Limited heat tolerance in an Arctic passerine: Thermoregulatory implications for cold‐specialized birds in a rapidly warming world

1. Arctic animals inhabit some of the coldest environments on the planet and have evolved physiological mechanisms for minimizing heat loss under extreme cold. However, the Arctic is warming faster than the global average and how well Arctic animals tolerate even moderately high air temperatures (T _a) is unknown.
2. Using flow‐through respirometry, we investigated the heat tolerance and evaporative cooling capacity of snow buntings (Plectrophenax nivalis; ≈31 g, N = 42), a cold specialist, Arctic songbird. We exposed buntings to increasing T _a and measured body temperature (T _b), resting metabolic rate (RMR), rates of evaporative water loss (EWL), and evaporative cooling efficiency (the ratio of evaporative heat loss to metabolic heat production).
3. Buntings had an average (±SD) T _b of 41.3 ± 0.2°C at thermoneutral T _a and increased T _b to a maximum of 43.5 ± 0.3°C. Buntings started panting at T _a of 33.2 ± 1.7°C, with rapid increases in EWL starting at T _a = 34.6°C, meaning they experienced heat stress when air temperatures were well below their body temperature. Maximum rates of EWL were only 2.9× baseline rates at thermoneutral T _a, a markedly lower increase than seen in more heat‐tolerant arid‐zone species (e.g., ≥4.7× baseline rates). Heat‐stressed buntings also had low evaporative cooling efficiencies, with 95% of individuals unable to evaporatively dissipate an amount of heat equivalent to their own metabolic heat production.
4. Our results suggest that buntings’ well‐developed cold tolerance may come at the cost of reduced heat tolerance. As the Arctic warms, and this and other species experience increased periods of heat stress, a limited capacity for evaporative cooling may force birds to increasingly rely on behavioral thermoregulation, such as minimizing activity, at the expense of diminished performance or reproductive investment.

     

Thermoregulation in a large bird,the emu (Dromaius novaehollandiae)

The emu is a large, flightless bird native to Australia. Its habitats range from the high snow country to the arid interior of the continent. Our experiments show that the emu maintains a constant body temperature within the ambient temperature range-5 to 45°C. The males regulate their body temperature about 0.5°C lower than the females. With falling ambient temperature the emu regulates its body temperature initially by reducing conductance and then by increasing heat production. At-5°C the cost of maintaining thermal balance is 2.6 times basal metabolic rate. By sitting down and reducing heat loss from the legs the cost of homeothermy at-5°C is reduced to 1.5 times basal metabolic rate. At high ambient temperatures the emu utilises cutaneous evaporative water loss in addition to panting. At 45°C evaporation is equal to 160% of heat production. Panting accounts for 70% of total evaporation at 45°C. The cost of utilising cutaneous evaporation for the other 30% appears to be an increase in dry conductance.Abbreviations A _r Effective radiating surface area - BMR basal metabolic rate - C _dry dry conductance - CEWL cutaneous evaporative water loss - EHL evaporative heat loss - EWL evaporative water loss - FECO₂ fractional concentration of CO₂ in excurrent air - FF_H2O water content of chamber excurrent air - FEO₂ fractional concentration of O₂ in chamber excurrent air - FICO₂ fractional concentration of CO₂ in incurrent air - FIO₂ fractional concentration of O₂ in chamber incurrent air - MHP metabolic heat production - MR metabolic rate - REWL respiratory evaporative water loss - RH relative humidity - RQ respiratory quotient ; - SA surface area - SEM standard error of the mean - SNK Student-Newman-Keuls multiple range test - STPD standard temperature and pressure dry - T _a ambient temperature(s) - T _b body temperature(s) - T _e surface temperature(s) - flow rate of air into the chamber - carbon dioxide production - oxygen consumption - vapour pressure of water     

Metabolism,thermoregulation and evaporative water loss in the Chaotung Vole (Eothenomys olitor) in Yunnan-Kweichow Plateau in summer

