Trade-offs between activity and thermoregulation in a small carnivore, the least weasel Mustela nivalis

Increasing ambient temperatures trigger shifts in activity patterns and temporal partitioning in a large carnivore guild

Shifts in species' interactions are implicated as an important proximate cause underpinning climate-change-related extinction. However, there is little empirical evidence on the pathways through which climate conditions, such as ambient temperature, impact community dynamics. The timing of activities is a widespread behavioural adaptation to environmental variability, and temporal partitioning is a key mechanism that facilitates coexistence, especially within large carnivore communities. We investigated temperature impacts on community dynamics through its influence on the diel activity of, and temporal partitioning amongst, four sympatric species of African large carnivores: lions (Panthera leo), leopards (Panthera pardus), cheetahs (Acinonyx jubatus) and African wild dogs (Lycaon pictus). Activity of all species was shaped by a combination of light availability and temperature, with most species becoming more nocturnal and decreasing activity levels with increasing temperatures. A nocturnal shift was most pronounced in cheetahs, the most diurnal species during median temperatures. This shift increased temporal overlap between cheetahs and other carnivore species by up to 15.92%, highlighting the importance of considering the responses of interacting sympatric species when inferring climate impacts on ecosystems. Our study provides evidence that temperature can significantly affect temporal partitioning within a carnivore guild by generating asymmetrical behavioural responses amongst functionally similar species.

An anthropogenic landscape reduces the influence of climate conditions and moonlight on carnivore activity

Abstract

Abiotic factors are limitations that can affect animal activity and distribution, whether directly or indirectly. The objective of this study was to evaluate how abiotic factors influence the activity of two mustelid species inhabiting the same region but different habitats in NE Poland-pine marten inhabits forests and stone marten occupy built-up areas. From 1991 to 2016, we obtained 23,639 year-round observations of 15 pine martens and 8524 observations of 47 stone martens. We explore the influence of ambient temperature, snow cover depth and moonlight reaching the ground and interactions between these variables on the probability of martens' activity. The activity of pine martens living in natural habitats is more affected by climate conditions and moonlight than that of stone martens living in anthropogenic areas. Pine martens inhabiting forests increased activity when the ambient temperature was above 0 °C and snow cover was absent, as well as when the ambient temperature dropped to - 15 °C and snow cover depth was about 10 cm. Stone marten occupying anthropogenic areas did not reduce their activity if the temperature dropped. Variation of activity in relation to ambient conditions is probably related to pine martens' behavioural thermoregulation. The pine marten was active more frequently on bright nights, while moonlight intensity did not affect the activity of the stone marten. Our study concludes that complex interactions among abiotic factors concerning different habitats play a synergetic role in shaping carnivore activity and suggest that climate warming may affect the behaviour of both martens.

Significance statement

The survival and reproduction of animals depends on their activity, which is subject to various constraints. We studied the influence of climate conditions and moonlight intensity on the ground on the activity of pine and stone marten. We found that pine martens in natural habitat were greatly impacted by ambient conditions, whereas stone martens in built-up areas were less so. Natural habitats involve limitations related to harsh winters but may mitigate the effects of high temperatures. In contrast, animals living in built-up areas are exposed to higher temperatures in summer, which is of particular importance in the face of climate change. Our results show that the combination of several environmental factors affects animal behaviour and these factors have varying effects in various habitats.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00265-023-03331-9.

The fire of evolution: energy expenditure and ecology in primates and other endotherms

Total energy expenditure (TEE) represents the total energy allocated to growth, reproduction and body maintenance, as well as the energy expended on physical activity. Early experimental work in animal energetics focused on the costs of specific tasks (basal metabolic rate, locomotion, reproduction), while determination of TEE was limited to estimates from activity budgets or measurements of subjects confined to metabolic chambers. Advances in recent decades have enabled measures of TEE in free-living animals, challenging traditional additive approaches to understanding animal energy budgets. Variation in lifestyle and activity level can impact individuals' TEE on short time scales, but interspecific differences in TEE are largely shaped by evolution. Here, we review work on energy expenditure across the animal kingdom, with a particular focus on endotherms, and examine recent advances in primate energetics. Relative to other placental mammals, primates have low TEE, which may drive their slow pace of life and be an evolved response to the challenges presented by their ecologies and environments. TEE variation among hominoid primates appears to reflect adaptive shifts in energy throughput and allocation in response to ecological pressures. As the taxonomic breadth and depth of TEE data expand, we will be able to test additional hypotheses about how energy budgets are shaped by environmental pressures and explore the more proximal mechanisms that drive intra-specific variation in energy expenditure.

