The smallest flying vertebrate pollinators, including hummingbirds and nectar bats, exist at an energetic extreme. Nectarivores must balance the need for high rates of metabolic power output to sustain comparatively high costs of thermoregulation and the intense energetic demands of forward and hovering flight with the constraint against building and carrying large, heavy energy stores. Work over the past half century has quantified metabolic rate and daily energy requirements for these animals and revealed that hummingbirds and nectar bats achieve energy homeostasis through exceptional physiological flexibility. They can rapidly and completely switch from fueling costly flight with lipid oxidation, when fasted, to oxidizing nectar sugar, ingested minutes prior, at rates that completely support hovering while foraging. This physiological capacity for rapid flux and oxidation of dietary sugar to completely fuel intense exercise, termed the 'sugar oxidation cascade', stands in stark contrast to models of fuel use in running mammals. Remarkably, the capacity for rapid absorption and oxidation of fructose is as elevated in hummingbirds and nectar bats as is their capacity to use glucose. Here, we review insights into convergently and divergently evolved features of the sugar oxidation cascade among hummingbirds and nectar bats, as revealed by advances in comparative genomic, molecular and biochemical techniques. We then review available evidence and hypothesize that additional groups of nectar and fruit-eating bats and birds exhibit similar fuel use patterns during exercise, and we call on researchers to develop techniques to assess fuel use during forward flight in these non-hovering taxa.

Allochrony can be a cause or consequence of speciation, either creating temporal reproductive isolation that reduces gene flow between diverging populations or reinforcing divergence that has already occurred through geographic isolation. The former appears to apply to band-rumped storm-petrels (Hydrobates castro) at some breeding sites, where there are genetically differentiated hot and cool season breeding populations. It is unclear, however, whether seasonally segregated but genetically similar populations retain the same habitat preferences or whether divergence in foraging behaviour is associated with the process of allochronic speciation. We quantified the foraging distribution of band-rumped storm-petrels at St Helena, the largest known breeding colony in the South Atlantic at which hot and cool season breeders do not appear to be genetically differentiated. Fifty-four GPS tags were deployed on experienced breeders across two hot and two cool breeding seasons. We compared foraging trip parameters, foraging effort and examined whether environmental (oceanographic and atmospheric) conditions and habitat selection varied between seasonal populations. Long foraging trips lasted up to 9 days and involved travel distances of up to 3,285 km. The trip durations and distances were similar between the two seasonal populations, but directions differed markedly, resulting in pronounced differences in at-sea distributions. Adults breeding in the cool season foraged across ~ 619,000 km2 southeast of St Helena selecting warmer waters (~ 23.1 ± 0.7 °C). In the hot season, adults used a similarly sized area (~ 600,000 km2) to the southwest, but selected cooler waters (~ 21.2 ± 0.4 °C) even though overall conditions at unused but available locations were warmer (~ 23.7 ± 0.7 °C) than in the cool season (~ 20.6 ± 0.5 °C). Seasonal differences in oceanographic conditions likely force hot season breeders to select cool nutrient-rich waters, whereas cool season breeders may select wind or temperature conditions that minimise travel or thermoregulatory costs. This clear segregation in foraging range and habitat selection suggests that the divergence in at-sea distributions between two genetically similar seasonal breeding populations may contribute to allochrony and ultimately to sympatric speciation in the band-rumped storm-petrel at St Helena and elsewhere.

Ongoing environmental changes are affecting behavioral responses of animal populations. Both warming temperatures and increased human disturbance may trigger adjustments in mammal activity patterns, for example, favoring activity switch to nighttime despite a greater risk of encountering nocturnal predators. Disentangling the relative roles of these stressors is critical for predicting the population-level consequences of environmental changes, yet the joint effect of multiple stressors is poorly understood. Here we investigated how ambient summer temperature, predators, and human presence influenced temporal responses in two herbivorous mammals (the roe deer Capreolus capreolus and the fallow deer Dama dama) across Mediterranean protected areas. By conducting intensive camera trapping (∼12,400 trapping days; 196 sites), we evaluated changes in daily activity level and nocturnality of deer species. Both herbivores reduced their daily activity with warmer temperatures, emphasizing the need to minimize thermoregulatory costs, yet only roe deer increased nocturnality following diel warming. Conversely, nocturnality of the more heat-tolerant fallow deer was only affected by wolf (Canis lupus) visitation rate, although weakly, suggesting that fallow deer traded off heat avoidance with predator avoidance. We found neither reductions in daily activity levels nor an increase in nocturnality in response to higher human visitation rate, possibly depending on our relatively undisturbed protected areas (i.e., areas with low human population density and sustainable levels of outdoor recreational activities) or the stronger effect of heat avoidance. Under the anticipated warming, species-specific consequences of these behavioral responses on population viability may be expected.

