ORTIZ, S.D.*; DIAMOND, J.: The Ontogeny of Metabolic Rate in Two Species of Peromyscus Mice.

Two congeneric North American mice, Peromyscus eremicus of southwestern deserts and P. leucopus of southeastern woodlands, are good models for investigating the adaptive significance of species variation because of the ecological differences between their habitats. Little is known about the ontogeny of metabolic rate in mammals in general, and nothing in these particular species. Although we know that growth is an energetically expensive process, reflected in high metabolic rates, we do not fully understand how metabolic rate changes with growth. This study examined the relationship between metabolic rate and postnatal growth of juvenile mice. We hypothesized that the two species would exhibit qualitatively similar relationships between metabolic rate and growth but with quantitative differences because of differences in activity levels and ancestral habitat. We measured basal metabolic rate (BMR) of 9 P. eremicus and 16 P. leucopus (all captive-born) from age 14 days to sexual maturity (60 d). Body mass was measured in these same animals from birth to 120 d to determine pre- and post-weaning growth rates. Growth rates were slower post-weaning than pre-weaning in both species. Both pre- and post-weaning growth rates of P. eremicus exceeded those of P. leucopus, possibly related to the larger average litter size in P. leucopus (and hence perhaps a lower per-pup milk supply), related in turn to a more productive ancestral habitat. Mass-specific metabolic rate of both species declined asymptotically from pre-weaning until day 50. However, BMR was lower in P. eremicus than P. leucopus at all measured stages of development, as expected from the more arid ancestral habitat of P. eremicus. Thus, species differences between congeners are amenable to adaptive interpretations.