Differential Expression of Metabolism-Related Genes in Plateau Pika (Ochotona curzoniae) at Different Altitudes on the Qinghai–Tibet Plateau

According to life history theory, animals living in extreme environments have evolved specific behavioral and physiological strategies for survival. However, the genetic mechanisms underpinning these strategies are unclear. As the highest geographical unit on Earth, the Qinghai–Tibet Plateau is characterized by an extreme environment and climate. During long-term evolutionary processes, animals that inhabit the plateau have evolved specialized morphological and physiological traits. The plateau pika (Ochotona curzoniae), one of the native small mammals that evolved on the Qinghai–Tibet Plateau, has adapted well to this cold and hypoxic environment. To explore the genetic mechanisms underlying the physiological adaptations of plateau pika to extremely cold ambient temperatures, we measured the differences in resting metabolic rate (RMR) and metabolism-related gene expression in individuals inhabiting three distinct altitudes (i.e., 3,321, 3,663, and 4,194 m). Results showed that the body mass and RMR of plateau pika at high- and medium-altitudes were significantly higher than those at the low-altitude. The expression levels of peroxisome proliferator-activated receptor α (pparα), peroxisome proliferator-activated receptor-γ coactivator-1α (pgc-1α), and the PR domain-containing 16 (PRDM16) in white (WAT) and brown (BAT) adipose tissues of plateau pika from high- and medium-altitudes were significantly higher than in pika from the low-altitude region. The enhanced expression levels of pgc-1α and pparα genes in the WAT of pika at high-altitude showed that WAT underwent “browning” and increased thermogenic properties. An increase in the expression of uncoupling protein 1 (UCP1) in the BAT of pika at high altitude indicated that BAT increased their thermogenic properties. The gene expression levels of pparα and pgc-1α in skeletal muscles were significantly higher in high-altitude pika. Simultaneously, the expression of the sarcolipin (SLN) gene in skeletal muscles significantly increased in high-altitude pika. Our results suggest that plateau pika adapted to an extremely cold environment via browning WAT, thereby activating BAT and enhancing SLN expression to increase non-shivering thermogenesis. This study demonstrates that plateau pika can increase thermogenic gene expression and energy metabolism to adapt to the extreme environments on the plateau.

Effect of sex, age, and reproductive status on daily activity levels and activity patterns in jaguars (Panthera onca)

All animals, including carnivores, adapt their daily activity duration and distribution to satisfy food demands, breed, or avoid mortality risk. We used the kernel density method to estimate daily movement activity levels and movement activity patterns of jaguars in Hato Piñero, in Venezuelan Western Llanos, based on 3,656 jaguar detection time records from two and a half years of camera trapping. Jaguars were active for 11.7 h per day on average and exhibited mostly nocturnal and crepuscular activity pattern, however, with marked differences between sex/age/reproductive groups. Reproductive females had the highest daily activity level (13.2 h/day), followed by adult males (10.9 h/day), non-reproductive females (10.5 h/day), and cubs (8.7 h/day). Activity patterns also differed, with males and reproductive females having activity peaks at the same hours after sunset and before sunrise, cubs in the night and after sunrise, while non-reproductive females were most active during night hours. This study was the first to document the effect of sex, age, and reproductive status on daily level and activity pattern in the jaguar.

Size is not everything: differing activity and foraging patterns between the sexes in a monomorphic mammal

Abstract

Animals balance foraging with other activities, and activity patterns may differ between sexes due to differing physical requirements and reproductive investments. Sex-specific behavioural differences are common in sexually dimorphic mammals, but have received limited research attention in monomorphic mammals where the sexes are similar in body size. Eurasian beavers (Castor fiber) are obligate monogamous and monomorphic mammals and a good model species to study sex-specific differences. As females increase energy expenditure during reproduction, we hypothesized differing seasonal activity budgets, circadian activity rhythms and foraging patterns between male and reproducing female beavers. To test this hypothesis, we equipped adult beavers with VHF transmitters (N=41; 16 female, 25 male) and observed them throughout their active period at night from spring to late summer. Occurrence of their main activities (foraging, travelling and being in lodge) and use of food items (trees/shrubs, aquatic vegetation and herbs/grasses) were modelled to investigate sex-specific seasonal activity budgets and circadian activity rhythms. The sexes did not differ in time spent foraging across the season or night, but during spring, females resided more in the lodge and travelled less. Males and females both foraged on aquatic vegetation during spring, but females used this food source also during late summer, whereas males mostly foraged on trees/shrubs throughout the year. We conclude that seasonal activity budgets and foraging differ subtly between the sexes, which may relate to different energy budgets associated with reproduction and nutritional requirements. Such subtle seasonal behavioural adaptions may be vital for survival and reproduction of monomorphic species.