Global climate change is increasing both average temperatures and the occurrence of extreme weather events, such as heatwaves. These changes pose major challenges for wildlife, especially during vulnerable life stages. In altricial bird species, nest-bound offspring are particularly susceptible to thermal stress due to limited behavioural thermoregulation, where high nest temperatures can impair development and induce elevated mortality, particularly when exceeding critical thresholds. We experimentally manipulated nest temperature and food availability in lesser kestrels (Falco naumanni) rearing their nestlings in nest boxes to investigate whether compensatory feeding offsets the negative effects of high nest temperatures on offspring size and pre-fledging survival. Nestlings were reared under three conditions: control (high nest temperatures), shaded (reduced temperature), and food-supplemented (high temperature with extra food). While parental provisioning rates did not differ among experimental groups, nestlings in food-supplemented boxes showed greater body size than control nestlings, despite similar thermal exposure, and matched the size of nestlings in shaded boxes. This indicates that increased food can buffer the energetic costs of thermoregulation under elevated, but non-lethal, temperatures. Mortality during development did not differ significantly among groups, as temperatures remained below critical thresholds. However, during a naturally occurring heatwave, mortality in control and food-supplemented nests was similarly high (∼40%), whereas it was very limited in shaded nest boxes (∼4%). These results suggest that food supplementation cannot prevent heatwave-induced mortality, likely caused by hyperthermia rather than starvation/dehydration. Nest microclimate may thus play a key role in mitigating the negative impacts of intense heatwaves in avian species.

Metabolic rates in animals scale allometrically with body mass, a relationship well-established in terrestrial mammals. Whether these scaling laws apply to fully aquatic mammals remains uncertain, due to key physiological and ecological differences. We estimated field metabolic rates (FMRs) for five sympatric cetaceans of varying sizes, inhabiting sub-Arctic Icelandic waters: harbour porpoises (Phocoena phocoena; mean body length ± s.d = 1.35 ± 0.19m), white-beaked dolphins (Lagenorhynchus albirostris; 2.42 ± 0.17m), minke whales (Balaenoptera acutorostrata; 7.53 ± 0.82m), humpback whales (Megaptera novaeangliae; 9.44 ± 1.13m) and blue whales (Balaenoptera musculus; 21.97 ± 0.96m). Unoccupied Aerial Vehicle (UAV) photogrammetry and published data were used to estimate body size, while respiration rates (breathes min-1) were obtained from UAV focal follows, biologging tags, and literature sources. From these data we predicted daily FMRs (MJ day-1) using existing bioenergetic models. As expected, mass-specific FMR declined with increasing body size among species, consistent with scaling laws. However, FMRs across all species were elevated relative to terrestrial predictions, likely reflecting the greater energetic demands of aquatic life. FMR also scaled positively with the surface-area-to-volume ratio (SVR) of each species, supporting the hypothesis that thermoregulatory costs are driven by body shape and size, and influence energy expenditure. This was further supported by the positive relationship between FMR and heat loss rates. Overall, our findings suggest that large mysticetes benefit from reduced mass-specific FMRs, enabling long migrations and extended fasting that broaden their habitat use. Smaller cetaceans face higher metabolic demands and may be more dependent on smaller, prey-rich habitats. These size-dependent energetic constraints may influence species plasticity and vulnerability to environmental changes.

Restricted elevational ranges are common across tropical montane species, but the mechanisms generating and maintaining these patterns remain poorly resolved. A long-standing hypothesis is that specialized thermal physiology explains these distributions. However, biotic factors such as habitat and interspecific competition have also been proposed to limit tropical species’ elevational ranges. We combined point-level abundances, respirometry-based measurements of metabolic rate, habitat surveys and playback experiments to simultaneously test these three hypotheses for four species of Central American cloud forest songbirds. Contrary to the physiological hypothesis, we found no evidence that thermoregulatory costs constrain species distributions. Instead, thermal conditions across each species’ elevational range remained well within sustainable limits, staying ≤65% of hypothesized thresholds for tropical birds, even at the highest elevations. By contrast, we found some support for a combined role of habitat and competition in shaping elevational ranges. In one related species pair, the dominant lower-elevation species appears restricted by microhabitat, while the higher-elevation species is likely prevented from expanding downslope by the presence of this congener. Taken together, we conclude that thermoregulatory costs are an inadequate explanation for elevational range limits of tropical birds at our site and suggest that biotic factors can be key in shaping these distributions. We provide a Spanish translation of the in the supplementary materials.

