MAXIMAL SPRINT SPEEDS AND MUSCLE-FIBER COMPOSITION OF WILD AND LABORATORY HOUSE MICE

We compared males from four groups of house mice (Mus domesticus), all bred and raised under common conditions in the laboratory: randombred Hsd:ICR; a wild population from Wisconsin; hybrids from lab darns; hy brids from wild dams. Wild mice were much faster sprinters (maximal fo rced sprint speeds over 1.0 m ranged from 2.38 to 3.34 m/s) than were lab mice (range = 0.89-1.68 m/s). Hybrids exhibited intermediate speed s (range = 1.54-2.70 m/s) and body masses, indicating largely additive inheritance. Type-specific mean muscle fiber cross-sectional areas of the gastrocnemius muscle did not differ significantly among groups. P ercentage cross-sectional areas occupied by each of the three identifi ed fiber types also did not differ significantly among groups, nor did they covary with body mass. For their body mass, however, lab mice ha d smaller gastrocnemius muscles than did wild and hybrid mice, which h ad muscles of similar size. Although we cannot rule out the possibilit y that smaller gastrocnemius muscles or slight differences in fiber co mposition account for the lower sprint speeds of the lab mice, we sugg est that differences in unmeasured physiological, behavioral or motiva tional factors are probably the primary cause. This interpretation is supported by a lack of correlation between individual differences in s print speed and either relative gastrocnemius muscle mass or muscle fi ber type composition.