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We tested the hypothesis that interspecific variation in chiropteran heat tolerance and evaporative cooling capacity is correlated with day-roost microclimates, using three vespertilionid bats that occur sympatrically during summer in Saskatchewan, Canada. We predicted that hoary bats (Lasiurus cinereus (Palisot de Beauvois, 1796); ~22 g) would have higher heat tolerance than little brown bats (Myotis luafugus (Le Conte, 1831); ~7 g) and silver-haired bats (Lasionycteris noctivagans (Le Conte, 1831); ~13 g), as the latter two species roost in tree crevices or cavities that are more thermally buffered than the foliage roosts of hoary bats. We measured core body temperature ([T.sub.b]; passive integrated transponder tags), evaporative water loss, and resting metabolic rate (flow-through respirometry) while exposing individuals to a stepped profile of increasing air temperature ([T.sub.a]) from ~30 [degrees]C in ~2 [degrees]C increments. Experiments were terminated when individuals became hyperthermic ([T.sub.b] [approximately equal to] 42.5 [degrees]C), with maximum [T.sub.a] ([T.sub.a,max]) ranging from 42.0 to 49.7 [degrees]C. As predicted, hoary bats had the highest heat tolerance and evaporative cooling capacity, reaching [T.sub.a,max] ~2.4 and 1.2 [degrees]C higher than little brown and silver-haired bats, respectively. Our results are consistent with the hypothesis that heat tolerance of bats is correlated with roost microclimates, although interspecific variation in body mass and phylogeny may confound these conclusions. Key words: heat tolerance, evaporative cooling capacity, hyperthermia, roost temperature, Myotis luafugus, little brown bat, Lasionycteris noctivagans, silver-haired bat, Lasiurus cinereus, hoary bat, Yinpterochiroptera. Nous avons vérifié l'hypothèse voulant que les variations interspécifiques de la tolérance à la chaleur et de la capacité de refroidissement par évaporation chez les chiroptères soient corrélées aux microclimats des gîtes diurnes, en utilisant trois chauves-souris de la famille des vespertilionidés présentes en sympatrie en été en Saskatchewan (Canada). Nous avions prédit que la tolérance à la chaleur des chauves-souris cendrées (Lasiurus cinereus (Palisot de Beauvois, 1796); ~22 g) serait plus grande que celles des vespertilions bruns (Myotis lucifugus (Le Conte, 1831); ~7 g) et des chauves-souris argentées (Lasionycteris noctivagans (Le Conte, 1831); ~13 g), puisque les gîtes diurnes de ces deux dernières espèces sont des fentes ou cavités dans les arbres caractérisées par des températures moins variables que les gîtes dans le feuillage des chauves-souris cendrées. Nous avons mesuré la température corporelle centrale ([T.sub.b]; radioétiquettes passives intégrées), la perte d'eau par évaporation et le métabolisme au repos (respirométrie dynamique), alors que les spécimens étaient exposés à des températures de l'air ([T.sub.a]) commençant à ~30 [degrees]C et augmentant par incréments de ~2 [degrees]C. Les manipulations cessaient que les spécimens devenaient hyperthermiques ([T.sub.b] [approximately equal to] 42,5 [degrees]C), les Ta maximums ([T.sub.a,max]) allant de 42,0 à 49,7 [degrees]C. Comme prévu, les chauves-souris cendrées présentent la plus forte tolérance à la chaleur et la plus grande capacité de refroidissement par évaporation, atteignant des [T.sub.a,max] de ~2,4 et 1,2 [degrees]C supérieures à celles des vespertilions bruns et des chauves-souris argentées, respectivement. Nos résultats concordent avec l'hypothèse selon laquelle la tolérance à la chaleur des chauves-souris est corrélée aux microclimats des gîtes, bien que des variations interspécifiques de la masse corporelle et la phylogénie puissent nécessiter de nuancer ces conclusions. [Traduit par la Rédaction] Mots-clés : tolerance a la chaleur, capacite de refroidissement par evaporation, hyperthermie, temperature du gite, Myotis lucifugus, vespertilion brun, Lasionycteris noctivagans, chauve-souris argentee, Lasiurus cinereus, chauve-souris cendree, yinpterochiropteres., Introduction Free-ranging endotherms frequently encounter environmental temperatures above their normothermic body temperature ([T.sub.b]), for example, while foraging in sunlit microsites or occupying hot roosts during the day (e.g., Henshaw and [...]

