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 ge netically correlated, we conducted an artificial selection experiment for i ncreased voluntary wheel-running activity in house mice (Mus domesticus). H ere, we compare body masses of mice from control and selected lines after 1 4 generations of selection. In both groups, beginning at weaning and then f or 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 t o prevent rotation. Mice from selected lines were more active than controls at weaning (21 days) and across the experiment (total revolutions during l ast 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 % l ess than 'sedentary' individuals; both effects were statistically significa nt and additive. Within the free-wheel-access group, individual variation i n 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 nega tive but not statistically significant (P>0.40). The narrow-sense genetic c orrelation between wheel-running and body mass after 8 weeks of wheel acces s was estimated to be -0.50. A negative genetic correlation could account f or the negative relationship between voluntary wheel-running and body mass that has been reported across 13 species of muroid rodents.