Metabolic rate and environmental productivity: Well-provisioned animals evolved to run and idle fast

Even among vertebrate species of the same body mass and higher-level taxono mic group, metabolic rates exhibit substantial differences, for which diver se explanatory factors-such as dietary energy content, latitude, altitude, temperature, and rainfall-have been postulated. A unifying underlying facto r could be food availability, in turn controlled by net primary productivit y (NPP) of the animal's natural environment. We tested this possibility by studying five North American species of Peromyscus mice, all of them simila r in diet (generalist omnivores) and in gut morphology but differing by fac tors of up to 13 in NPP of their habitat of origin. We maintained breeding colonies of all five species in the laboratory under identical conditions a nd consuming identical diets. Basal metabolic rate (BMR) and daily ad libit um food intake both increased with NPP, which explained 88% and 90% of thei r variances, respectively. High-metabolism mouse species from high-NPP envi ronments were behaviorally more active than were low-metabolism species fro m low-NPP environments. Intestinal glucose uptake capacity also increased w ith NPP (and with BMR and food intake), because species of high-NPP environ ments had larger small intestines and higher uptake rates. For metabolic ra tes of our five species, the driving environmental variable is environmenta l productivity itself (and hence food availability), rather than temporal v ariability of productivity. Thus, species that have evolved in the presence of abundant food run their metabolism "fast," both while active and while idling, as compared with species of less productive environments, even when all species are given access to unlimited food.