Proper adjustment of thermoregulatory mechanisms ensures the survival of mammals when they are subjected to seasonal changes in their natural environment. To understand the physiological and ecological adaptations of Eothenomys olitor, we measured their metabolic rate, thermal conductance, body temperature (T_b) and evaporative water loss at a temperature range of 5–30 °C in summer. The thermal neutral zone (TNZ) of E. olitor was 20–27.5 °C, and the mean body temperature was 35.81±0.15 °C. Basal metabolic rate (BMR) was 2.81±0.11 ml O₂/g h and mean minimum thermal conductance (C_m) was 0.18±0.01 ml O₂/g h °C. Evaporative water loss (EWL) in E. olitor increased when the ambient temperature increased. The maximal evaporative water loss was 6.74±0.19 mg H₂O/g h at 30 °C. These results indicated that E. olitor have relatively high BMR, low body temperature, low lower critical temperature, and normal thermal conductance. EWL plays an inportant role in temperature regulation. These characteristics are closely related to the living habitat of the species, and represent its adaptive strategy to the climate of the Yunnan-Kweichow Plateau, a low-latitude, high-altitude region where annual temperature fluctuations are small, but daily temperature fluctuations are greater.     

Thermoregulation of water foraging honeybees—Balancing of endothermic activity with radiative heat gain and functional requirements

Foraging honeybees are subjected to considerable variations of microclimatic conditions challenging their thermoregulatory ability. Solar heat is a gain in the cold but may be a burden in the heat. We investigated the balancing of endothermic activity with radiative heat gain and physiological functions of water foraging Apis mellifera carnica honeybees in the whole range of ambient temperatures (T_a) and solar radiation they are likely to be exposed in their natural environment in Middle Europe.The mean thorax temperature (T_th) during foraging stays was regulated at a constantly high level (37.0-38.5 °C) in a broad range of T_a (3-30 °C). At warmer conditions (T_a = 30-39 °C) T_th increased to a maximal level of 45.3 °C. The endothermic temperature excess (difference of T_body − T_a of living and dead bees) was used to assess the endogenously generated temperature elevation as a correlate of energy turnover. Up to a T_a of ∼30 °C bees used solar heat gain for a double purpose: to reduce energetic expenditure and to increase T_th by about 1-3 °C to improve force production of flight muscles. At higher T_a they exhibited cooling efforts to get rid of excess heat. A high T_th also allowed regulation of the head temperature high enough to guarantee proper function of the bees’ suction pump even at low T_a. This shortened the foraging stays and this way reduced energetic costs. With decreasing T_a bees also reduced arrival body weight and crop loading to do both minimize costs and optimize flight performance.     

Effects of flight behaviour on body temperature and kinematics during inter-male mate competition in the solitary desert bee Centris pallida

Abstract. Body temperatures and kinematics are measured for male Centris pallida bees engaged in a variety of flight behaviours (hovering, patrolling, pursuit) at a nest aggregation site in the Sonoran Desert. The aim of the study is to test for evidence of thermoregulatory variation in convective heat loss and metabolic heat production and to assess the mechanisms of acceleration and forward flight in field conditions. Patrolling males have slightly (1–3 °C) cooler body temperatures than hoverers, despite similar wingbeat frequencies and larger body masses, suggesting that convective heat loss is likely to be greater during patrolling flight than during hovering. Comparisons of thorax and head temperature as a function of air temperature (T_a) indicate that C. pallida males are thermoregulating the head by increasing heat transfer from the thorax to the head at cool T_a. During patrolling flight and hovering, wingbeat frequency significantly decreases as T_a increases, indicating that variation in metabolic heat production contributes to thermal stability during these behaviours, as has been previously demonstrated for this species during flight in a metabolic chamber. However, wingbeat frequency during brief (1–2 s) pursuits is significantly higher than during other flight behaviours and independent of T_a. Unlike most other hovering insects, C. pallida males hover with extremely inclined stroke plane angles and nearly horizontal body angles, suggesting that its ability to vary flight speed depends on changes in wingbeat frequency and other kinematic mechanisms that are not yet described.     

