Maximal metabolic rates during voluntary exercise, forced exercise, and cold exposure in house mice selectively bred for high wheel-running

Selective breeding for high wheel-running activity has generated four lines of laboratory house mice (S lines) that run about 170% more than their control counterparts (C lines) on a daily basis, mostly because they run faster. We tested whether maximum aerobic metabolic rates (V(O2)max) have evolved in concert with wheel-running, using 48 females from generation 35. Voluntary activity and metabolic rates were measured on days 5+6 of wheel access (mimicking conditions during selection), using wheels enclosed in metabolic chambers. Following this, V(O2)max was measured twice on a motorized treadmill and twice during cold-exposure in a heliox atmosphere (He-O-2). Almost all measurements, except heliox V(O2)max, were significantly repeatable. After accounting for differences in body mass (S < C) and variation in age at testing, S and C did not differ in V-O2max during forced exercise or in heliox nor in maximal running speeds on the treadmill. However, running speeds and V-O2max during voluntary exercise were significantly higher in S lines. Nevertheless, S mice never voluntarily achieved the V-O2max elicited during their forced treadmill trials, suggesting that aerobic capacity per se is not limiting the evolution of even higher wheel-running speeds in these lines. Our results support the hypothesis that S mice have genetically higher motivation for wheel-running and they demonstrate that behavior can sometimes evolve independently of performance capacities. We also discuss the possible importance of domestication as a confounding factor to extrapolate results from this animal model to natural populations.