The repeatability of a trait is a critical factor in determining how that trait is affected by natural selection. We examined the repeatability of a key physiological trait, maximum oxygen consumption (O 2max), in a wild population of Belding's Ground Squirrels, Spermophilus beldingi. O 2max is an integrated measure of organismal metabolic performance. It can be intuitively related to fitness because it sets an upper limit to sustainable power output during ecologically important activites such as locomotion and thermoregulatory heat production. 2. We used respirometry to determine O 2max during exercise and thermogenesis. Exercise O 2max was elicited in an enclosed running wheel. Thermogenic O 2max was obtained with acute cold exposure in a helium-oxygen gas mixture. 3. Repeatability of both exercise and thermogenic O 2max was high over 2 h intervals but declined over longer test periods (6-18 days and 1-2 years). In general, repeatability was higher for exercise O 2max than for thermogenic O 2max. 4. We found no repeatability for animals tested initially as juveniles and then 1 or 2 years later as adults; evidently there is sufficient plasticity in O 2max to decouple aerobic performance between these life stages. A small number of adults tested in successive years showed significant repeatability of exercise O 2max but no repeatability of thermogenic O 2max .

We measured maximal oxygen consumption (VO(2max)) and burst speed in populations of Trinidadian guppies (Poecilia reticulata) from contrasting high- and low-predation habitats but reared in "common garden" conditions. We tested two hypothesis: first, that predation, which causes rapid life-history evolution in guppies, also impacts locomotor physiology, and second, that trade-offs would occur between burst and aerobic performance. VO(2max) was higher than predicted from allometry, and resting VO(2) was lower than predicted. There were small interdrainage differences in male VO(2max), but predation did not affect VO(2max) in either sex. Maximum burst speed was correlated with size; absolute burst speed was higher in females, but size-adjusted speed was greater in males. For both sexes, burst speed conformed to allometric predictions. There were differences in burst speed between drainages in females, but predation regime did not affect burst speed in either sex. We did not find a significant correlation between burst speed and VO(2max), suggesting no trade-off between these traits. These results indicate that predation-mediated evolution of guppy life history does not produce concomitant evolution in aerobic capacity and maximum burst speed. However, other aspects of swimming performance (response latencies or acceleration) might show adaptive divergence in contrasting predation regimes.

Intra-and interspecific variation in locomotor performance has long been of interest to comparative physiologists. Interspecific comparative studies of trait variation are often used to provide insights into both evolutionary adaptation and the mechanistic foundations of performance. A related approach (which reduces the problem of phylogenetic nonindependence among related species) is to use correlative analyses of natural intraspecific variation at different levels of integration (e.g. enzymes, organelles, cells, organs, organ systems and the intact animal) to yield insights into the functional basis of whole-animal performance (e.g. Hulbert and Else, 1981;Garland, 1984;Bennett, 1987Bennett, , 1997Bennett et al., 1989;Mangum and Hochachka, 1998). For example, several recent papers have shown correlative links between avian energy metabolism (basal metabolism and maximal aerobic capacity) and organ size and muscle enzymology (Kersten and Piersma, 1987;Daan et al., 1990;Piersma et al., 1996;Burness et al., 1998), and explore the role of central and peripheral organs in setting the limits to performance (e.g. Chappell et al., 1999;Hammond et al., 2000). These results can be used to predict what organs or organ systems are likely to be impacted by selection on whole-animal performance, or by acclimation, seasonal changes or other types of phenotypic plasticity. Of special interest in both mechanistic and We examined the mechanistic basis for two wholeanimal performance traits, aerobic capacity and burst speed, in six laboratory-reared Trinidadian guppy populations from different native drainages with contrasting levels of predation. Using within-and between-population variation, we tested whether variation in organs and organ systems (heart, gill and swimming motor mass) and the activities of several enzymes that support locomotion (citrate synthetase, lactate dehydrogenase and myofibrillar ATPase) are correlated with aerobic performance (maximum rates of oxygen consumption, VO∑max) or burst performance (maximum swim speed during escape responses). We also tested for associations between physiological traits and habitat type (different drainages and predation levels).Organ size and enzyme activities showed substantial size-independent variation, and both performance measures were strongly correlated to body size. After accounting for size effects, neither burst nor aerobic performance was strongly correlated to any organ size or enzymatic variable, or to each other. Two principal components (PCI, PC2) in both males and females accounted for most of the variance in the organ size and enzymatic variables. In both sexes, heart and gill mass tended to covary and were negatively associated with citrate synthetase and lactate dehydrogenase activity. In males (but not females), variation in aerobic performance was weakly but significantly correlated to variation in PC1, suggesting that heart and gill mass scale positively with V O∑max. Neither of the component variables and no single morphological or enzymatic trait was ...

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