Significance statement

Activity budgets and foraging patterns of animals are key to their survival and may differ between males and females with different body sizes and physical requirements. In monomorphic species, where males and females have similar body sizes, fewer differences are expected, but may still be pronounced during certain times of the year. We modelled sex-specific seasonal activity budgets and circadian activity rhythms and use of food items in a monomorphic mammal, the Eurasian beaver. By treating season and time of day as a continuous variable rather than modelling differences within distinct predefined periods, we identified subtle sex-specific seasonal trends in activity budgets and use of food items.

Heat and dehydration induced oxidative damage and antioxidant defenses following incubator heat stress and a simulated heat wave in wild caught four-striped field mice Rhabdomys dilectus

Heat waves are known for their disastrous mass die-off effects due to dehydration and cell damage, but little is known about the non-lethal consequences of surviving severe heat exposure. Severe heat exposure can cause oxidative stress which can have negative consequences on animal cognition, reproduction and life expectancy. We investigated the current oxidative stress experienced by a mesic mouse species, the four striped field mouse, Rhabdomys dilectus through a heat wave simulation with ad lib water and a more severe temperature exposure with minimal water. Wild four striped field mice were caught between 2017 and 2019. We predicted that wild four striped field mice in the heat wave simulation would show less susceptibility to oxidative stress as compared to a more severe heat stress which is likely to occur in the future. Oxidative stress was determined in the liver, kidney and brain using malondialdehyde (MDA) and protein carbonyl (PC) as markers for oxidative damage, and superoxide dismutase (SOD) and total antioxidant capacity (TAC) as markers of antioxidant defense. Incubator heat stress was brought about by increasing the body temperatures of animals to 39-40.8°C for 6 hours. A heat wave (one hot day, followed by a 3-day heatwave) was simulated by using temperature cycle that wild four striped field mice would experience in their local habitat (determined through weather station data using temperature and humidity), with maximal ambient temperature of 39°C. The liver and kidney demonstrated no changes in the simulated heat wave, but the liver had significantly higher SOD activity and the kidney had significantly higher lipid peroxidation in the incubator experiment. Dehydration significantly contributed to the increase of these markers, as is evident from the decrease in body mass after the experiment. The brain only showed significantly higher lipid peroxidation following the simulated heat wave with no significant changes following the incubator experiment. The significant increase in lipid peroxidation was not correlated to body mass after the experiment. The magnitude and duration of heat stress, in conjunction with dehydration, played a critical role in the oxidative stress experienced by each tissue, with the results demonstrating the importance of measuring multiple tissues to determine the physiological state of an animal. Current heat waves in this species have the potential of causing oxidative stress in the brain with future heat waves to possibly stress the kidney and liver depending on the hydration state of animals.

Coevolution of body size and metabolic rate in vertebrates: a life-history perspective

Despite many decades of research, the allometric scaling of metabolic rates (MRs) remains poorly understood. Here, we argue that scaling exponents of these allometries do not themselves mirror one universal law of nature but instead statistically approximate the non-linearity of the relationship between MR and body mass. This 'statistical' view must be replaced with the life-history perspective that 'allows' organisms to evolve myriad different life strategies with distinct physiological features. We posit that the hypoallometric allometry of MRs (mass scaling with an exponent smaller than 1) is an indirect outcome of the selective pressure of ecological mortality on allocation 'decisions' that divide resources among growth, reproduction, and the basic metabolic costs of repair and maintenance reflected in the standard or basal metabolic rate (SMR or BMR), which are customarily subjected to allometric analyses. Those 'decisions' form a wealth of life-history variation that can be defined based on the axis dictated by ecological mortality and the axis governed by the efficiency of energy use. We link this variation as well as hypoallometric scaling to the mechanistic determinants of MR, such as metabolically inert component proportions, internal organ relative size and activity, cell size and cell membrane composition, and muscle contributions to dramatic metabolic shifts between the resting and active states. The multitude of mechanisms determining MR leads us to conclude that the quest for a single-cause explanation of the mass scaling of MRs is futile. We argue that an explanation based on the theory of life-history evolution is the best way forward.

