The Physiology of Heat Tolerance in Small Endotherms

Understanding the heat tolerances of small mammals and birds has taken on new urgency with the advent of climate change. Here, we review heat tolerance limits, pathways of evaporative heat dissipation that permit the defense of body temperature during heat exposure, and mechanisms operating at tissue, cellular, and molecular levels.

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Thermoregulation in desert birds: scaling and phylogenetic variation in heat tolerance and evaporative cooling

Journal of Experimental Biology ◽

10.1242/jeb.229211 ◽

2021 ◽

Vol 224 (Suppl 1) ◽

pp. jeb229211

Author(s):

Andrew E. McKechnie ◽

Alexander R. Gerson ◽

Blair O. Wolf

Keyword(s):

Heat Tolerance ◽

Heat Dissipation ◽

Arid Zone ◽

Resting Metabolic Rate ◽

Heat Exposure ◽

Published Data ◽

Evaporative Heat Loss ◽

Evaporative Water ◽

Data Set ◽

The Difference

ABSTRACTEvaporative heat dissipation is a key aspect of avian thermoregulation in hot environments. We quantified variation in avian thermoregulatory performance at high air temperatures (Ta) using published data on body temperature (Tb), evaporative water loss (EWL) and resting metabolic rate (RMR) measured under standardized conditions of very low humidity in 56 arid-zone species. Maximum Tb during acute heat exposure varied from 42.5±1.3°C in caprimulgids to 44.5±0.5°C in passerines. Among passerines, both maximum Tb and the difference between maximum and normothermic Tb decreased significantly with body mass (Mb). Scaling exponents for minimum thermoneutral EWL and maximum EWL were 0.825 and 0.801, respectively, even though evaporative scope (ratio of maximum to minimum EWL) varied widely among species. Upper critical limits of thermoneutrality (Tuc) varied by >20°C and maximum RMR during acute heat exposure scaled to Mb0.75 in both the overall data set and among passerines. The slope of RMR at Ta>Tuc increased significantly with Mb but was substantially higher among passerines, which rely on panting, compared with columbids, in which cutaneous evaporation predominates. Our analysis supports recent arguments that interspecific within-taxon variation in heat tolerance is functionally linked to evaporative scope and maximum ratios of evaporative heat loss (EHL) to metabolic heat production (MHP). We provide predictive equations for most variables related to avian heat tolerance. Metabolic costs of heat dissipation pathways, rather than capacity to increase EWL above baseline levels, appear to represent the major constraint on the upper limits of avian heat tolerance.

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Effects on heat tolerance of physical training in water and on land

Journal of Applied Physiology ◽

10.1152/jappl.1982.53.5.1291 ◽

1982 ◽

Vol 53 (5) ◽

pp. 1291-1298 ◽

Cited By ~ 32

Author(s):

B. A. Avellini ◽

Y. Shapiro ◽

S. M. Fortney ◽

C. B. Wenger ◽

K. B. Pandolf

Keyword(s):

Heat Tolerance ◽

Physical Training ◽

Heat Dissipation ◽

Sweat Rate ◽

Cardiovascular Response ◽

Training Stimulus ◽

Heat Exposure ◽

Maximal Aerobic Capacity ◽

Group Iii ◽

Group I

A 4-wk training program was undertaken by 15 untrained non-heat-acclimated males who were divided into three groups matched on maximal aerobic capacity (VO2max) and trained either in water or on land to determine how physical training (PT) in these different media affects heat tolerance. Subjects trained on a cycle ergometer for 1 h/day, 5 days/wk at 75% VO2max, with the exercise intensity progressively increased to maintain a constant training stimulus. Group I exercised on land, whereas groups II and III exercised while immersed to the neck in water of either 32 degrees C (II) or 20 degrees C (III). Daily exercise increased core temperature (Tc) in groups I and II but not in group III. Training elicited similar increases (approximately 15%) in VO2max in the three groups. Before and after PT, all subjects exercised at approximately 30% VO2max for 3 h at 49 degrees C, 20% rh. Compared with before training, groups I and II showed a decrease in final Tc and heart rate (HR) in the posttraining heat exposure. Sweat rate increased 25% in group II but remained the same in group I. Group III demonstrated a decrease in final HR, but final Tc was higher than before training. Sweat rate did not increase in group III and was lower than the other groups. It was concluded that PT can improve the cardiovascular response to dry heat without affecting thermoregulatory capacity. PT appears to enhance heat tolerance only if Tc is permitted to rise during exercise, thus stimulating the temperature-regulating center for heat dissipation.

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The evolution of critical thermal limits of life on Earth

Nature Communications ◽

10.1038/s41467-021-21263-8 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Joanne M. Bennett ◽

Jennifer Sunday ◽

Piero Calosi ◽

Fabricio Villalobos ◽

Brezo Martínez ◽

...

Keyword(s):

Climate Change ◽

Cold Tolerance ◽

Heat Tolerance ◽

Thermal Tolerance ◽

Adaptive Responses ◽

Tolerance Limits ◽

Deep Time ◽

Thermal Limits ◽

Critical Thermal Limits ◽

Life On Earth

AbstractUnderstanding how species’ thermal limits have evolved across the tree of life is central to predicting species’ responses to climate change. Here, using experimentally-derived estimates of thermal tolerance limits for over 2000 terrestrial and aquatic species, we show that most of the variation in thermal tolerance can be attributed to a combination of adaptation to current climatic extremes, and the existence of evolutionary ‘attractors’ that reflect either boundaries or optima in thermal tolerance limits. Our results also reveal deep-time climate legacies in ectotherms, whereby orders that originated in cold paleoclimates have presently lower cold tolerance limits than those with warm thermal ancestry. Conversely, heat tolerance appears unrelated to climate ancestry. Cold tolerance has evolved more quickly than heat tolerance in endotherms and ectotherms. If the past tempo of evolution for upper thermal limits continues, adaptive responses in thermal limits will have limited potential to rescue the large majority of species given the unprecedented rate of contemporary climate change.

