Selective breeding for high voluntary exercise in mice increases maximal (VO2max), but not basal metabolic rate.

In general, sustained high rates of physical activity require a high maximal aerobic capacity (VO2max), which may also necessitate a high basal aerobic metabolism (BMR), given that the two metabolic states are linked via shared organ systems, cellular properties, and metabolic pathways. We tested the hypotheses that (a) selective breeding for high voluntary exercise in mice would elevate both VO2max and BMR, and (b) these increases are accompanied by increases in the sizes of some internal organs (ventricle, triceps surae muscle, liver, kidney, spleen, lung, brain). We measured 72 females from generations 88 and 96 of an ongoing artificial selection experiment comprising 4 replicate High Runner (HR) lines bred for voluntary daily wheel-running distance and 4 non-selected Control (C) lines. With body mass as a covariate, HR lines as a group had significantly higher VO2max (+13.6%, P<0.0001), consistent with previous studies, but BMR did not significantly differ between HR and C lines (+6.5%, P=0.181). Additionally, HR mice did not statistically differ from C mice for whole-body lean or fat mass, or for the mass of any organ collected (with body mass as a covariate). Finally, mass-independent VO2max and BMR were uncorrelated (r=0.073, P=0.552) and the only statistically significant correlation with an organ mass was for VO2max and ventricle mass (r=0.285, P=0.015). Overall, our results indicate that selection for a behavioral trait can yield large changes in behavior without proportional modifications to underlying morphological or physiological traits.