Researchers from diverse disciplines, including organismal and cellular physiology, sports science, human nutrition, evolution and ecology, have sought to understand the causes and consequences of the surprising variation in metabolic rate found among and within individual animals of the same species. Research in this area has been hampered by differences in approach, terminology and methodology, and the context in which measurements are made. Recent advances provide important opportunities to identify and address the key questions in the field. By bringing together researchers from different areas of biology and biomedicine, we describe and evaluate these developments and the insights they could yield, highlighting the need for more standardisation across disciplines. We conclude with a list of important questions that can now be addressed by developing a common conceptual and methodological toolkit for studies on metabolic variation in animals. [ABSTRACT FROM AUTHOR]

Anthropogenic land use changes, such as deforestation and commercial forestry, have substantially reduced natural roost sites for European bats. A common conservation solution is to provide artificial roosts (i.e. bat boxes), but there are concerns that these can become hotter than natural roosts in summer and could be death traps during heat waves. Nevertheless, females of several bat species form maternity colonies in these boxes, thus occupying hotter and more humid microclimates than solitarily roosting males. We tested if cooling efficiency and heat tolerance differ between sexes in European bats, and estimated the evaporative water requirements for bats living in bat boxes during hot summer days.We used indirect calorimetry and thermometry to quantify thermoregulation at high air temperatures (Ta) in four species of verspitilionid bats that regularly occupy artificial roosts. We measured resting metabolic heat production, evaporative water loss rates (EWL) and body temperature (Tb) at Ta between 28°C and 48°C during summer. We predicted that females have higher evaporative cooling efficiency (evaporative heat loss/metabolic heat production) than males, allowing them to reach their heat tolerance limit at higher Ta.We found no sex differences in maximum evaporative cooling efficiency, maximum Tb, and maximum Ta tolerated. However, the patterns of increasing EWL with Ta differed between sexes. Females tolerated higher Ta before increasing EWL than males and then rapidly increased EWL to higher values than males at the maximum Ta tolerated. These sex differences in heat dissipation strategies may reflect varying ecological and physiological constraints associated with different summer roosting habits.Synthesis and applications. Our study revealed that some small European bat species are already at risk of succumbing to lethal dehydration during present‐day heat waves, with daytime evaporative water requirements equivalent to ~30% of body mass in sun‐exposed boxes. For conservation managers working with common European bat species, particularly those in monoculture forests with woodcrete bat‐boxes, our physiologically informed recommendations include positioning boxes in diverse locations varying in aspect and sun exposure. This will ensure thermal heterogeneity of roost sites and provide a wide gradient of microclimate conditions, allowing for roost switching when necessary. [ABSTRACT FROM AUTHOR]