Metabolic rate in common shrews is unaffected by temperature, leading to lower energetic costs through seasonal size reduction

Small endothermic mammals have high metabolisms, particularly at cold temperatures. In the light of this, some species have evolved a seemingly illogical strategy: they reduce the size of the brain and several organs to become even smaller in winter. To test how this morphological strategy affects energy consumption across seasonally shifting ambient temperatures, we measured oxygen consumption and behaviour in the three seasonal phenotypes of the common shrew (Sorex araneus), which differ in size by about 20%. Body mass was the main driver of oxygen consumption, not the reduction of metabolically expensive brain mass. Against our expectations, we found no change in relative oxygen consumption with low ambient temperature. Thus, smaller body size in winter resulted in significant absolute energy savings. This finding could only partly be explained by an increase of lower cost behaviours in the activity budgets. Our findings highlight that these shrews manage to avoid one of the most fundamental and intuitive rules of ecology, allowing them to subsist with lower resource availability and successfully survive the harsh conditions of winter.

Winter activity patterns of the North American badger (Taxidea taxus) at its northwestern periphery

The North American badger (Taxidea taxus) is a nonhibernating carnivore that occurs in areas with highly seasonal climates, such as in Canada where the animal reaches its northern limits. There, winter climate is harsh and conventional habitat is limited and patchy, possibly leading to additional energetic constraints. Using radiotelemetry and remote cameras, we documented winter activities of 16 badgers (12 females, 4 males) between 2007 and 2011 in British Columbia, at the northwestern periphery of the species’ range. In comparison to their summer ecology, the animals reduced home range sizes by up to 98% and showed fidelity to particular burrows. Burrow fidelity was variable (mean 27 days; range 0–108) and coincided with reduction in body temperatures, suggesting the onset of torpor. Despite lengthy use of the same burrow, badgers emerged frequently and conducted excursions that appeared to represent foraging behavior. Burrow emergence was not related to ambient temperature or snow depth; rather, it was best explained by the amount of days elapsed since the start of winter. Despite these trends, winter activity also was variable among individual badgers. It may be that limited fat reserves and higher energetic costs in northwestern ranges require winter foraging activities.

Climate and landscape changes as driving forces for future range shift in southern populations of the European badger

Human-Induced Rapid Environmental Change (HIREC), particularly climate change and habitat conversion, affects species distributions worldwide. Here, we aimed to (i) assess the factors that determine range patterns of European badger (Meles meles) at the southwestern edge of their distribution and (ii) forecast the possible impacts of future climate and landcover changes on those patterns. We surveyed 272 cells of 5 × 5 km, to assess badger presence and confirmed its occurrence in 95 cells (35%). Our models estimate that badger’s presence is promoted by the occurrence of herbaceous fields and shrublands (5%–10%), and low proportions of Eucalyptus plantations (<~15%). Regions with >50% of podzols and eruptive rocks, higher sheep/goat density (>4 ind/km²), an absence of cattle, intermediate precipitation regimes (800–1000 mm/year) and mild mean temperatures (15–16 °C) are also more likely to host badgers. We predict a decrease in favourability of southern areas for hosting badgers under forecasted climate and landcover change scenarios, which may lead to a northwards retraction of the species southern distribution limit, but the overall landscape favourability is predicted to slightly increase. The forecasted retraction may affect community functional integrity, as its role in southern ecological networks will be vacant.

Seasonal variation in energy expenditure in a rodent inhabiting a winter-rainfall desert

Animals that spend more energy than they obtain risk entering allostatic overload, reducing survival and fitness. They are predicted to adjust their daily energy expenditure (DEE) during periods of food scarcity. Adjustments of DEE to changes in food availability have been well-studied in species in temperate zones during winter, but less so in species enduring seasonal droughts. Likely mechanisms regulating DEE involve adjustments of activity and maintenance metabolism. Species that experience seasonal droughts and changes in food availability, like the African striped mouse (Rhabdomys pumilio), are appropriate model organisms to study the regulation of seasonal changes of DEE. We quantified DEE using the 'doubly labelled water' method, measured resting metabolic rate (RMR), and concomitantly determined activity levels using all-day focal observations of 69 free-living striped mice in the cold moist season with high food availability and the hot dry season with low food availability. Striped mice decreased their DEE in the food scarce dry season using multiple mechanisms, especially reductions in RMR, and reduced overall physical activity. This was further facilitated passively by reduced thermoregulatory costs. Our study demonstrates that animals reduce DEE via active and passive mechanisms in food-restricted environments, and highlights that several environmental factors should be considered simultaneously when aiming to understand how animals cope with harsh environments.

