An intra-population heterothermy continuum: notable repeatability of body temperature variation in food-deprived yellow-necked mice

Predation can shape the cascade interplay between heterothermy, exploration and maintenance metabolism under high food availability

Maintenance metabolism as the minimum energy expenditure needed to maintain homeothermy (a high and stable body temperature, T b), reflects the magnitude of metabolic machinery and the associated costs of self-maintenance in endotherms (organisms able to produce heat endogenously). Therefore, it can interact with most, if not all, organismal functions, including the behavior-fitness linkage. Many endothermic animals can avoid the costs of maintaining homeothermy and temporally reduce T b and metabolism by entering heterothermic states like torpor, the most effective energy-saving strategy. Variations in BMR, behavior, and torpor use are considered to be shaped by food resources, but those conclusions are based on research studying these traits in isolation. We tested the effect of ecological contexts (food availability and predation risk) on the interplay between the maintenance costs of homeothermy, heterothermy, and exploration in a wild mammal-the yellow-necked mouse. We measured maintenance metabolism as basal metabolic rate (BMR) using respirometry, distance moved (exploration) in the open-field test, and variation in T b (heterothermy) during short-term fasting in animals captured at different locations of known natural food availability and predator presence, and with or without supplementary food resources. We found that in winter, heterothermy and exploration (but not BMR) negatively correlated with natural food availability (determined in autumn). Supplementary feeding increased mouse density, predation risk and finally had a positive effect on heterothermy (but not on BMR or exploration). The path analysis testing plausible causal relationships between the studied traits indicated that elevated predation risk increased heterothermy, which in turn negatively affected exploration, which positively correlated with BMR. Our study indicates that adaptive heterothermy is a compensation strategy for balancing the energy budget in endothermic animals experiencing low natural food availability. This study also suggests that under environmental challenges like increased predation risk, the use of an effective energy-saving strategy predicts behavioral expression better than self-maintenance costs under homeothermy.

Modeling heterothermic fitness landscapes in a marsupial hibernator using changes in body composition

Hibernation is an adaptive strategy that allows animals to enter a hypometabolic state, conserving energy and enhancing their fitness by surviving harsh environmental conditions. However, addressing the adaptive value of hibernation, at the individual level and in natural populations, has been challenging. Here, we applied a non-invasive technique, body composition analysis by quantitative magnetic resonance (qMR), to calculate energy savings by hibernation in a population of hibernating marsupials (Dromiciops gliroides). Using outdoor enclosures installed in a temperate rainforest, and measuring qMR periodically, we determined the amount of fat and lean mass consumed during a whole hibernation cycle. With this information, we estimated the daily energy expenditure of hibernation (DEEH) at the individual level and related to previous fat accumulation. Using model selection approaches and phenotypic selection analysis, we calculated linear (directional, β), quadratic (stabilizing or disruptive, γ) and correlational (ρ) coefficients for DEEH and fat accumulation. We found significant, negative directional selection for DEEH (βDEEH = - 0.58 ± 0.09), a positive value for fat accumulation (βFAT = 0.34 ± 0.07), and positive correlational selection between both traits (ρDEEH × FAT = 0.24 ± 0.07). Then, individuals maximizing previous fat accumulation and minimizing DEEH were promoted by selection, which is visualized by a bi-variate selection surface estimated by generalized additive models. At the comparative level, results fall within the isometric allometry known for hibernation metabolic rate in mammals. Thus, by a combination of a non-invasive technique for body composition analysis and semi-natural enclosures, we were characterized the heterothermic fitness landscape in a semi-natural population of hibernators.

Energy allocation shifts from sperm production to self-maintenance at low temperatures in male bats

The ability of animals to produce endogenous heat provides a buffer against environmental changes but also incurs high energetic costs. Especially small endothermic mammals have high energy demands. Some temperate-zone species (heterotherms) regularly use torpor, which slows down their entire metabolism but also potentially delays reproduction, to compensate for this. We used a unique experimental approach to test the consequences of extended low and high ambient temperatures on the trade-off in energy allocation to body mass maintenance, thermoregulation effort and seasonal sexual maturation in temperate zone male bats. We showed that long exposure to low ambient temperature shifts energy allocation away from sexual maturation to self-maintenance and results in a delay of sperm maturation by as much as an entire month. This effect was partially buffered by higher body mass. Heavier bats were able to afford more intensive thermoregulation and consequently speed up maturation. Interestingly, bats at constant high temperatures avoided deep torpor and matured faster than those at low temperatures, but sperm production was also slower than under natural conditions. Our results show that not only low, but also constant high ambient temperatures are detrimental during seasonal sexual maturation and the trade-off between investing into self-maintenance and fitness is a finely tuned compromise.

Body temperature is a repeatable trait in a free-ranging passerine bird

Body temperature (Tb) affects animal function through its influence on rates of biochemical and biophysical reactions, the molecular structures of proteins and tissues, and, ultimately, organismal performance. Despite its importance in driving physiological processes, there are few data on how much variation in Tb exists within populations of organisms, and whether this variation consistently differs among individuals over time (i.e. repeatability of a trait). Here, using thermal radio-frequency identification implants, we quantified the repeatability of Tb, both in the context of a fixed average environment (∼21°C) and across ambient temperatures (6-31°C), in a free-living population of tree swallows (Tachycineta bicolor, n=16). By experimentally trimming the ventral plumage of a subset of female swallows (n=8), we also asked whether the repeatability of Tb is influenced by the capacity to dissipate body heat. We found that both female and male tree swallow Tb was repeatable at 21°C (R=0.89-92), but female Tb was less repeatable than male Tb across ambient temperature (Rfemale=0.10, Rmale=0.58), which may be due to differences in parental investment. Trimmed birds had on average lower Tb than control birds (by ∼0.5°C), but the repeatability of female Tb did not differ as a function of heat dissipation capacity. This suggests that trimmed individuals adjusted their Tb to account for the effects of heat loss on Tb. Our study provides a first critical step toward understanding whether Tb is responsive to natural selection, and for predicting how animal populations will respond to climatic warming.

Continuous growth through winter correlates with increased resting metabolic rate but does not affect daily energy budgets due to torpor use

Small mammals that are specialists in homeothermic thermoregulation reduce their self-maintenance costs of normothermy to survive the winter. By contrast, heterothermic ones that are considered generalists in thermoregulation can lower energy expenditure by entering torpor. It is well known that different species vary the use of their strategies to cope with harsh winters in temperate zones; however, little is still known about the intraspecific variation within populations and the associated external and internal factors. We hypothesized that yellow-necked mice Apodemus flavicollis decrease their resting metabolic rate (RMR) from autumn to winter, and then increase it during spring. However, since the alternative for seasonal reduction of RMR could be the development of heterothermy, we also considered the use of this strategy. We measured body mass (m_b), RMR, and body temperature (T_b) of mice during 2 consecutive years. In the 1st year, mice decreased whole animal RMR in winter, but did not do so in the 2nd year. All mice entered torpor during the 2nd winter, whereas only a few did so during the first one. Mice showed a continuous increase of m_b, which was steepest during the 2nd year. The relationship between RMR and m_b varied among seasons and years most likely due to different mouse development stages. The m_b gain at the individual level was correlated positively with RMR and heterothermy. This indicates that high metabolism in winter supports the growth of smaller animals, which use torpor as a compensatory mechanism. Isotope composition of mice hair suggests that in the 1st year they fed mainly on seeds, while in the 2nd, they likely consumed significant amounts of less digestible herbs. The study suggests that the use of specialist or generalist thermoregulatory strategies can differ with environmental variation and associated differences in developmental processes.