Individual Variation in Thermogenic Capacity Affects Above-Ground Activity of High-Altitude Deer Mice

1. Understanding an animal's ecology requires knowledge of how individual variation in behaviour and physiology interact with each other and with the environment that an animal experiences. 2. Environmental variation affects behaviour, but whether individual variation in physiological performance also affects behaviour is poorly known. 3. We studied a high-altitude population of Deer Mice (Peromyscus maniculatus) inhabiting an environment cold enough that above-ground activity (behaviour) may be limited by the thermogenic capacity (maximal rate of oxygen consumption$[V_{\text{O}_{2\text{max}}}]$during cold exposure) of mice. 4. We measured thermogenic capacity and operative environmental temperature (an integrated measure of the thermal environment), and then used robust-design capture-mark-recapture (CMR) models to test whether the thermal environmental and individual variation in thermogenic capacity affected capture probabilities (a likely indicator of above-ground activity). 5. Models including environmental covariates and thermogenic capacity were strongly favoured over models that did not include them. 6. Our results demonstrate that individual variation in physiological performance may constrain behaviour in nature. 7. Besides contributing to our understanding of interactions in the multivariate phenotype, our results suggest that it may be possible to elucidate the mechanistic factors influencing capture probabilities. Such information could be valuable to ecologists, life historians and wildlife managers.