Climate change is affecting mortality of weasels due to camouflage mismatch

Direct phenological mismatch caused by climate change can occur in mammals that moult seasonally. Two colour morphs of the weasel Mustela nivalis (M. n.) occur sympatrically in Białowieża Forest (NE Poland) and differ in their winter pelage colour: white in M. n. nivalis and brown in M. n. vulgaris. Due to their small body size, weasels are vulnerable to attacks by a range of different predators; thus cryptic coat colour may increase their winter survival. By analysing trapping data, we found that the share of white subspecies in the weasel population inhabiting Białowieża Forest decreases with decreasing numbers of days with snow cover. This led us to hypothesise that selective predation pressure should favour one of the two phenotypes, according to the prevailing weather conditions in winter. A simple field experiment with weasel models (white and brown), exposed against different background colours, revealed that contrasting models faced significantly higher detection by predators. Our observations also confirmed earlier findings that the plasticity of moult in M. n. nivalis is very limited. This means that climate change will strongly influence the mortality of the nivalis-type due to prolonged camouflage mismatch, which will directly affect the abundance and geographical distribution of this subspecies.

Profound seasonal shrinking and regrowth of the ossified braincase in phylogenetically distant mammals with similar life histories

Ontogenetic changes in skull shape and size are ubiquitous in altricial vertebrates, but typically unidirectional and minimal in full-grown animals. Red-toothed shrews exhibit a rare exception, where the shape, mass and size of the skull, brain, and several major organs, show significant bidirectional seasonal changes. We now show a similar but male-biased shrinking (16%) and regrowth (8%) in the standardized braincase depth of least weasels (Mustela nivalis). Juvenile weasels also exhibit a growth overshoot, followed by a shrinkage period lasting until the end of their first winter. Only male weasels then regrow during their second summer. High-resolution CT scans suggest areas of the skull are affected differently during shrinking and regrowth in both species. This suggests multiple evolutionary drivers: while the shrinking likely facilitates survival during seasonal low resource availability in these high-metabolic mammals with year-round activity, the regrowth may be most strongly influenced by high investment into reproduction and territories, which is male-biased in the weasels. Our data provide evidence for convergent evolution of skull and thus brain shrinkage and regrowth, with important implications for understanding adaptations to changing environments and for applied research on the correlated changes in bone structure, brain size and the many other affected organs.

Growth overshoot and seasonal size changes in the skulls of two weasel species

Ontogenetic changes in mammalian skulls are complex. For a very few species (i.e. some Sorex shrews), these also include seasonally driven, bidirectional size changes within individuals, presumably to reduce energy requirements during low resource availabilities. These patterns are poorly understood, but are likely most pronounced in high-metabolic species with limited means for energy conservation. We used generalized additive models to quantify the effect of location, Julian day, age and sex on the length and depth of 512 and 847 skulls of stoat (Mustela erminea) and weasel (M. nivalis) specimens collected throughout the northern hemisphere. Skull length of both species varies between sexes and geographically, with stoat skull length positively correlated with latitude. Both species demonstrate seasonal and ontogenetic patterns, including a rare, absolute growth overshoot in juvenile braincase depth. Standardized braincase depths of both species peak in their first summer, then decrease in their first winter, followed by a remarkable regrowth that peaks again during their second summer. This seasonal pattern varies in magnitude and timing between geographical regions and the sexes, matching predictions of Dehnel's phenomenon. This suggests implications for the evolution of over-wintering strategies in mammals, justifying further research on their mechanisms and value, with implications for applied osteology research.

Thyroid hormones correlate with field metabolic rate in ponies, Equus ferus caballus

During winter free living herbivores are often exposed to reduced energy supply at the same time that energy needs for thermoregulation increase. Several wild herbivores as well as robust horse breeds reduce their metabolism during times of low ambient temperature and food shortage. Thyroid hormones (TH) affect metabolic intensity and a positive effect of TH on basal metabolic rate (BMR) has been demonstrated in mammals and birds. As BMR and field metabolic rate (FMR) are often assumed to be intrinsically linked, TH may represent a reliable indicator for FMR. To test this hypothesis, ten Shetland pony mares were kept under semi-extensive central European conditions. During the winter month one group was fed 60% and one group 100% of their maintenance energy requirements. We measured FMR, locomotor activity, resting heart rate and TH levels in summer and winter. FMR, locomotor activity, resting heart rate and total T3 concentrations decreased substantially in winter compared to summer, whereas total T4 increased. Feed restriction led to a reduced FMR and resting heart rate, while TH and locomotor activity were not affected. Across both seasons FMR, resting heart and locomotor activity were positively correlated with total T3 but negatively and more weakly related with total T4.