Bat thermoregulation in the heat: Limits to evaporative cooling capacity in three southern African bats

High environmental temperatures pose significant physiological challenges related to energy and water balance for small endotherms. Although there is a growing literature on the effect of high temperatures on birds, comparable data are scarcer for bats. Those data that do exist suggest that roost microsite may predict tolerance of high air temperatures. To examine this possibility further, we quantified the upper limits to heat tolerance and evaporative cooling capacity in three southern African bat species inhabiting the same hot environment but using different roost types (crevice, foliage or cave). We used flow-through respirometry and compared heat tolerance limits (highest air temperature (T_a) tolerated before the onset of severe hyperthermia), body temperature (T_b), evaporative water loss, metabolic rate, and maximum cooling capacity (i.e., evaporative heat loss/metabolic heat production). Heat tolerance limits for the two bats roosting in more exposed sites, Taphozous mauritianus (foliage-roosting) and Eptesicus hottentotus (crevice-roosting), were T_a = ~44 °C and those individuals defended maximum T_b between 41 °C and 43 °C. The heat tolerance limit for the bat roosting in a more buffered site, Rousettus aegyptiacus (cave-roosting), was T_a = ~38 °C with a corresponding T_b of ~38 °C. These interspecific differences, together with a similar trend for higher evaporative cooling efficiency in species occupying warmer roost microsites, add further support to the notion that ecological factors like roost choice may have profound influences on physiological traits related to thermoregulation.     

Thermogenic characteristics and evaporative water loss in the tree shrew (Tupaia belangeri)

Thermogenic characteristics and evaporative water loss were measured at different temperatures in Tupaia belangeri. The thermal neutral zone (TNZ) of T. belangeri was 30–35 °C. Mean body temperature was 39.76±0.27 °C and mean body mass was 100.86±9.09 g. Basal metabolic rate (BMR) was 1.38±0.03 ml O₂/g h. Average minimum thermal conductance (C_m) was 0.13±0.01 ml O₂/g h °C. Evaporative water loss in T. belangeri increased when the temperature rose; the maximal evaporative water loss was 3.88±0.41 mg H₂O/g h at 37.5 °C. The results may reflect features of small mammals in the sub-tropical plateau region: T. belangeri had high basal metabolic rate and high total thermal conductance, compared with the predicted values based on their body mass whilst their body temperatures are relatively high; T. belangeri has high levels of evaporative water loss and poor water-retention capacity. Evaporative water loss plays an important role in temperature regulation.     

Temperature and humidity dynamics of cutaneous and respiratory evaporation in pigeons,Columba livia

Summary Using a two-compartment metabolism chamber, we measured oxygen consumption simultaneously with evaporative water loss (EWL) separately from the skin and respiratory tract of pigeons exposed to various air temperatures and humidities. Both respiratory (REWL) and cutaneous (CEWL) water loss increased markedly with increasing air temperature, and latent heat loss through both routes dissipated large fractions of internal heat production during mild heat stress. CEWL as a percentage of total EWL significantly exceeded REWL (60±1.5%) at thermoneutral air temperatures, and was also a substantial fraction of total EWL at lower and higher temperatures. Both REWL and CEWL were inverse functions (apparently linear) of ambient humidity at 20 and 30 °C. These observations verify suggestions by other investigators that CEWL in birds plays a greater role in water balance and in counteracting heat stress than was previously believed.Abbreviations EWL evaporative water loss - CEWL cutaneous EWL - REWL Respiratory EWL - Oxygen consumption (cm³ g^–1 h^–1) - metabolic heat production per unit external surface area (W/m²) - Water vapor density (g/m³)     

Dehydration,with and without heat,in kangaroos from mesic and arid habitats: different thermal responses including varying patterns in heterothermy in the field and laboratory

Field data showing the daily patterns in body temperature (T _b) of kangaroos in hot, arid conditions, with and without water, indicate the use of adaptive heterothermy, i.e. large variation in T _b. However, daily T _b variation was greater in the Eastern Grey Kangaroo (Macropus giganteus), a species of mesic origin, than in the desert-adapted Red Kangaroo (Macropus rufus). The nature of such responses was studied by an examination of their thermal adjustments to dehydration in thermoneutral temperatures (25°C) and at high temperature (45°C) via the use of tame, habituated animals in a climate chamber. At the same level of dehydration M. rufus was less impacted, in that its T _b changed less than that for M. giganteus while it evaporated significantly less water. At a T _a of 45°C with water restriction T _b reached 38.9 ± 0.3°C in M. rufus compared with 40.2 ± 0.4°C for M. giganteus. The ability of M. rufus to reduce dry conductance in the heat while dehydrated was central to its superior thermal control. While M. giganteus showed more heterothermy, i.e. its T _b varied more, this seemed due to a lower tolerance of dehydration in concert with a strong thermal challenge. The benefits of heterothermy to M. giganteus were also limited because of thermal (Q₁₀) effects on metabolic heat production and evaporative heat loss. The impacts of T _b on heat production were such that low morning T _b’s seen in the field may be associated with energy saving, as well as water saving. Kangaroos respond to dehydration and heat similarly to many ungulates, and it is apparent that the accepted notions about adaptive heterothermy in large desert mammals may need revisiting.     

