During their lifetime, most animals can compensate for changing environmental conditions by altering functional capacities of physiological systems (physiological plasticity), by changing behavior (behavioral plasticity), or both (Garland and Carter, 1994; Huey and Berrigan, 1996). These adjustments help match performance capacity to environmental demands, and hence may be crucial ecologically. The timing and magnitude of physiological plasticity is also important in an evolutionary context, because the ability to change performance limits in concert with changing demands may have drastic consequences for fitness in a fluctuating environment (Huey and Berrigan, 1996; DeWitt et al., 1998; Wilson and Franklin, 2002). For several reasons, thermal acclimation in small endotherms is a useful system for studying physiological plasticity. First, it can be induced simply by changing ambient temperature. Second, the response can be easily measured as maximal rates of oxygen consumption (V . O ∑ max ). Third, thermal acclimation is ecologically relevant in highly seasonal habitats (Rosenmann et al., 1975; Cygan, 1985; Zegers and Merritt, 1988; Hayes, 1989; Bozinovic et al., 1990; Merritt, 1995; Kronfeld-Schor et al., 2000). Fourth, there are considerable data on the mechanistic basis of thermal acclimation at different levels of organization, from organ size (e.g. McDevitt and Speakman, 1994; Speakman and McQueenie, 1996; Derting and Austin, 1998; Hammond and Kristan, 2000), to physiology and biochemistry (Golozoubova et al., 2001; Nedergaard et al., 2001; Deveci et al., 2001; Shmeeda et al., 2002), to gene expression (Jacobsson et al., 1994; Yu et al., 2002). Finally, recent studies have found significant selection on V . O ∑ max in wild populations, re-emphasizing its evolutionary and ecological relevance (Hayes and O’Connor, 1999; E. L. Rezende, F. Bozinovic and T. Garland, unpublished results). Despite considerable study, some aspects of thermal acclimation merit additional work. There are few data on within-individual performance consistency across acclimatory events (Hayes and Chappell, 1990; Nespolo and Rosenmann, 1997). Individual consistency (repeatability) over time is a prerequisite for natural selection to affect trait variation, and it may set the upper limit on the narrow sense heritability of the 295 The Journal of Experimental Biology 207, 295-305 Published by The Company of Biologists 2004 doi:10.1242/jeb.00760 Thermal acclimation in small endotherms provides an excellent model for the study of physiological plasticity, as energy requirements can be easily manipulated and the results are relevant for natural conditions. Nevertheless, how physiology changes throughout acclimation, and how individuals vary in their response to acclimation, remain poorly understood. Here we describe a high temporal- resolution study of cold acclimation in the deer mouse Peromyscus maniculatus. The experimental design was based on repeated measures at short intervals throughout cold acclimation, with controls (maintained at constant temperature) for measurement artifacts. We monitored body mass, maximum metabolic rate in cold exposure and ventilatory traits (respiratory frequency, tidal and minute volume and oxygen extraction) for 3 weeks at 23°C. Then, half of the individuals were held for 7 weeks at 5°C. Body mass was differently affected by cold acclimation depending on sex. Maximal metabolism (V . O ∑ max ) increased significantly during the first week of cold acclimation, ‘overshot’ after 5 weeks and dropped to a plateau about 34% above control values at week 7. Similarly, ventilatory traits increased during cold acclimation, though responses were different in their kinetics and magnitude. Body mass, maximum metabolism, and most ventilatory traits were repeatable after 7 weeks in control and cold-acclimated animals. However, repeatability tended to be lower in the cold-acclimated group, especially while animals were still acclimating. Our results show that acclimation effects may be under- and/or overestimated, depending on when trials are performed, and that different traits respond differently, and at different rates, to acclimation. Hence, future studies should be designed to ensure that animals have attained steady-state values in acclimation experiments. Key words: acclimation, ambient temperature, maximal oxygen consumption, physiological plasticity, Peromyscus maniculatus, repeatability, thermogenesis, ventilation. Summary Introduction Cold-acclimation in Peromyscus: temporal effects and individual variation in maximum metabolism and ventilatory traits Enrico L. Rezende*, Mark A. Chappell and Kimberly A. Hammond Department of Biology, University of California, Riverside, California 92521, USA *Author for correspondence (e-mail: erezende@citrus.ucr.edu) Accepted 20 October 2003