Thermoregulation in endotherms: physiological principles and ecological consequences

In a seminal study published nearly 70 years ago, Scholander et al. (Biol Bull 99:259-271, 1950) employed Newton's law of cooling to describe how metabolic rates (MR) in birds and mammals vary predictably with ambient temperature (T a). Here, we explore the theoretical consequences of Newton's law of cooling and show that a thermoregulatory polygon provides an intuitively simple and yet useful description of thermoregulatory responses in endothermic organisms. This polygon encapsulates the region in which heat production and dissipation are in equilibrium and, therefore, the range of conditions in which thermoregulation is possible. Whereas the typical U-shaped curve describes the relationship between T a and MR at rest, thermoregulatory polygons expand this framework to incorporate the impact of activity, other behaviors and environmental conditions on thermoregulation and energy balance. We discuss how this framework can be employed to study the limits to effective thermoregulation and their ecological repercussions, allometric effects and residual variation in MR and thermal insulation, and how thermoregulatory requirements might constrain locomotor or reproductive performance (as proposed, for instance, by the heat dissipation limit theory). In many systems the limited empirical knowledge on how organismal traits may respond to environmental changes prevents physiological ecology from becoming a fully developed predictive science. In endotherms, however, we contend that the lack of theoretical developments that translate current physiological understanding into formal mechanistic models remains the main impediment to study the ecological and evolutionary repercussions of thermoregulation. In spite of the inherent limitations of Newton's law of cooling as an oversimplified description of the mechanics of heat transfer, we argue that understanding how systems that obey this approximation work can be enlightening on conceptual grounds and relevant as an analytical and predictive tool to study ecological phenomena. As such, the proposed approach may constitute a powerful tool to study the impact of thermoregulatory constraints on variables related to fitness, such as survival and reproductive output, and help elucidating how species will be affected by ongoing climate change.

Seasonal variation of resting metabolic rate and body mass in free-living weasels Mustela nivalis

Metabolic rates and body mass of mammals vary seasonally along with ambient temperatures and food availability. At the population level, seasonal changes in metabolic rate and mass can be due to selective mortality or emigration of individuals whose metabolic rate or mass differs from the average for the population. Alternatively, the metabolic rates of individuals can change seasonally, such that the population average increases or decreases due to shifts in the physiology of the overall population. The latter implies that individuals respond in a similar manner to changing seasonal conditions. We studied seasonal changes in body mass (BM) and resting metabolic rate (RMR) in free-ranging male weasels (Mustela nivalis) to test the consistency of these traits in individuals caught in different seasons of the year. At the population level, BM was remarkably stable across the seasons (F(3, 124)=0.25, P=0.9). In contrast, BM- corrected RMR varied significantly between seasons and was the lowest in winter (F(3, 135)=9.13, P<0.0001). We demonstrated that individual weasels were consistent in how their BM and RMR deviated from the seasonal means for the population (intraclass correlation, τ=0.78 and 0.33, respectively). This variation among individuals explained ~76% and 27% of the total variation of BM and basal metabolic rate, respectively. Hence, the relatively constant BM at the population level across seasons is due to a relative constancy of BM in individuals. Our study is one of relatively few research projects that demonstrate that seasonal changes in RMR observed in the wild population are in part due to a consistency in individual responses to changing environmental conditions.

Male weasels decrease activity and energy expenditure in response to high ambient temperatures

The heat dissipation limit (HDL) hypothesis suggests that the capacity of endotherms to dissipate body heat may impose constraints on their energy expenditure. Specifically, this hypothesis predicts that endotherms should avoid the detrimental consequences of hyperthermia by lowering their energy expenditure and reducing their activity in response to high ambient temperatures (T(a)). We used an extensive data set on the daily energy expenditure (DEE, n = 27) and the daily activity time (AT, n = 48) of male weasels (Mustela nivalis) during the spring and summer breeding season to test these predictions. We found that T(a) was related in a "hump-shaped" (i.e. convex) manner to AT, DEE, resting metabolic rate (RMR) and metabolic scope (the ratio of DEE to RMR). These results support the HDL hypothesis because in response to warm Tas male weasels reduced their AT, DEE, and RMR. Although the activity and energy expenditure of large endotherms are most likely to be constrained in response to warm Tas because they are less able to dissipate heat, our results suggest that small endotherms may also experience constraints consistent with the HDL hypothesis.