Abstract Primates exhibit thermal behaviors (use of postures in tree stratum) that help regulate body temperature in response to environmental conditions, reducing the energetic and water costs of thermoregulation. This study examined the relationship between ambient temperature and thermal behavioral strategies during resting periods in mantled howler monkeys (Alouatta palliata) across 3 sites in Veracruz, Mexico: Agaltepec (AGA; 28.4 ± 4.7 °C), Mirador Pilapa (MP; 28.8 ± 2.1 °C), and Zapoapan (ZAP; 28.9 ± 2.5 °C). We used focal-animal sampling, randomly selecting individuals and recording the time invested in each posture and the tree stratum in which they rested, and the ambient temperature. All statistical analyses were performed independently between sites. The results indicate that, across the 3 study sites, a consistent pattern of posture adopted within specific tree strata was observed, despite differences in altitude, vegetation, and ambient temperature. We found a strong positive correlation between lower temperatures and the time recorded in the semi-fetal posture in the high strata (Rc > 0.7547), and between higher temperatures and the time recorded in the extended posture in the low strata (Rc > 0.6803). These results suggest howler monkeys combine posture and vertical positioning to optimize thermoregulation. However, as temperatures rise, the preference for extended postures in lower strata—where temperatures may be cooler—leads to longer rest periods. This behavioral shift could reduce feeding and hydration, potentially increasing the risk of dehydration or heat stress under future climate conditions.

Phenotypes reflect the adaptations of organisms to their environments, with common rules defining how coloration and body size should vary based on climate and latitude. Hong Kong (HK) cattle present an opportunity to study these adaptations in one of the very few cattle populations not directly controlled by humans. These cattle are free-ranging in a subtropical climate, characterized by high humidity and temperatures during the wet season, and scarce precipitation during the dry season. We studied seasonal coat colour changes in HK feral cattle, and sexual dimorphism in body size and horn length. We provide the first evidence of seasonal changes in coat colour in cattle, with paler coats being more common in the wet season, while darker coats prevailed in the dry season. These seasonal changes were influenced by temperature, wind speed, solar radiation and body condition. We found that males were larger and had longer horns than females. Our results show a male-biased sex dimorphism in the HK feral cattle. Additionally, our findings suggest that thermoregulation costs drive coloration in these cattle. The phenotypic plasticity we demonstrate in these subtropical feral cattle improves our knowledge of the adaptations of ungulates to their habitat.

Abstract Daily activity patterns of free-living subterranean rodents have often been associated with temperature fluctuations in shallow soil layers, but their seasonal variation has been understudied. We analysed activity data from free-living silvery mole-rats Heliophobius argenteocinereus radio-tracked over six periods, including the coldest and hottest periods of the year, to investigate whether their activity is concentrated during parts of the day when thermoregulation costs are presumed to be lowest. During the coldest period, mole-rat activity correlated most strongly and positively with the temperature at depths of 5–10 cm, corresponding to shallow burrow branches leading to mounds or food resources. In contrast, during the two hottest periods, activity was more closely and negatively related to temperatures at a depth of 25 cm. While the activity pattern detected during the coldest period aligns with the expected greater daily variation in thermoregulatory costs associated with burrowing closer to the soil surface, the patterns from the hottest periods are more difficult to interpret. We hypothesise that during the hottest periods of the year, mole-rats either (i) preferentially construct and use deeper burrow sections, adjusting their daily activity patterns to the small temperature fluctuations found there, or (ii) concentrate their activity to a part of the day when temperatures across a range of soil depths converge, provided these remain below their thermal neutral zone (TNZ).