Adaptive plasticity in avian thermal physiology is increasingly apparent, with a well-studied example being metabolic upregulation during cold winters in small birds inhabiting temperate and boreal latitudes. Recent studies have revealed greater variation in the direction and magnitude of seasonal metabolic adjustments among subtropical/tropical birds experiencing milder winters compared with higher-latitude counterparts, suggesting that patterns could vary among years within populations. We quantified seasonal metabolic variation (summer vs. winter) in Kalahari Desert populations of two Afrotropical passerines, the white-browed sparrow-weaver (WBSW; Plocepasser mahali; ∼40 g) and the scaly-feathered weaver (SFW; Sporopipes squamifrons; ∼10 g) over subsequent years (2014–2017). We used flow-through respirometry to measure basal metabolic rate (BMR) and summit metabolism (Msum; maximum cold-induced resting metabolic rate) and quantified seasonal fluctuations in air temperature (Ta) and food abundance (arthropod and grass seed abundance) at the study site. Our data reveal that the direction and magnitude of seasonal metabolic acclimatization vary among years in both species, with the winter BMR of WBSWs ranging from ∼20% lower to 68% higher compared with the summer BMR. In contrast to higher-latitude species, Msum was not related to the cold-limit temperature of birds or to winter minimum Ta at the study site, but interannual variation in BMR and Msum was significantly lower in seasons with lower food abundance in both WBSWs and SFWs. Our data support the idea that patterns of seasonal acclimatization are more variable in birds from lower latitudes and that there is considerable phenotypic flexibility in avian thermal physiology. [ABSTRACT FROM AUTHOR]

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Heterothermic responses characterised by pronounced hypometabolism and reductions in body temperature (Tb) are one of the most effective ways in which small endotherms can offset the energetic cost of endothermic homeothermy. It remains unclear, therefore, why daily torpor and hibernation are restricted to only a subset of avian lineages. To further our understanding of the phylogenetic distribution of avian torpor, we investigated winter thermoregulation in the southern African ground woodpecker Geocolaptes olivaceus. We considered this species a good candidate for heterothermy, because it is resident year-round in mountainous regions with cold winters and reliant on small ectothermic prey. We recorded Tb patterns in free-ranging individuals and measured Tb and metabolic rates in captive individuals. Neither free-ranging nor captive woodpeckers showed any indication of daily torpor or even shallow rest-phase hypothermia. All birds maintained bimodally distributed Tb characteristic of classic endothemic homeothermy, with a mean rest-phase Tb of 37.9 ± 0.2°C and no data below 37.0°C. The mean circadian amplitude of Tb was 4.2°C, equivalent to approximately twice the expected value. There was some evidence of seasonal acclimatisation in Tb, with a small decrease in rest-phase Tb with the onset of the austral winter. Captive birds showed patterns of resting metabolic rate and Tb consistent with the classic model of endothermic homeothermy. The apparent absence of torpor in G. olivaceus supports the notion that, unlike the case in mammals, many avian taxa that may a priori be expected to benefit from deep heterothermy do not use it. [ABSTRACT FROM AUTHOR]

Sandgrouse (Pterocliformes) are quintessential examples of avian adaptation to desert environments, but relatively little is known about the limits to their heat tolerance and evaporative cooling capacity. We predicted that evaporative cooling in Burchell's sandgrouse (Pterocles burchelli) is highly efficient and provides the basis for tolerance of very high air temperature (Ta). We measured body temperature (Tb), resting metabolic rate (RMR) and evaporative water loss (EWL) at Ta between 25°C and ~58°C in birds exposed to successive increments in Ta. Normothermic Tb averaged 39.0°C, lower than typical avian values. At Ta>34.5°C, Tb increased linearly to a maximum of 43.6°C at Ta=56°C. The upper critical limit of thermoneutrality (Tuc) was Ta=43.8°C, closely coinciding with the onset of panting and gular flutter. Above the Tuc, RMR increased 2.5-fold to 2.89 W at Ta=56°C, a fractional increase far exceeding that of many other species under comparable conditions. Rates of EWL increased rapidly at Ta>42.9°C to 7.84±0.90 g h-1 at Ta=56°C, an 11-fold increase above minimal levels. Maximum evaporative cooling efficiency (ratio of evaporative heat loss to metabolic heat production) was 2.03, but could be as high as 2.70 if our assumption that the birds were metabolising lipids is incorrect. Thermoregulation at very high Ta> in P. burchelli was characterised by large increases in RMR and EWL, and is much less efficient than in taxa such as columbids and caprimulgids. [ABSTRACT FROM AUTHOR]