Environmental and genetic influences on body mass and resting metabolic rates (RMR) in a natural population of weasel Mustela nivalis

Body mass (BM) and resting metabolic rates (RMR) are two inexorably linked traits strongly related to mammalian life histories. Yet, there have been no studies attempting to estimate heritable variation and covariation of BM and RMR in natural populations. We used a marker-based approach to construct a pedigree and then the 'animal model' to estimate narrow sense heritability (h(2) ) of these traits in a free-living population of weasels Mustela nivalis--a small carnivore characterised by a wide range of BM and extremely high RMR. The most important factors affecting BM of weasels were sex and habitat type, whereas RMR was significantly affected only by seasonal variation of this trait. All environmental factors had only small effect on estimates of additive genetic variance of both BM and RMR. The amount of additive genetic variance associated with BM and estimates of heritability were high and significant in males (h(2)  = 0.61), but low and not significant in females (h(2 ) =( ) 0.32), probably due to small sample size for the latter sex. The results from the two-trait model revealed significant phenotypic (r(P)  = 0.62) and genetic correlation (r(A)  = 0.89) between BM and whole body RMR. The estimate of heritability of whole body RMR (0.54) and BM corrected RMR (0.45) were lower than estimates of heritability for BM. Both phenotypic and genetic correlations between BM corrected RMR and BM had negative signals (r(P)  = -0.42 and r(A)  = -0.58). Our results indicate that total energy expenditures of individuals can quickly evolve through concerted changes in BM and RMR.

Is there a linkage between metabolism and personality in small mammals? The root vole (Microtus oeconomus) example

Significant inter-individual variation in the rate of animal metabolism is a widespread phenomenon that has started to accumulate general interest. Here we follow recent calls to focus on linkage between the variation in energy metabolism and animal personality. By using wild caught root voles as a study species, we examined the relationship between the behavioral patterns (assessed in open field test) and resting metabolic rate (RMR), both of which are known to show large individual differences and intra-individual consistency in voles. Our results showed only a weak relationship between personality traits and metabolism, since the most parsimonious model (according to AICc) explaining RMR included only body mass and season as factors (explaining 84.8% of variation in RMR). However, the next two alternative models (within ΔAICc=2) also included the personality trait reflecting proactive behaviors (PC1) in addition to body mass, sex and season (85.2 and 85.8% of RMR variance explained, respectively). In all, our study does not provide compelling support for recent ideas of close linkage between behavior and metabolism. Still, our study highlights that even in the case of wild caught individuals, when behavior and metabolism often carry effects of both intrinsic and extrinsic conditions, the potential metabolic effects of varying energetically costly behaviors cannot be neglected.

Effect of energetic constraints on distribution and winter survival of weasel males

1. The absolute energy needs of small animals are generally lower than those of larger animals. This should drive higher mortality of larger animals, when the environmental conditions deteriorate. However, demonstration of the effect of energy constraints on survivals proved difficult, because the range of body mass within species is generally too small to produce enough variation for studying such an effect. An opportunity for an intraspecific study comes from weasels inhabiting the Białowieża Forest (north-eastern Poland), which are characterized by a threefold variation in body mass. 2. We assumed that in summer larger weasel males are favoured by sexual selection, because they are more successful when competing for mates. We then tested whether they suffer higher mortality in winter, because they have difficulty finding sufficient food to satisfy their energy needs and/or because the additional foraging time would result in increased exposure to predation. 3. We measured daily energy expenditures (DEE) of overwintering weasel males using the doubly labelled water (DLW) technique. We constructed an energetic model predicting how individuals of different size are able to balance their energy budgets feeding on large and small prey while minimizing time spent hunting, thereby reducing their own exposure to predation. 4. The range of body mass in overwintering weasels predicted by our model corresponded very well with the distribution of prey body mass in three different habitats within our study area. Larger individuals were able to compensate for higher food requirements by using habitats with larger prey species than those available to smaller male weasels. This effectively offset the expected negative association between body mass and winter survival predicted from considerations of energy balance. 5. Our results show how energetic constraints affect body mass and spatial segregation of a species at the intra-specific level not only across large geographical ranges, but also within a relatively small area.

