Individual (Co)variation in Resting and Maximal Metabolic Rates in Wild Mice

Abstract

Basal metabolic rate (BMR) represents the lowest level of aerobic metabolism in a resting, postabsorptive endotherm as measured within the thermoneutral zone. By contrast, maximal metabolic rate (VO2max) reflects the upper limit of aerobic metabolism achieved during intensive exercise. As BMR and _VO2max define the boundaries of the possible levels of aerobic metabolism expressed by a normothermic individual, a key question is whether BMR and _VO2max are correlated. In the present study, we took repeated paired measurements of thermoneutral resting metabolic rate (RMRt) and VO2max on 165 white-footed mice (Peromyscus leucopus). Over a single summer (May–October), repeatability (R ± SE) was low but statistically significant (P < 0:05) for both RMRt and VO2max (R=0:15 ± 0:08 for RMRt; R=0:27 ± 0:12 for VO2max). Willingness to run during the forcedexercise trials was also significantly repeatable (R p 0:16± 0:08). At the residual level (within individual), RMRt and VO2max tended to be positively correlated (re =0:23 ± 0:11, P=0:051), suggesting the presence of correlated phenotypic plasticity. By contrast, RMRt and _VO2max were significantly negatively correlated at the among-individual level (rind =-0:84 ± 0:40). To the extent that variation in RMRt reflects variation in BMR, the negative among-individual correlation does not corroborate the idea that a costly metabolic machinery is needed to support a high _VO2max. Future research should investigate the (genetic) relationship between RMRt (and BMR) and other energetically expensive behaviors and activities to better understand how energy is allocated within individuals.