ABSTRACTUnderstanding insectivorous bat diversity and activity is crucial for conservation efforts, particularly in under‐researched regions like sub‐tropical savannas. Our study assessed bat species richness and seasonal activity (i.e., number of passes) in MalaMala Game Reserve, located within the south‐western region of the Greater Kruger National Park (KNP), South Africa, a hitherto unsampled site. We conducted acoustic monitoring using ultrasonic detectors over two distinct seasons: the wet season (January to March) and the dry season (June to August) in 2022. Our findings revealed 16 species from six families, representing 40% of the bat species known in KNP. The Molossidae family was the most dominant, followed by Vespertilionidae, while the Hipposideridae family recorded the fewest calls, likely due to their high‐frequency echolocation calls, which attenuate rapidly, or possibly because these bats were less active in our study area, or a combination of both factors. Seasonal variations in bat activity were observed, with significantly higher activity during the wet season, likely due to increased insect abundance and reduced thermoregulatory costs. By targeting a previously unsurveyed region and incorporating seasonal comparisons, our study addresses a critical spatial and temporal knowledge gap and provides a valuable foundation for the development of long‐term, standardized bat monitoring across the Greater KNP landscape.

Increasing brain size is a hallmark of human evolution. While a larger brain offers evolutionary advantages driven by social and cognitive adaptations, it also imposes considerable energetic, metabolic, and thermoregulatory costs. As a result, brain size may have biological limits that impose survival pressures during periods of extreme environmental change. Here, temporal trends in absolute brain size across the genus Homo are analyzed, with a focus on a marked slowdown in growth beginning around 300,000years ago. The results suggest that strong directional selection for brain expansion in early Homo was followed by a shift toward stabilizing selection in later populations. Comparisons across glacial and interglacial periods indicate that the physiological costs of large brains may have become especially disadvantageous during warming interglacial periods in the last 100,000 years, potentially increasing extinction risk. This evolutionary shift coincides with the emergence of cognitive and cultural innovations-such as symbolic tools and language-that may have enabled cognitive offloading, reducing selective pressure for continued encephalization. Together, these findings support the hypothesis that stabilizing selection, mediated in part by behavioral and technological adaptations, buffered later Homo populations against the ecological and physiological costs associated with large brains.

Abstract Animal architecture has received considerable attention as an extended phenotype that buffers organisms against environmental harshness. However, few studies have described animal architecture in a representative sample, assessed and then tested its function experimentally. We adopted such an approach for stick lodges built by the bush Karoo rat (Otomys unisulcatus), a solitary rodent inhabiting semi-arid regions of southern Africa. These lodges are among the largest animal-built structures by solitary mammals, yet their structural variation and ecological function remain poorly quantified. We conducted the first population-level survey of all lodges in our 4.5 ha field site to assess external and internal lodge structure, assess their effects on microclimate, and tested experimentally whether lodge size affects microclimate. Lodges were several hundred times larger than their builders and featured complex structure, including basking platforms, nesting, food and latrine chambers. Inside lodges, temperatures were closer to the bush Karoo rat’s thermoneutral zone and exhibited higher humidity than ambient conditions, which could reduce costs of thermoregulation and reduce water loss. These benefits correlated with initial lodge size and decreased when lodge size was experimentally reduced. Tunnels were often blocked off with thorny branches, possibly to deter snakes. Platforms typically faced eastwards and were regularly used for basking, likely enhancing passive heat gain. We conclude that bush Karoo rat lodges are complex, and costly to build and maintain. They improve thermoregulation and provide predator protection and food storage. Bush Karoo rat lodges (1) are external structures, adding to the builder’s body (2) are functional, (3) probably increasing fitness, (4) result from behavioural activity, and (5) are likely genetically encoded. We therefore consider them an extended phenotype shaped by ecological pressures and evolutionary history.

Temperature profoundly influences the distribution and diversity of ectotherms, yet in natural settings, interactions between environmental temperatures, behaviour, physiological function and the influence of these factors on individual survival remain poorly understood. In particular, it is unclear as to how trade‐offs between these factors are optimised in wild, free‐ranging species.We combined temperature‐sensitive radio transmitters and accelerometers to measure in situ body temperatures and field‐based thermal locomotor performance, estimating thermal optimum and maximum performance. This allowed us to quantify the effectiveness of thermoregulation in the wild and determine whether seasonal trade‐offs in thermoregulatory behaviour shape thermal performance and influence survival in the Australian central bearded dragon (Pogona vitticeps).Lizards adjusted their behaviour to maintain optimal body temperatures, achieving greater thermoregulatory precision in spring and summer when environmental costs of thermoregulation were low, but reducing that precision in winter when costs were higher. Activity time and maximum locomotor performance were higher during seasons when thermoregulatory precision was high.Maximum locomotor performance in the field was a strong predictor of survival, regardless of sex, even though survival probabilities were higher in males than females. Higher locomotor performance was associated with increased mortality risk, but survival was not influenced by activity levels or thermoregulatory indices.These findings highlight the complex trade‐offs that ectotherms must navigate to balance behavioural thermoregulation and survival. Our data demonstrate the important influence of seasonal and sex‐specific variation on behaviour and fitness‐related outcomes. Interpreting field‐derived thermal performance curves alongside laboratory measures is crucial for distinguishing ‘true’ physiological capacity from the integrated ecological contexts that shape performance and fitness in nature. Such insights are vital for predicting how ectotherms may respond to future climate warming.

