Non-genetic phenotypic variation within and among individuals is a pervasive theme in physiology, ecology and evolutionary biology. A familiar example is comparative physiology's long and productive tradition of exploring the influence of environmental factors (e.g. temperature, water availability and oxygen partial pressure) or conditioning regimes (e.g. diet, exercise training and cold exposure) on traits affecting thermal biology, osmoregulation, locomotor capacity, aerobic performance, etc.

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... The ecological and evolutionary relevance of environmental influences on physiological traits (as well as the magnitude of their effects) varies considerably from species to species and from trait to trait, but one phenotypic factor common to all multicellular animals is age. Much research has focused on changes in physiology, morphology and behavior associated with the transition from juvenile stages to adulthood and on senescence effects near the end of the life span. With a few exceptions (e.g. age effects on human physiology, such as exercise and athletic performance, have been extensively documented; Stones and Kozma, 1985; Goldberg et al., 1996) , comparative physiologists have generally paid less attention to age-related changes in non-senescent adult animals. Age impacts nearly all aspects of physiology, often indirectly. For example, many animals continue to grow after attaining reproductive maturity, and this mass gain over the life span will influence the wide spectrum of physiological traits that vary with body size. However, age may have a significant physiological impact even in species with relatively determinate growth (such as humans). As well as being interesting from a functional perspective, an understanding of age effects is relevant for field studies of adaptation -especially those that hinge on performance comparisons between contrasting environments or selective regimes -because population demographic structure is often complex and it is usually difficult to determine the age of freeliving animals. Without data on age effects and demography, results from inter-population comparisons may be difficult to interpret. In this paper, we use a small mammal, the North American deer mouse Peromyscus maniculatus, as a model system to examine age effects on several frequently measured aspects of aerobic performance: basal metabolism (BMR), maximum aerobic capacity in exercise (V . O∑max) and