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.     

Evaporative and excretory water loss during free flight in pigeons

Body water conservation is important in flying birds because the very high metabolic demands and heat dissipation requirements during flight depend on plasma-volume integrity. Wind tunnel experiments and theoretical model predictions show that evaporative water loss (EWL) depends on air temperature (T _a) and water vapor density (ρ_a), but these relationships have not been examined in free-flying birds. The contribution of excretory water loss to the total water loss of a flying bird is thought to be negligible but this assumption is untested. To study the dependence of water losses on environmental conditions in free-flying birds and to quantify the contribution of excretory water loss to total water loss, we estimated evaporative and excretory water losses in 16 trained, free-flying tippler pigeons (Columba livia, 250–340 g). We collected excreta by attaching a light latex, water-impermeable receptacle around each bird's vent. By gravimetry, we measured evaporative and excretory water losses of birds for eight flights at different T _as and compared flying to resting (control) birds for two of these flights. EWL was constant with respect to T _a when less than 15 °C, and increased with increasing T _a above 19 °C, indicating that evaporative cooling was invoked when the heat load increased. EWL increased with increasing ρ_a, possibly due to the strong correlation between ρ_a and T _a. Excretory water loss was independent of ρ_a or T _a and averaged almost 10% of the total water loss. Measurements of EWL made on pigeons during wind tunnel experiments and previous free-flight studies are consistent with our free-flight measurements made at similar T _a s. Accepted: 13 April 1999     

Metabolic thermogenesis and evaporative water loss in the Hwamei Garrulax canorus

Basal metabolic rate (BMR) is thought to be a major hub in the network of physiological mechanisms connecting life history traits. Evaporative water loss (EWL) is a physiological indicator that is widely used to measure water relations in inter- or intraspecific studies of birds in different environments. In this study, we examined the physiological responses of summer-acclimatized Hwamei Garrulax canorus to temperature by measuring their body temperature (T_b), metabolic rate (MR) and EWL at ambient temperatures (T_a) between 5 and 40 °C. Overall, we found that mean body temperature was 42.4 °C and average minimum thermal conductance (C) was 0.15 ml O₂ g⁻¹ h⁻¹ °C⁻¹ measured between 5 and 20 °C. The thermal neutral zone (TNZ) was 31.8–35.3 °C and BMR was 181.83 ml O₂ h⁻¹. Below the lower critical temperature, MR increased linearly with decreasing T_a according to the relationship: MR (ml O₂ h⁻¹)=266.59–2.66 T_a. At T_as above the upper critical temperature, MR increased with T_a according to the relationship: MR (ml O₂ h⁻¹)=−271.26+12.85 T_a. EWL increased with T_a according to the relationship: EWL (mg H₂O h⁻¹)=−19.16+12.64 T_a and exceeded metabolic water production at T_a>14.0 °C. The high T_b and thermal conductance, low BMR, narrow TNZ, and high evaporative water production/metabolic water production (EWP/MWP) ratio in the Hwamei are consistent with the idea that this species is adapted to warm, mesic climates, where metabolic thermogenesis and water conservation are not strong selective pressures.     

Participation of book lungs in evaporative water loss in Paraphysa parvula, a migalomorph spider from Chilean Andes

Small animals need efficient water conservation mechanisms for survival and reproduction, which is relevant for the spiders that have large book lungs with large respiratory surface. If lung evaporation is relevant to limit water loss, adjustments of the spiracle opening to metabolic demands should be expected. In this study, we measured the metabolic rate and total evaporative water loss mediated by the opening of the spiracles in the migalomorph spider Paraphysa parvula, a resident of fluctuating Mediterranean environments of the mountains of central Chile. We found that the metabolism of P. parvula was similar to other Theraphosidae and low compared to other arthropods. Carbon dioxide production and evaporative water loss increased with temperature, particularly at 40 °C. The total evaporative water loss at 40 °C increased dramatically to about 10 times that found with the lower temperatures. Thus, 40 °C will be the limit temperature for this species after which evaporative water loss starts to become damaging, so it has to avoid it. The exposition to hypercapnic environments had as a consequence an increase in evaporative water loss and the involvement of the book lungs in this loss was about 60%. The possibility of losing water could condition this species to seek temperate and oxygenated shelters under rocks.     

