 degrees C x h(-1) until the thresholds for vasoconstriction and shivering were clearly established. Water temperature was then increased to 37.5 degrees C. Subjects remained immersed for approximately 20 min, after which they exited the water, were towel-dried and sat in room air (22 degrees C) until both esophageal temperature and mean skin temperature (Tsk) returned to near-baseline values. Subjects then either performed 15 min of cycle ergometry (at 65% maximal oxygen consumption) followed by 30 min of recovery (Exercise), or remained seated with no exercise for 45 min (Control). Subjects were then cooled again. The core temperature thresholds for both vasoconstriction and shivering increased significantly by 0.2 degrees C Post-Exercise (P < 0.05). Because the Tsk at the onset of vasoconstriction and shivering was different during Pre- and Post-Exercise Cooling, we compensated mathematically for changes in skin temperatures using the established linear cutaneous contribution of skin to the control of vasoconstriction and shivering (20%). The calculated core temperature threshold (at a designated skin temperature of 32.0 degrees C) for vasoconstriction increased significantly from 37.1 (0.3) degrees C to 37.5 ( 0.3) degrees C post-exercise (P < 0.05). Likewise, the shivering threshold increased from 36.2 (0.3) degrees C to 36.5 (0.3) degrees C post-exercise (P < 0.05). In contrast to the post-exercise increase in cold thermal response thresholds, sequential measurements demonstrated a time-dependent similarity in the Pre- and Post-Control thresholds for vasoconstriction and shivering. These data indicate that exercise has a prolonged effect on the post-exercise thresholds for both cold thermoregulatory responses.     

Effects of thermal environment on sleep and circadian rhythm

ABSTRACT: The thermal environment is one of the most important factors that can affect human sleep. The stereotypical effects of heat or cold exposure are increased wakefulness and decreased rapid eye movement sleep and slow wave sleep. These effects of the thermal environment on sleep stages are strongly linked to thermoregulation, which affects the mechanism regulating sleep. The effects on sleep stages also differ depending on the use of bedding and/or clothing. In semi-nude subjects, sleep stages are more affected by cold exposure than heat exposure. In real-life situations where bedding and clothing are used, heat exposure increases wakefulness and decreases slow wave sleep and rapid eye movement sleep. Humid heat exposure further increases thermal load during sleep and affects sleep stages and thermoregulation. On the other hand, cold exposure does not affect sleep stages, though the use of beddings and clothing during sleep is critical in supporting thermoregulation and sleep in cold exposure. However, cold exposure affects cardiac autonomic response during sleep without affecting sleep stages and subjective sensations. These results indicate that the impact of cold exposure may be greater than that of heat exposure in real-life situations; thus, further studies are warranted that consider the effect of cold exposure on sleep and other physiological parameters.     

Spatial variation of heat flux in Steller sea lions: evidence for consistent avenues of heat exchange along the body trunk

Maintaining insulative fat stores is vital for homeothermic marine mammals foraging in cold polar waters. To accomplish this, animals must balance acquisition and expenditure of energy. If this balance is shifted, body condition can decrease, challenging thermal homeostasis and further affecting energy balance. Prior studies of temperature regulation in sea lions have neither quantified basic all-inclusive heat flux values for animals swimming in cold water, nor determined whether they exhibit consistent spatial patterns of heat flux. Heat flux and skin temperature data were thus collected from four captive Steller sea lions using heat flux sensors (HFSs) with embedded thermistors. Optimal sensor placement was established using infrared thermography to locate the major areas of heat flux along the surface of the animals. Experiments were conducted on swimming animals in a large habitat tank with and without a drag harness, and on stationary animals in a temperature- and current-controlled swim flume. All heat flux measurements were corrected by a previously determined correction factor of 3.42 to account for insulative effects of the HFSs and attachment mechanism. Heat flux from shoulders and hips was consistently greater than from mid-trunk and axillary areas in both swimming and stationary animals, suggesting that certain areas of the body are preferentially used to offload excess heat. Mean heat flux for animals swimming with a drag harness was significantly greater than for unencumbered animals, indicating a likely increase in heat production beyond minimum heat loss. Thus, thermal stress does not appear to constitute significant costs for Steller sea lions swimming under conditions of increased drag at speeds of approximately 1 m/s in water temperatures of approximately 8.0 °C.     