Thermal physiology and energetics in male desert hamsters (Phodopus roborovskii) during cold acclimation

The adjustments in thermal physiology and energetics were investigated in male desert hamsters (Phodopus roborovskii) which were acclimated to 5°C for 4 weeks. Mean core body temperature in cold acclimated animals decreased by 0.21°C compared with controls. Further analysis revealed that the decrease mainly occurred in the scotophase, while in the photophase core body temperature remained constant during the whole cold acclimation. Thermogenic capacity, represented by resting metabolic rate and nonshivering thermogenesis increased in cold acclimated hamsters from initial values of 1.38 ± 0.05 and 5.32 ± 0.30 to 1.77 ± 0.08 and 8.79 ± 0.31 mlO(2) g(-1) h(-1), respectively. After cold acclimation, desert hamsters maintained a relative stable body mass of 21.7 ± 0.1 g very similar to the controls kept at 23°C (21.8 ± 0.1 g). The mean values of food intake and digestible energy (metabolisable energy) in cold acclimated hamsters were 5.3 ± 0.1 g day(-1) and 76.3 ± 0.9 kJ day(-1) (74.8 ± 0.9), respectively, which were significantly elevated by 76.7 and 80.4% compared to that in control group. The apparent digestibility was 81.0 ± 0.3% in cold acclimated animals which was also higher than the 79.7 ± 0.2% observed in controls. This increase corresponded with adaptive adjustments in morphology of digestive tracts with 20.2 and 36.8% increases in total length and wet mass, respectively. Body fat mass and serum leptin levels in cold acclimated hamsters decreased by 40.7 and 67.1%, respectively. The wheel running turns and the onset of wheel running remained unchanged. Our study indicated that desert hamsters remained very active during cold acclimation and displayed adaptive changes in thermal physiology and energy metabolism, such as enhanced thermogenic and energy processing capacities.

Maximal heat dissipation capacity and hyperthermia risk: neglected key factors in the ecology of endotherms

1. The role of energy in ecological processes has hitherto been considered primarily from the standpoint that energy supply is limited. That is, traditional resource-based ecological and evolutionary theories and the recent 'metabolic theory of ecology' (MTE) all assume that energetic constraints operate on the supply side of the energy balance equation. 2. For endothermic animals, we provide evidence suggesting that an upper boundary on total energy expenditure is imposed by the maximal capacity to dissipate body heat and therefore avoid the detrimental consequences of hyperthermia--the heat dissipation limit (HDL) theory. We contend that the HDL is a major constraint operating on the expenditure side of the energy balance equation, and that processes that generate heat compete and trade-off within a total boundary defined by heat dissipation capacity, rather than competing for limited energy supply. 3. The HDL theory predicts that daily energy expenditure should scale in relation to body mass (M(b)) with an exponent of about 0.63. This contrasts the prediction of the MTE of an exponent of 0.75. 4. We compiled empirical data on field metabolic rate (FMR) measured by the doubly-labelled water method, and found that they scale to M(b) with exponents of 0.647 in mammals and 0.658 in birds, not significantly different from the HDL prediction (P > 0.05) but lower than predicted by the MTE (P < 0.001). The same statistical result was obtained using phylogenetically independent contrasts analysis. Quantitative predictions of the model matched the empirical data for both mammals and birds. There was no indication of curvature in the relationship between Log(e) FMR and Log(e)M(b). 5. Together, these data provide strong support for the HDL theory and allow us to reject the MTE, at least when applied to endothermic animals. 6. The HDL theory provides a novel conceptual framework that demands a reframing of our views of the interplay between energy and the environment in endothermic animals, and provides many new interpretations of ecological and evolutionary phenomena.

Evidence for an intrinsic energetic ceiling in free-ranging kittiwakes Rissa tridactyla

1. The rate at which free-living animals can expend energy is limited but the causes of this limitation are not well understood. Theoretically, energy expenditure may be intrinsically limited by physiological properties of the animal constraining its capacity to process energy. Alternatively, the limitation could be set extrinsically by the amount of energy available in the environment or by a fitness trade-off in terms of reduced future survival associated with elevated metabolism. 2. We measured daily energy expenditure (DEE) using the doubly labelled water method in chick-rearing black-legged kittiwakes (Rissa tridactyla) at a study site close to the northern limit of their breeding range over 5 years. We measured breeding success, foraging trip duration and diet composition as proxies of resource availability during these years and estimated the probability of parent kittiwakes to return to the colony in relation to their energy expenditure in order to determine whether kittiwakes adjust their DEE in response to variation in prey availability and whether elevated DEE is associated with a decrease in adult survival. 3. We found that DEE was strikingly similar across all five study years. There was no evidence that energy expenditure was limited by resource availability that varied considerably among study years. Furthermore, there was no evidence of a negative effect of DEE on adult return rate, which does not support the hypothesis of a survival cost connected to elevated energy expenditure. 4. The additional lack of variation in DEE with respect to ambient temperature, brood size or between sexes suggests that kittiwakes at a time of peak energy demands may operate close to an intrinsic metabolic ceiling independent of extrinsic factors.