Artificial selection for increased wheel-running activity in house mice results in decreased body mass at maturity

To test the hypothesis that body size and activity levels are negatively genetically correlated, we conducted an artificial selection experiment for increased voluntary wheel-running activity in house mice (Mus domesticus). Here, we compare body masses of mice from control and selected lines after 14 generations of selection. In both groups, beginning at weaning and then for 8 weeks, we housed half of the individuals with access to running wheels that were free to rotate and the other half with wheels that were locked to prevent rotation. Mice from selected lines were more active than controls at weaning (21 days) and across the experiment (total revolutions during last week: females 2.5-fold higher, males 2.1-fold higher). At weaning, mice from selected and control lines did not differ significantly in body mass. At 79 days of age, mice from selected lines weighed 13.6 % less than mice from control lines, whereas mice with access to free wheels weighed 4.5 % less than 'sedentary' individuals; both effects were statistically significant and additive. Within the free-wheel-access group, individual variation in body mass of males was negatively correlated with amount of wheel-running during the last week (P<0.01); for females, the relationship was also negative but not statistically significant (P>0.40). The narrow-sense genetic correlation between wheel-running and body mass after 8 weeks of wheel access was estimated to be -0. 50. A negative genetic correlation could account for the negative relationship between voluntary wheel-running and body mass that has been reported across 13 species of muroid rodents.