Transpiration and canopy conductance in an inner alpine Scots pine (Pinus sylvestris L.) forest

Canopy transpiration (E_c) of a 150-year-old Pinus sylvestris L. stand in an inner Alpine dry valley, Tyrol, Austria was estimated throughout two growing seasons 2011 and 2012 by means of xylem sap flow measurements. Although there were prolonged periods of limited soil water availability, E_c did not show a clear trend with respect to soil water availability and averaged 0.4 ± 0.19 mm day⁻¹ under conditions of non-limiting soil water availability and 0.37 ± 0.17 mm day⁻¹ when soil water availability was limited. This is because canopy conductance declined significantly with increasing evaporative demand and thus significantly reduced tree water loss. The growing season total of E_c was 74 mm and 88 mm in 2011 and 2012, respectively, which is significantly below the values estimated for other P. sylvestris forest ecosystems in Central Europe, and thus reflecting a strong adaptation to soil drought during periods of high evaporative demands.     

Evaporative water loss and energy metabolic in two small mammals,voles (Eothenomys miletus) and mice (Apodemus chevrieri), in Hengduan mountains region

Evaporative water loss (EWL) and energy metabolism were measured at different temperatures in Eothenomys miletus and Apodemus chevrieri in dry air. The thermal neutral zone (TNZ) of E. miletus was 22.5–30 °C and that of A. chevrieri was 20–27.5 °C. Mean body temperatures of the two species were 35.75±0.5 and 36.54±0.61 °C. Basal metabolic rates (BMR) were 1.92±0.17 and 2.7±0.5 ml O₂/g h, respectively. Average minimum thermal conductance (C_m) were 0.23±0.08 and 0.25±0.06 ml O₂/g h °C. EWL in E. miletus and A. chevrieri increased with the increase in temperature; the maximal EWL at 35 °C was 4.78±0.6 mg H₂O/g h in E. miletus, and 5.92±0.43 mg H₂O/g h in A. chevrieri. Percentage of evaporative heat loss to total heat production (EHL/HP) increased with the increase in temperature; the maximal EHL/HP was 22.45% at 30 °C in E. miletus, and in A. chevrieri it was 19.96% at 27.5 °C. The results may reflect features of small rodents in the Hengduan mountains region: both E. miletus and A. chevrieri have high levels of BMR and high levels of total thermal conductance, compared with the predicted values based on their body masses, while their body temperatures are relatively low. EWL plays an important role in temperature regulation.     

Genetic diversity of the endangered species Kirengeshoma palmata (Saxifragaceae) in China

In order to investigate the levels of genetic diversity of the endangered species Kirengeshoma palmata (Saxifragaceae), four extant populations were sampled and analyzed using inter-simple sequence repeats (ISSR) markers. We expected a low genetic diversity level, but our results revealed a high level of intraspecific genetic diversity, probably resulting from this species being in a refuge during the last glaciation (at population level: P = 63.25%, A_e = 1.47, H_E = 0.26 and H_O = 0.37; at species level: P = 79.00%, A = 1.5538, H_T = 0.2586 and H_sp = 0.3104). A low level of genetic differentiation among populations was detected based on Nei's genetic diversity analysis (16.69%) and AMOVA (19.36%). Populations shared high levels of genetic identity. Insect pollination and seed dispersal by wind may have facilitated extensive gene flow and are likely responsible for this present structure of genetic variation.     

Lizards on ice: Comparative thermal tolerances of the world's southernmost gecko

(1) We measured thermal tolerances (critical thermal minimum, CT_min and panting threshold, T_pant) for four populations of Homonota darwinii spanning most of the latitudinal range of the species. (2) CT_min differed across populations, but not latitudinally as predicted, likely because latitude was not as good a proxy for operative temperatures (T_e). (3) Some populations had subzero CT_min indicating supercooling or freeze tolerance—the first time either phenomenon has been reported for a gecko. (4) T_pant did not differ significantly among populations. (5) The thermal tolerance breadth appears to be correlated with thermal variability in the environment. (6) Annual T_e data indicate gecko retreats play a crucial role in surviving extreme surface temperatures (<0 or >50 °C).     