Respiratory heat loss of Holstein cows in a tropical environment

In order to develop statistical models to predict respiratory heat loss in dairy cattle using simple physiological and environmental measurements, 15 Holstein cows were observed under field conditions in a tropical environment, in which the air temperature reached up to 40°C. The measurements of latent and sensible heat loss from the respiratory tract of the animals were made by using a respiratory mask. The results showed that under air temperatures between 10 and 35°C sensible heat loss by convection decreased from 8.24 to 1.09 W m^–2, while the latent heat loss by evaporation increased from 1.03 to 56.51 W m^–2. The evaporation increased together with the air temperature in almost a linear fashion until 20°C, but it became increasingly high as the air temperature rose above 25°C. Convection was a mechanism of minor importance for respiratory heat transfer. In contrast, respiratory evaporation was an effective means of thermoregulation for Holsteins in a hot environment. Mathematical models were developed to predict both the sensible and latent heat loss from the respiratory tract in Holstein cows under field conditions, based on measurements of the ambient temperature, and other models were developed to predict respiration rate, tidal volume, mass flow rate and expired air temperature as functions of the ambient temperature and other variables.This paper forms part of A. S. Campos Maias doctoral thesis.     

Why do vultures have bald heads? The role of postural adjustment and bare skin areas in thermoregulation

Griffon vultures (Genus Gyps) have large areas of bare skin on their body, and by changing their posture they can vary the extent to which these are covered by feathers. We used a mathematical model to estimate the amount of bare skin exposed in the postures adopted in cold and hot conditions. Measurements of heat flow through different parts of museum skins, which differed in their feather density, were then used together with the estimates of proportions of body surface covered by each feather density type, to determine rates of heat loss from the whole body. Postural change can cause the proportion of body surface composed of bare skin areas to change from 32% to 7%, and in cold conditions these changes are sufficient to account for a 52% saving in heat loss from the body. We suggest that the bare skin areas in griffon vultures may play an important role in thermoregulation.     

The effect of fleece on core and rumen temperature in sheep

Temperature loggers were implanted to record core body temperature (T_core) and rumen temperature (T_rumen) in sheep. The relationship between T_core and T_rumen was compared for fleeced and shorn Merino sheep over a range of environmental temperatures and during stressors involved with shearing. Fleeced sheep maintained higher T_core and T_rumen than shorn sheep in all environmental conditions tested (from thermoneutral up to 33 °C and 55% relative humidity). Shearing of the fleeced sheep resulted in those sheep having a lower T_core when exposed to hot conditions, compared to the previously shorn sheep. Respiratory rates of fleeced sheep followed similar patterns and were higher than shorn sheep under all environmental conditions. After the fleeced sheep were shorn, their respiratory rates decreased to rates similar to the previously shorn sheep when under heat load, suggesting heat loss other than respiratory evaporative heat loss was augmented.     

Development and validity of a universal empirical equation to predict skin surface temperature on thermal manikins

Clothing evaporative resistance is an important input in thermal comfort models. Thermal manikin tests give the most accurate and reliable evaporative resistance values for clothing. The calculation methods of clothing evaporative resistance require the sweating skin surface temperature (i.e., options 1 and 2). However, prevailing calculation methods of clothing evaporative resistance (i.e., options 3 and 4) are based on the controlled nude manikin surface temperature due to the sensory measurement difficulty. In order to overcome the difficulty of attaching temperature sensors to the wet skin surface and to enhance the calculation accuracy on evaporative resistance, we conducted an intensive skin study on a thermal manikin ‘Tore’. The relationship among the nude manikin surface temperature, the total heat loss and the wet skin surface temperature in three ambient conditions was investigated. A universal empirical equation to predict the wet skin surface temperature of a sweating thermal manikin was developed and validated on the manikin dressed in six different clothing ensembles. The skin surface temperature prediction equation in an ambient temperature range between 25.0 and 34.0 °C is T_sk=34.0–0.0132HL. It is demonstrated that the universal empirical equation is a good alternative to predicting the wet skin surface temperature and facilitates calculating the evaporative resistance of permeable clothing ensembles. Further studies on the validation of the empirical equation on different thermal manikins are needed however.     

Maximum voluntary temperature of insect larvae reveals differences in their thermal biology

We investigate the thermoregulatory behaviors of larvae of four species of Drosophila (D. melanogaster, D. subobscura, D. pseudoobscura, and D. mojavensis), a thermotolerant strain of Drosophila melanogaster (T strain) known to differ in thermal biology, and two mutant stocks of D. melanogaster that have (as adults) defective thermoregulatory behavior. We describe and evaluate new techniques to measure two indices of maximum voluntary temperature of insect larvae. Both measures were highly repeatable within lines (species, strains, or mutants). One measure (temperature at which larvae stood upright) differed among lines consistent with expectations based on adult thermal ecology, suggesting that this measure will be useful measures of thermoregulatory set-points of larvae. The second measure (temperature of emergence from media) is less discriminatory.     