Abstract Energy is a central resource for life, but animals' energy budgets are limited by their ability to collect, process, and redistribute energetic assets to diverse functions, including behavior. Yet none of the existing bioenergetic models unequivocally explain behavioral variation, suggesting that they are not mutually exclusive or that they are incomplete and overlook important processes. To describe a means by which an endotherm regulates its metabolism and behavior in varying environments, we simultaneously analyzed several components of energetic budgets. To accurately assess the costs of body maintenance, thermoregulation, locomotory movement, and shyness of wild mice, we measured resting metabolism at different temperatures and repeatedly quantified basal metabolic rate (BMR) and behavior. At the among-individual level, we found no significant associations between personality traits and metabolic rates. At the within-individual level, we found that when including the most accurate estimates of BMR, higher BMR was associated with reduced shyness. The among-individual trends of the associations between thermoregulatory cost and behavior were stronger than those between BMR and behavior. Our results suggest that temperature affects shyness and that behavior and BMR may covary over time within individuals, reinforcing context dependence of energetic regulation of behavioral responses.

Abstract Endothermy is an energetically expensive trait, yet it has posed an evolutionary advantage across different lineages-a paradox that remains puzzling to biologists. Here, I investigate whether endothermy can evolve through life history optimization using a model of the balance between energy assimilation and energy allocation to somatic maintenance, thermoregulation, growth, or reproduction. The model displays bistable strategies when assimilation rates and thermoregulatory costs increase, respectively, exponentially and linearly with body temperature: the "heterothermic strategy" consists of minimizing the costs of thermoregulation by maintaining body temperature close to ambient temperature, and the "homeothermic strategy" consists of increasing body temperature until the costs of thermoregulation are fully compensated by the increased assimilation capacity at higher temperatures. These strategies produce similar fitness outcomes and thus emerge as alternative stable states of the system, maintained by strong stabilizing selection preventing transitions between them. Using quantitative genetics simulations, I show that a drop in ambient temperature may push populations toward an evolutionary branching point, enabling the rapid radiation of homeothermic lineages coupled with body size reductions. I thus propose that life history optimization of energy balance can explain the radiation of homeothermic endothermy associated with either climate cooling or migration to colder regions by early endothermic lineages.

The HDL (heat dissipation limitation) hypothesis posits that mammalian energy budgets (SuSMR, sustained metabolic rate) are limited by the ability to dissipate metabolic heat. The HDL hypothesis has been tested in lactating mice but rarely systematically differs in SuSMR. Here, we used lines of laboratory mice divergently selected for basal metabolic rate (BMR) and effectively co-selected for SuSMR. We exposed lactating females to 23 and 30°C and manipulated their heat dissipation abilities by fur shaving. Exposure to 30°C did not affect the high BMR mice's litter mass but increased litter mass in the low BMR mice. Fur shaving did not affect litter mass. However, it decreased body temperature (Tb) by 0.2°C in the shaved mice, independent of line affiliation and ambient temperature. In both lines exposed to 30°C, the Tb increased by 0.2°C, while daily energy expenditure (a proxy of SuSMR) decreased by 20% and still was higher in the high BMR mice. These results do not support the HDL hypothesis. Low SuSMR individuals may benefit from higher ambient temperatures because of reduced costs of thermoregulation. It may change the course of natural selection towards reducing SuSMR and BMR.