Intraspecific variation in avian thermoregulatory responses to heat stress has received little attention, despite increasing evidence that endothermic animals show considerable physiological variation among populations. We investigated seasonal (summer versus winter) variation in heat tolerance and evaporative cooling in an Afrotropical ploceid passerine, the white-browed sparrow-weaver (Plocepasser mahali; ~47 g) at three sites along a climatic gradient with more than 10°C variation in mid-summer maximum air temperature (Ta). We measured resting metabolic rate (RMR) and total evaporative water loss (TEWL) using open flow-through respirometry, and core body temperature (Tb) using passive integrated transponder tags. Sparrow-weavers were exposed to a ramped profile of progressively higher Ta between 30 and 52°C to elicit maximum evaporative cooling capacity (N=10 per site per season); the maximum Ta birds tolerated before the onset of severe hyperthermia (Tb≈44°C) was considered to be their hyperthermia threshold Ta (Ta,HT). Our data reveal significant seasonal acclimatisation of heat tolerance, with a desert population of sparrow-weavers reaching significantly higher Ta in summer (49.5± 1.4°C, i.e. higher Ta,HT) than in winter (46.8±0.9°C), reflecting enhanced evaporative cooling during summer. Moreover, desert sparrow-weavers had significantly higher heat tolerance and evaporative cooling capacity during summer compared with populations from more mesic sites (Ta,HT=47.3±1.5 and 47.6±1.3°C). A better understanding of the contributions of local adaptation versus phenotypic plasticity to intraspecific variation in avian heat tolerance and evaporative cooling capacity is needed for modelling species' responses to changing climates. [ABSTRACT FROM AUTHOR]

Energy conservation is a clear function of torpor. Although many studies imply that torpor is also a water-saving strategy, the experimental evidence linking water availability with torpor is inconclusive. We tested the relative roles of water and energy shortages in driving torpor, using the Siberian hamster Phodopus sungorus as a model species. To account for the seasonal development of spontaneous heterothermy, we used male hamsters acclimated to short (8L:16D, SP; n = 40) and long (16L:8D, LP; n = 36) photoperiods. We continuously measured body temperature (T b) during consecutive 32 h of complete removal of water, food, or both, separated by 7.5 d recovery periods. We predicted that all deprivation types would increase the frequency of spontaneous torpor in SP, and induce torpor in LP-acclimated hamsters. Individuals underwent each deprivation type twice in random orders. Food and water deprivation did not induce torpor in LP-acclimated P. sungorus. Patterns of torpor expression varied among deprivation types in SP individuals. Torpor frequency was significantly lower, but bouts were ∼2 h longer and 2.5 °C deeper, during water deprivation compared to food and food-and-water deprivation. Heterothermic responses to all deprivation types were repeatable among individuals. Different torpor patterns during water and food deprivation suggest that water and energy shortages are distinct physiological challenges. Deeper and longer bouts during water deprivation likely led to higher energy and water savings, while shorter and shallower bouts during fasting may reflect a trade-off between energy conservation and food-seeking activity. The lack of a difference between food- and food-and-water-deprived hamsters suggests a higher sensitivity to food than water shortage. This supports the traditional view that energy conservation is the major function of torpor, but suggests that water shortages may also modulate torpor use. The high repeatability of thermoregulatory responses to resource deprivation suggests that these may be heritable traits subject to natural selection. • Removal of food and water does not induce torpor in long-day acclimated hamsters. • Removal of food or water induce different torpor use patterns in short-day hamsters. • Water deprivation induces deeper and longer torpor bouts than food deprivation. • Thermoregulatory responses to resource deprivation are repeatable among individuals. • High repeatability of heterothermy suggests it may be subject to natural selection. [ABSTRACT FROM AUTHOR]