Sensible and latent heat loss from the body surface of Holstein cows in a tropical environment

The general principles of the mechanisms of heat transfer are well known, but knowledge of the transition between evaporative and non-evaporative heat loss by Holstein cows in field conditions must be improved, especially for low-latitude environments. With this aim 15 Holstein cows managed in open pasture were observed in a tropical region. The latent heat loss from the body surface of the animals was measured by means of a ventilated capsule, while convective heat transfer was estimated by the theory of convection from a horizontal cylinder and by the long-wave radiation exchange based on the Stefan–Boltzmann law. When the air temperature was between 10 and 36°C the sensible heat transfer varied from 160 to –30 W m^–2, while the latent heat loss by cutaneous evaporation increased from 30 to 350 W m^–2. Heat loss by cutaneous evaporation accounted for 20–30% of the total heat loss when air temperatures ranged from 10 to 20°C. At air temperatures >30°C cutaneous evaporation becomes the main avenue of heat loss, accounting for approximately 85% of the total heat loss, while the rest is lost by respiratory evaporation.Part of first authors doctoral thesis     

Heat tolerance of short-beaked echidnas (Tachyglossus aculeatus) in the field

(1) Echidnas occur throughout the hot arid zone of Australia yet laboratory studies have concluded that they are ill equipped physiologically to manage T_a higher than 35°C. (2) Consequently, it is generally assumed that echidnas must rely on behavioural thermoregulation, being nocturnal in hot weather and seeking less extreme microclimates during the day. (3) By monitoring T_b of echidnas in the field and relating these to T_a within their day time shelters in Western Queensland during summer, this study showed that echidnas are able to tolerate T_a of 35–40°C in hollow logs for up to 10 h. (4) Further, as T_b remains < T_a in these situations, echidnas may have physiological mechanisms for dealing with the heat after all.     

Evaporative water loss in seven species of fossorial rodents: Does effect of degree of fossoriality and sociality exist?

In terrestrial endotherms, evaporation is a significant mechanism of water loss in hot environments. Although water is passively lost by evaporation, individuals can regulate it at different levels. Inhabiting a relatively stable environment characterized by mild ambient temperature (T_a) and high humidity can ensure a balanced water budget. Many fossorial rodents are well adapted to live in such conditions. In this study, evaporative water loss (EWL) of fossorial rodent species with different degree of adaptations to underground life (from strictly subterranean to those with regular surface activity) was evaluated. By measuring EWL, the specific contribution of either evaporative or non-evaporative components of heat loss can be determined. With the exception of the silvery mole-rat (Heliophobius argenteocinereus), in all tested rodents EWL is relatively stable below and within the thermoneutral zone (TNZ). As T_as increase above TNZ, EWL increases as does total thermal conductance, but conductance increases several times more than EWL. In addition, non-evaporative routes seem to be more important than evaporative heat loss in the analyzed species. No clear pattern of EWL in relation to a species degree of fossoriality or sociality was detected. In this context, atmosphere of burrows could affect EWL, since the high humidity found inside tunnels can establish limits on evaporation to favor water rather than thermal balance.     

Thermal status of wet-suited divers using closed circuit O2 apparatus in sea water of 17–18.5°C

A wet suit may not provide adequate thermal protection when diving in moderately cold water (17–18°C), and any resultant mild hypothermia may impair performance during prolonged diving. We studied heat exchange during a dive to a depth of 5 m in sea water (17–18.5°C) in divers wearing a full wet suit and using closed-circuit oxygen breathing apparatus. Eight fin swimmers dived for 3.1 h and six underwater scooter (UWS) divers propelled themselves through the water for 3.7 h. The measurements taken throughout the dive were the oxygen pressure in the cylinder and skin and rectal temperatures (T _re). Each subject also completed a cold score questionnaire. The T _re decreased continuously in all subjects. Oxygen consumption in the fin divers (1.40 l · min⁻¹) was higher than that of the UWS divers (1.05 l · min⁻¹). The mean total insulation was 0.087°C · m² · W⁻¹ in both groups. Mean body insulation was 37% of the total insulation (suit insulation was 63%). The reduction in T _re over the 1st hour was related to subcutaneous fat thickness. There was a correlation between cold score and T _re at the end of 1 h, but not after that. A full wet suit does not appear to provide adequate thermal protection when diving in moderately cold water. Accepted: 21 January 1997     