Do changes in feed intake or ambient temperature cause changes in cattle rumen temperature relative to core temperature?

A technique was developed to monitor and describe the relationship between core body temperature (T_c) and rumen temperature (T_rum) in cattle. This relationship was assessed in cattle subjected to varying environmental temperatures and subsequent variations in dry matter and water intake. Increasing the environmental wet bulb temperature (WBT) from ambient conditions (approximately 15 °C WBT) to mild heat stress conditions (25 °C WBT) caused an increase in both T_c and T_rum with significant decreases in feed intake and increases in water consumption. Despite increases in both T_c and T_rum, reductions in dry matter intake, and an increase in water consumption, the relationship between T_c and T_rum did not change.     

Evaporative heat loss from group-housed growing pigs at high ambient temperatures

The effects of relative humidity and ambient temperature on evaporative heat loss were studied in 12 trials each with a group of 10 gilts with an initial BW of 61.7 kg (58.0–65.5 kg). The ambient temperature inside a respiration chamber was increased by 2 °C d⁻¹ starting at 16 °C and ending at 32 °C. Relative humidity was set at 50%, 65% or 80% and remained constant within each trial. The animals had free access to feed and water. Skin temperature (SkinT), total heat production (HP), evaporative heat loss (EvapH), respiration rate (RR), and wallowing of the animals were recorded. SkinT was lowest at 80% relative humidity (P<0.05). For each degree Celsius rise in SkinT, wallowing increased by 0.19% (P<0.05). For each degree Celsius rise in ambient temperature, total HP decreased by 115 kJ pig⁻¹ d⁻¹ and EvapH increased by 290 kJ pig⁻¹ d⁻¹ (P<0.05). It was concluded that under constant high ambient temperature and relative humidity, the pigs clearly employ respiratory evaporation to lose heat. Wallowing showed the importance of skin EvapH with higher temperatures, especially at high relative humidity. This study shows the importance of evaporative cooling from the skin. The implication is that pigs at high ambient temperatures, especially in combination with a high relative humidity, should be able to wet themselves. For animal welfare and environmental reasons, it is important that they are able to wet their skin.     

Oxygen consumption and body temperature in yellow-bellied marmot populations from montane-mesic and lowland-xeric environments

Summary Yellow-bellied marmots characteristically live in montane-mesic environments, but in several areas in western North America, this species extended its range into lowland-xeric habitats. Body mass was significantly smaller in the lowland-xeric population from eastern Washington at 393 m than in the montane-mesic population from western Colorado at 2900 m. Oxygen consumption of marmots from montane-mesic and lowland-xeric environments was signiflcantly affected by ambient temperature (T_A) water regimen, population, and a population x water regimen x temperature interaction. Lowland-xeric animals had a higher metabolic rate at low T_As, but a lower metabolic rate at higher T_As than the montane-mesic aminals. Oxygen consumption was lower on a restricted-water regimen than on ad libitum water in both populations. Coefficients relating oxygen consumption to body mass were affected by T_A, water regimen, and population. These intraspecific coefficients are larger than the interspecific coefficients for all mammals. Body temperature (T_B) was affected significantly by T_A, water regimen, and population. T_A body mass, and a population x water regimen interaction significantly affected conductance. Conductance generally was higher in the lowland-xeric than in the montane-mesic marmots. Both populations increased conductance at high T_A, but the lowland-xeric population dissipated a much higher proportion of the heat by evaporative water loss (EWL) than did the montane-mesic population. Metabolic water production exceeded or equaled EWL at 5–20°C. Smaller body size, reduced metabolism at high T_A, and increased EWL at high T_A characterized the lowland-xeric population.Metabolic rates of yellow-bellied marmots were higher than predicted from body size during the reproductive season but decreased to 67% of that predicted from the Kleiber curve by late summer. Marmots minimize thermoregulatory costs by concentrating activity at times when the microclimate is favorable, by tolerating hyperthermia at high T_A in the field, and by having a conductance lower than that predicted from body size.Abbreviations DHC dry-heat conductance - EHL evaporative heat loss - EWL evaporative water loss - HP heat produced - T _A ambient temperature - T _n body temperature - M body mass