CHAPPELL, M.A.**; RUSSELL, G.A.; HAMMOND, K.A.; Univ. of California, Riverside; Univ. of California, Riverside; Univ. of California, Riverside: BMR is not repeatable over extended periods in deer mice

Basal metabolic rate (BMR) is probably the most thoroughly investigated aspect of energy metabolism in vertebrate endotherms. Hundreds of studies have explored its mass allometry, phylogenetic and ecological relationships, ontogeny, mechanistic underpinnings, and so forth. Implicit in many of these studies, (and explicit in a few) is the view that BMR responds to selection, either direct selection on metabolism or energy economy, or indirect selection on correlated traits. Selection on any trait presumes repeatability and heritability but BMR is known to be highly plastic, responding to circadian cycles, altitude hypoxia, acclimation to temperature, and numerous other physiological, environmental, or life history factors. Given all the attention it has received, especially in the context of evolution, there have been surprisingly few studies of BMR repeatability, especially in mammals. We studied BMR repeatability in captive deer mice (Peromyscus maniculatus) across intervals of 33-88 days, and also examined the role of birth altitude (3800 versus 340 m) and cold acclimation (8 weeks at ~ 4 �C) on BMR, and relationships between BMR and organ size. BMR was not affected by birth altitude but significantly increased in response to cold acclimation. Organ masses were not correlated with BMR, and BMR was not repeatable. Absence of consistent relationships between organ size and BMR suggests that no single organ or organ system (either central or peripheral) �controls� variation in BMR. Lack of repeatability � although in sharp contrast to a recent report of high BMR repeatability in bank voles � indicates little potential for selection to act on BMR in deer mice.