Adaptive explanations that rely on physiological arguments are common, but tests of hypotheses about the significance of whole-animal physiological performance (e.g., aerobic capacities) are rare. We studied phenotypic selection on the thermogenic capacity (i.e., maximal rate of oxygen consumption [V0 2 max] elicited via cold exposure) of high-altitude (-3800 m) deer mice (Peromyscus maniculatus). A high V0 2 max equates to a high capacity for heat production and should favor survival in the cold environments prevalent at high altitude. Strong directional selection favored high V0 2 max, at least in one year. The selection for increased V0 2 max is consistent with predictions derived from incorporating our physiological data into a biophysical model. During another year, we found weak evidence of selection for decreased body mass. Nonlinear selection was not significant for any of the selection episodes we studied. The strong directional selection for V0 2 max that we observed suggests that-given ample genetic variation-aerobic metabolism and perhaps endothermy may have evolved rapidly on the geological time scale.

Adaptive explanations that rely on physiological arguments are common, but tests of hypotheses about the significance of whole-animal physiological performance (e.g., aerobic capacities) are rare. We studied phenotypic selection on the thermogenic capacity (i.e., maximal rate of oxygen consumption [VO max] elicited via cold exposure) of high-altitude (~3800 m) deer mice (Peromyscus maniculatus). A high VO max equates to a high capacity for heat production and should favor survival in the cold environments prevalent at high altitude. Strong directional selection favored high VO max, at least in one year. The selection for increased VO max is consistent with predictions derived from incorporating our physiological data into a biophysical model. During another year, we found weak evidence of selection for decreased body mass. Nonlinear selection was not significant for any of the selection episodes we studied. The strong directional selection for VO max that we observed suggests that-given ample genetic variation-aerobic metabolism and perhaps endothermy may have evolved rapidly on the geological time scale.

Summary 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 maniculatu s) inhabiting an environment cold enough that above-ground activity (behaviour) may be limited by the thermogenic capacity (maximal rate of oxygen consumption [ V o 2 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 capturemark-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.

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Two inbred mouse strains, C57BL/6J (B6) and DBA/2J (D2), were evaluated for effects of ethanol on thermoregulation. Continuous recording of core temperature (Tc) from undisturbed animals at an ambient temperature (Ta) of 27 degrees C indicated Tc was similar for both strains during active (approximately 38.0 degrees C) and inactive (approximately 36.7 degrees C) periods. Ethanol-injections of 1.5, 2.5, 3.5, and 4.5 g/kg in an environment where Ta rose and fell at 6-min intervals, reaching extremes of 14 and 42 degrees C, produced dose-dependent falls in Tc for both strains. The changes in Ta produced fluctuations in Tc under all conditions. The amplitude of these fluctuations in Tc was used as a measure of physiological disruption. Dose-dependent increases in disruption were found for both strains. At a constant 26 degrees C Ta, ethanol produced dose-related increases in tail temperature. Responses after ethanol administration were different for B6 and D2 mice. The results indicate regulated temperature is similar for B6 and D2 strains. Regulated temperature is decreased more by ethanol for B6 mice, whereas disruption of thermoregulation by ethanol is greater for D2 mice.