Regulation of local sweating in sleep-deprived exercising humans

Thermoregulatory sweating [total body (m _sw,b), chest (m _sw,c) and thigh (m _sw,t) sweating], body temperatures [oesophageal (T _oes) and mean skin temperature (T _sk)] and heart rate were investigated in five sleep-deprived subjects (kept awake for 27 h) while exercising on a cycle (45 min at approximately 50% maximal oxygen consumption) in moderate heat (T _air andT _wall at 35° C. Them _sw,c andm _sw,t were measured under local thermal clamp (T _sk,1), set at 35.5° C. After sleep deprivation, neither the levels of body temperatures (T _oes,T _sk) nor the levels ofm _{sw, b},m _{sw, c} orm _{sw, t} differed from control at rest or during exercise steady state. During the transient phase of exercise (whenT _sk andT _sk,1 were unvarying), them _{sw, c} andm _{sw, t} changes were positively correlated with those ofT _oes. The slopes of them _{sw, c} versusT _oes, orm _{sw, t} versusT _oes relationships remained unchanged between control and sleep-loss experiments. Thus the slopes of the local sweating versusT _oes, relationships (m _{sw, c} andm _{sw, t} sweating data pooled which reached 1.05 (SEM 0.14) mg·cm^–2·min^–1°C^–1 and 1.14 (SEM 0.18) mg·cm^–2·min^–1·°C^–1 before and after sleep deprivation) respectively did not differ. However, in our experiment, sleep deprivation significantly increased theT _oes threshold for the onset of bothm _{sw, c} andm _{sw, t} (+0.3° C,P<0.001). From our investigations it would seem that the delayed core temperature for sweating onset in sleep-deprived humans, while exercising moderately in the heat, is likely to have been due to alterations occurring at the central level.     

Monte Carlo simulations of individual variability and their effects on simulated heat stress using thermoregulatory modeling

This paper addresses a variable-dependence (VD) MC method developed based on a previous attempt (VI-MC method) (J. Therm. Biol. 29 (2004), 515) to be incorporated in a thermoregulatory model. Simulated individuals with anthropometrics by VI- and VD-MC methods for US Army population were compared using principal component analysis and Fisher's exact tests. The results indicated that VD-MC data represented overall body size as the primary component and body shape as the secondary component that were more realistic and similar to the measured US Army data (p>0.05) rather than VI-MC data (p<0.05). Such differences consequently affected individual thermoregulatory responses to simulated heat stress. The VD-MC method provides a more realistic representation of individual variability and thus underpins more realistic predictions of individual thermoregulatory responses.     

Mathematical prediction of core body temperature from environment,activity, and clothing: The heat strain decision aid (HSDA)

Physiological models provide useful summaries of complex interrelated regulatory functions. These can often be reduced to simple input requirements and simple predictions for pragmatic applications. This paper demonstrates this modeling efficiency by tracing the development of one such simple model, the Heat Strain Decision Aid (HSDA), originally developed to address Army needs. The HSDA, which derives from the Givoni-Goldman equilibrium body core temperature prediction model, uses 16 inputs from four elements: individual characteristics, physical activity, clothing biophysics, and environmental conditions. These inputs are used to mathematically predict core temperature (T_c) rise over time and can estimate water turnover from sweat loss. Based on a history of military applications such as derivation of training and mission planning tools, we conclude that the HSDA model is a robust integration of physiological rules that can guide a variety of useful predictions. The HSDA model is limited to generalized predictions of thermal strain and does not provide individualized predictions that could be obtained from physiological sensor data-driven predictive models. This fully transparent physiological model should be improved and extended with new findings and new challenging scenarios.     

Design and performance of a rugged standard operative temperature thermometer for avian studies

The lack of a truly satisfactory sensor which can characterize the thermal environment at the spatial scale experienced by small endotherms has hindered study of their thermoregulatory behavior. We describe a general design for a rugged, easily constructed sensor to measure standard operative temperature, T_es. We present specific designs for adult dark-eyed juncos (Junco hyemalis) and hatchling mallards (Anas platyrhynchos). Sensor response was stable and repeatable (±1.4%) over the course of several months. Over the range of conditions for which validation data were available (variable air temperature and wind with negligible net radiation), sensors predicted the mean net heat production of live animals to within ±0.023 W (equivalent to ±1°C at T_es=15°C). The main limit on accuracy was scatter in the data on metabolism and evaporative water loss in live animals. These sensors are far more rugged and easily constructed than the heated taxidermic mounts previously used to measure T_es. These characteristics facilitate the use of significant numbers of sensors in thermal mapping studies of endotherms.