Small mammals inhabiting cold climates face high heat losses and thus, high energy demands for body temperature regulation. However, behavioral adaptations, such as seeking refuge from extreme cold in the subnivean space, can reduce the energetic cost of thermoregulation. Using automated cameras, we monitored collared lemmings (Dicrostonyx groenlandicus) at the northern limit of their range, as they surfaced from their snow burrows in spring, to assess the effect of weather variables on the occurrence of this behavior. We hypothesized that lemmings reduce the energetic cost of thermoregulation when they come to the surface of the snow in spring. As expected, the frequency of surface activity increased with air temperature, but decreased with cloud cover and wind speed. In addition, the operative temperature was higher above the snow than below, while snow profiles showed the absence of liquid water in the snowpack. These findings support that lemming surface activity in spring is a behavioral thermoregulation strategy. However, observations of several predators in the study area, combined with vigilance behavior observed in lemmings at the snow surface, stress the risks associated with such exposure on the snow. We therefore suggest that lemmings may face a trade-off between thermoregulation and predation risk. Given that data on lemming winter ecology are scarce, we also exploited this behavior to gather valuable knowledge on molting phenology and reproduction. Overall, our results provide insight into the complex trade-off between thermoregulation and other needs in small mammals inhabiting cold climates, and highlight some potential implications for arctic ecosystem dynamics based on predator-prey interactions.

Abstract Daily activity patterns of free-living subterranean rodents have often been associated with temperature fluctuations in shallow soil layers, but their seasonal variation has been poorly studied. We analysed activity data from free-living silvery mole-rats Heliophobius argenteocinereus radio-tracked over six periods, including the coldest and hottest periods of the year, to investigate whether their activity is concentrated into parts of the day when thermoregulation costs are supposed to be lowest. During the coldest period, mole-rat activity correlated most strongly with the temperature at depths of 5–10 cm (positively), corresponding to the superficially situated burrow branches leading to mounds or food resources located in the shallow soil layers. In contrast, during two hottest periods, activity was more closely related to temperatures at a depth of 25 cm (negatively). While the activity pattern detected during the coldest period aligns with the expected greater daily variation in thermoregulatory costs associated with burrowing closer to the soil surface, the patterns from the hottest periods are more difficult to interpret. We hypothesise that during the hottest periods of the year, mole-rats either (i) preferentially construct and use deeper burrow sections, adjusting their daily activity patterns to small temperature fluctuations found there, or (ii) concentrate their activity to a part of the day when temperatures across a range of soil depths converge, provided they remain below their thermal neutral zone (TNZ).

Energy can be limiting, especially for small animals with high metabolisms, particularly if they rely on ephemeral resources. Some energy-saving strategies, such as torpor, can impair physiological processes. Alternatively, group living can reduce energetic costs through social thermoregulation. This may allow individuals to maintain a high metabolism as well as processes such as gamete production. Although group living is common, its energetic benefits for heterothermic individuals during the season of sperm production have yet to be investigated. We remotely quantified the daily energy expenditure of individual parti-coloured bats (Vespertilio murinus) kept solitarily and in groups during the period of spermatogenesis, using high-resolution heart rate monitoring. The data showed that the energetic benefits of group living are complex. In groups, individual daily energy expenditure was more than 50% lower. Group roosting reduced the cost of thermoregulation during normothermia and allowed for a decrease in the depth but not the duration of torpor. Group living may enable bats to buffer unfavourable environmental conditions. Energy saved this way can then be invested in fitness-relevant processes, potentially making this a driver of the evolution of male sociality.

Animal thermoregulation may have significant costs and compete directly or indirectly with other energetically demanding processes, such as immune function. Although the subterranean environment is characterized by thermally stable conditions, small changes in ambient temperature could be critical in shaping immunity. However, little is known about the effects of ambient temperature, in naturally varying ranges, on immunity of wild species. Therefore, to evaluate the effect of short-term exposure to ambient temperatures on energy metabolism and body temperature during the acute phase immune response (APR) in the subterranean rodent Ctenomys talarum, 70 adult animals were divided into three experimental groups and exposed twice for 1 h to 15, 25 or 32°C (below, at or near the upper limit of the thermoneutral zone, respectively) before and after injection with saline (control) or lipopolysaccharide (LPS, which induces the APR). Animals exposed to 25 and 32°C showed a similar APR pattern, characterized by fever (average: 37.1 and 37.7°C, respectively), a 16% increase in O2 consumption and an increase in the neutrophil/lymphocyte ratio (N/L). Body mass loss and symptoms of sickness behavior were detected from 3 and 1 h post-injection, respectively. Individuals exposed to 15°C increased their metabolic rate by 60%, showed frequent hypothermia (34.3°C on average) and the characteristic N/L increase was attenuated. Body mass loss and sickness behavior were mostly detected 24 h post-injection. Our results suggest that the thermoregulation costs in C. talarum may limit the energy available for immunity, leading to different strategies to cope with infection.