Phenotypic plasticity to chronic cold exposure in two species of Peromyscus from different environments

Highland deer mice support increased thermogenesis in response to chronic cold hypoxia by shifting uptake of circulating fatty acids from muscles to brown adipose tissue

During maximal cold challenge (cold-induced V̇O2,max) in hypoxia, highland deer mice (Peromyscus maniculatus) show higher rates of circulatory fatty acid delivery compared with lowland deer mice. Fatty acid delivery also increases with acclimation to cold hypoxia (CH) and probably plays a major role in supporting the high rates of thermogenesis observed in highland deer mice. However, it is unknown which tissues take up these fatty acids and their relative contribution to thermogenesis. The goal of this study was to determine the uptake of circulating fatty acids into 24 different tissues during hypoxic cold-induced V̇O2,max, by using [1-14C]2-bromopalmitic acid. To uncover evolved and environment-induced changes in fatty acid uptake, we compared lab-born and -raised highland and lowland deer mice, acclimated to either thermoneutral (30°C, 21 kPa O2) or CH (5°C, 12 kPa O2) conditions. During hypoxic cold-induced V̇O2,max, CH-acclimated highlanders decreased muscle fatty acid uptake and increased uptake into brown adipose tissue (BAT) relative to thermoneutral highlanders, a response that was absent in lowlanders. CH acclimation was also associated with increased activities of enzymes citrate synthase and β-hydroxyacyl-CoA dehydrogenase in the BAT of highlanders, and higher levels of fatty acid translocase CD36 (FAT/CD36) in both populations. This is the first study to show that cold-induced fatty acid uptake is distributed across a wide range of tissues. Highland deer mice show plasticity in this fatty acid distribution in response to chronic cold hypoxia, and combined with higher rates of tissue delivery, this contributes to their survival in the cold high alpine environment.

Thermoregulatory trade-offs underlie the effects of warming summer temperatures on deer mice

Climate warming could challenge the ability of endotherms to thermoregulate and maintain normal body temperature (Tb), but the effects of warming summer temperatures on activity and thermoregulatory physiology in many small mammals remain poorly understood. We examined this issue in deer mice (Peromyscus maniculatus), an active nocturnal species. Mice were exposed in the lab to simulated seasonal warming, in which an environmentally realistic diel cycle of ambient temperature (Ta) was gradually warmed from spring conditions to summer conditions (controls were maintained in spring conditions). Activity (voluntary wheel running) and Tb (implanted bio-loggers) were measured throughout, and indices of thermoregulatory physiology (thermoneutral zone, thermogenic capacity) were assessed after exposure. In control mice, activity was almost entirely restricted to the night-time, and Tb fluctuated ∼1.7°C between daytime lows and night-time highs. Activity, body mass and food consumption were reduced and water consumption was increased in later stages of summer warming. This was accompanied by strong Tb dysregulation that culminated in a complete reversal of the diel pattern of Tb variation, with Tb reaching extreme highs (∼40°C) during daytime heat but extreme lows (∼34°C) at cooler night-time temperatures. Summer warming was also associated with reduced ability to generate body heat, as reflected by decreased thermogenic capacity and decreased mass and uncoupling protein (UCP1) content of brown adipose tissue. Our findings suggest that thermoregulatory trade-offs associated with daytime heat exposure can affect Tb and activity at cooler night-time temperatures, impacting the ability of nocturnal mammals to perform behaviours important for fitness in the wild.

Individual variation in heat substitution: is activity in the cold energetically cheaper for some individuals than others?

In many endotherms, a potentially important yet often overlooked mechanism to save energy is the use of the heat generated by active skeletal muscles to replace heat that would have been generated by thermogenesis (i.e., "activity-thermoregulatory heat substitution"). While substitution has been documented numerous times, the extent of individual variation in substitution has never been quantified. Here, we used a home-cage respirometry system to repeatedly measure substitution through the concomitant monitoring of metabolic rate (MR) and locomotor activity in 46 female white-footed mice (Peromyscus leucopus). A total of 117 measures of substitution were taken by quantifying the difference in the slope of the relationship between MR and locomotor activity speed at two different ambient temperatures. Consistency repeatability (±se) of substitution was 0.313±0.131 - hence, about a third of the variation in substitution occurs at the among-individual level. Body length and heart mass were positively correlated with substitution whereas surface area was negatively correlated with substitution. These two sub-organismal traits accounted for the majority of the among-individual variation (i.e., individual differences in substitution were not significant after accounting for these traits). Overall, our results imply that the energetic cost of activity below the thermoneutral zone is consistently cheaper from some individuals than others, and that the energy saved from substitution might be available to invest in fitness-enhancing activities.