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Torpor and hibernation

10.1093/oso/9780199551668.003.0011 ◽

2017 ◽

Author(s):

Andrew Clarke

Keyword(s):

Body Temperature ◽

Small Mammals ◽

Energy Input ◽

The Arctic ◽

Arid Areas ◽

Daily Torpor ◽

High Latitudes ◽

Small Species ◽

Universal Feature ◽

Deep Hibernation

A diurnal (circadian) rhythm in body temperature is a widespread, and possibly universal, feature of endotherms. Some mammals and birds down-regulate their metabolic rate significantly by night, allowing their body temperature to drop sufficiently that they become inactive and enter torpor. Both the minimum temperature achieved and the duration of torpor are highly variable. Daily torpor is principally a response to reduced energy intake, and a drop in ambient temperature. Hibernation is essentially an extreme form of torpor. Small mammals hibernating at high latitudes have regular arousals during which they urinate and may feed. Bears hibernate with relatively high body temperature, and do not undergo arousal. Only one bird, the poorwill, is known to hibernate. Rewarming during arousal may be fuelled exclusively by metabolism (for example in small mammals in the Arctic) or with significant energy input from basking (for example in subtropical arid areas). The capacity for torpor appears to be an ancestral character in both mammals and birds, possibly related to the origin of endothermy in small species subject to marked diurnal and/or seasonal variation in body temperature. Both deep hibernation and strict endothermy are probably derived characteristics.

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Climate change and infrastructure risk: Indoor heat exposure during a concurrent heat wave and blackout event in Phoenix, Arizona

Urban Climate ◽

10.1016/j.uclim.2021.100787 ◽

2021 ◽

Vol 36 ◽

pp. 100787

Author(s):

Brian Stone ◽

Evan Mallen ◽

Mayuri Rajput ◽

Ashley Broadbent ◽

E. Scott Krayenhoff ◽

...

Keyword(s):

Climate Change ◽

Heat Wave ◽

Heat Exposure

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Changes in Body Temperature of Small Mammals and Birds in a Few Minutes Range as Reflection of Environmental Influences

Bulletin of Experimental Biology and Medicine ◽

10.1007/s10517-021-05234-z ◽

2021 ◽

Author(s):

M. E. Diatroptov

Keyword(s):

Body Temperature ◽

Small Mammals ◽

Environmental Influences

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A novel mouse model of heatstroke accounting for ambient temperature and relative humidity

Journal of Intensive Care ◽

10.1186/s40560-021-00546-8 ◽

2021 ◽

Vol 9 (1) ◽

Author(s):

Kazuyuki Miyamoto ◽

Keisuke Suzuki ◽

Hirokazu Ohtaki ◽

Motoyasu Nakamura ◽

Hiroki Yamaga ◽

...

Keyword(s):

Gene Expression ◽

Body Temperature ◽

Relative Humidity ◽

Ambient Temperature ◽

Core Body Temperature ◽

Heat Exposure ◽

Organ Damage ◽

Core Body ◽

The Impact ◽

Treatment Water

Abstract Background Heatstroke is associated with exposure to high ambient temperature (AT) and relative humidity (RH), and an increased risk of organ damage or death. Previously proposed animal models of heatstroke disregard the impact of RH. Therefore, we aimed to establish and validate an animal model of heatstroke considering RH. To validate our model, we also examined the effect of hydration and investigated gene expression of cotransporter proteins in the intestinal membranes after heat exposure. Methods Mildly dehydrated adult male C57/BL6J mice were subjected to three AT conditions (37 °C, 41 °C, or 43 °C) at RH > 99% and monitored with WetBulb globe temperature (WBGT) for 1 h. The survival rate, body weight, core body temperature, blood parameters, and histologically confirmed tissue damage were evaluated to establish a mouse heatstroke model. Then, the mice received no treatment, water, or oral rehydration solution (ORS) before and after heat exposure; subsequent organ damage was compared using our model. Thereafter, we investigated cotransporter protein gene expressions in the intestinal membranes of mice that received no treatment, water, or ORS. Results The survival rates of mice exposed to ATs of 37 °C, 41 °C, and 43 °C were 100%, 83.3%, and 0%, respectively. From this result, we excluded AT43. Mice in the AT 41 °C group appeared to be more dehydrated than those in the AT 37 °C group. WBGT in the AT 41 °C group was > 44 °C; core body temperature in this group reached 41.3 ± 0.08 °C during heat exposure and decreased to 34.0 ± 0.18 °C, returning to baseline after 8 h which showed a biphasic thermal dysregulation response. The AT 41 °C group presented with greater hepatic, renal, and musculoskeletal damage than did the other groups. The impact of ORS on recovery was greater than that of water or no treatment. The administration of ORS with heat exposure increased cotransporter gene expression in the intestines and reduced heatstroke-related damage. Conclusions We developed a novel mouse heatstroke model that considered AT and RH. We found that ORS administration improved inadequate circulation and reduced tissue injury by increasing cotransporter gene expression in the intestines.

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