GARLAND, T; SWALLOW, J G; Univ. of California, Riverside; Univ. of South Dakota: Exercise Physiology of House Mice Selectively Bred for High Voluntary Activity

From an outbred base population, within-family selection was used to produce 4 replicate lines that exhibit high voluntary wheel running (S lines), while also maintaining 4 randombred lines as controls (C). S lines responded with a realized heritability of 0.3 and reached an apparent limit after 16 generations, running 2.7-fold more revs/day than C lines (both sexes), similar to the range reported by Dewsbury (1980) among 13 species of murids. Although selection continued, the factorial difference was maintained through generation 31. S lines mainly run faster, rather than longer, especially in females. The evolution of higher activity levels must have required an increase in motivation and/or ability to run; evidence suggests that both have increased. With respect to traits that may affect wheel-running ability, S lines show reduced total body mass and length, reduced retroperitoneal and total body fat, reduced hindlimb muscle mass (especially in two lines that segregate for a Mendelian recessive allele that halves hindlimb muscle mass), increased relative ventricle and kidney mass, increased symmetry of hindlimb bones, increased insulin-stimulated glucose uptake in some hindlimb muscles, and elevated plasma corticosterone levels at rest during the day. Traits that have not changed include hematocrit, maximal oxygen consumption during forced treadmill exercise (measured just prior to routine wheel testing) or in response to cold exposure in a helium-oxygen atmosphere, basal metabolic rate, swimming endurance, maximal sprint speed on a photocell-timed racetrack, and rota-rod performance. S vs. C differences in some of the foregoing traits vary with housing conditions (e.g., prolonged wheel access) and hence exhibit genotype-environment interaction. Supported by NSF IBN-0212567 to T.G.