There is increasing evidence for considerable phenotypic flexibility in endotherm thermal physiology, a phenomenon with far-reaching implications for the evolution of traits related to heat tolerance. Numerous studies have documented intraspecific variation in avian thermoregulatory traits, but few have revealed the shapes of thermoregulatory reaction norms or how these might vary among populations. We investigated phenotypic flexibility in the ability of a model Afrotropical passerine bird (the white-browed sparrow-weaver, Plocepasser mahali) to handle high air temperatures (T a). We allocated birds from three sites varying by ~ 11 °C in mean daily summer maximum T a to three acclimation temperature (T accl) treatments (daytime T accl ≈ 30 °C, 36 °C or 42 °C respectively; n ≈ 10 per site per T accl). After an acclimation period of 30 days, heat tolerance and evaporative cooling capacity was quantified by exposing birds to progressively higher T a until they approached severe hyperthermia (body temperature [ T b ] = 44.5 °C; T a range: 38–54 °C). We measured metabolic rate and evaporative water loss using open flow-through respirometry, and T b using temperature-sensitive passive-integrated transponder tags. Hyperthermia threshold T a (T a,HT) was significantly higher and T b significantly lower in birds acclimated to the hottest T accl compared to those from milder acclimation treatments. Population (i.e., site of capture) was not a significant predictor of any thermoregulatory variables or hyperthermia threshold T a (T a,HT) after acclimation, revealing that the shape of reaction norms for heat tolerance and evaporative cooling capacity does not vary among these three populations. Unlabelled Image • We investigated flexibility in the ability of a passerine to handle high temperatures. • Birds from the hottest acclimation treatment had the highest heat tolerance. • Ability to handle high temperatures did not vary among populations after acclimation. • Heat tolerance reaction norms do not vary across a climatic gradient in this species. [ABSTRACT FROM AUTHOR]

Researchers in the Global South (GS, developing countries) make valuable contributions to the field of comparative physiology, but face economic and scientific disparities and several unique challenges compared with colleagues in the Global North (developed countries). This Perspective highlights some of the challenges, knowledge gaps and disparities in opportunity faced by GS researchers, especially those at early-career stages. We propose collaborative solutions to help address these issues, and advocate for promoting investment and cultural and societal change for a more inclusive research community. Additionally, we highlight the role of GS researchers in contributing expert knowledge on local biodiversity and the environment; this knowledge can help to shape the future of comparative physiology, allowing us to achieve a better understanding of the evolution of physiological mechanisms and to develop innovative solutions to environmental and biomedical challenges. With this Perspective, we hope to highlight the need to foster a more diverse, equitable and inclusive research landscape in comparative physiology; one that empowers GS scientists to address the global challenges associated with biodiversity loss, climate change and environmental pollution., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2024. Published by The Company of Biologists Ltd.)

Small birds inhabiting northern temperate and boreal latitudes typically increase metabolic rates during cold winters or acclimation to low air temperatures ( T accl ). Recent studies suggest considerable variation in patterns of seasonal metabolic acclimatization in birds from subtropical and tropical regions with milder winters, but there remains a dearth of acclimation studies investigating metabolic flexibility among lower-latitude birds. We used short-term thermal acclimation experiments to investigate phenotypic flexibility in basal metabolic rate (BMR), thermoneutral evaporative water loss (EWL) and summit metabolism ( M sum ) in three populations of white-browed sparrow-weavers ( Plocepasser mahali ) along a climatic and aridity gradient. We allocated individuals to one of three T accl treatments (5, 20 and 35°C; n =11 per population per T accl ) for 28 days, and measured post-acclimation BMR, EWL and M sum using flow-through respirometry. Our data reveal the expected pattern of lower BMR and EWL (∼12% and 25% lower, respectively) in birds at T accl =35°C compared with cooler T accl treatments, as observed in previous acclimation studies on subtropical birds. We found no variation in the reaction norms of BMR and EWL among populations in response to acclimation, suggesting previously documented differences in seasonal BMR acclimatization are the result of phenotypic flexibility. In contrast to higher-latitude species, M sum did not significantly vary in response to thermal acclimation. These findings support the idea that factors other than enhancing cold tolerance may be driving patterns of metabolic variation in subtropical birds., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2020. Published by The Company of Biologists Ltd.)