Meeting Abstract
Intense physical competition between males for access to mating opportunities is widespread among mammals. In such agonistic encounters, dominant males often have greater reproductive fitness. However, the specific physical traits that facilitate social dominance are poorly understood. Body size is often correlated with reproductive fitness in mammals. Interestingly, body mass only weakly predicts fitness in male house mice (Mus musculus). We hypothesized that the distribution of both muscle and bone mass influences dominance status. We tested whether muscle mass differed when corrected for body size in dominant versus non-dominant males in two different competition experiments. In both studies, male mice competed in semi-natural environments for access to females residing in optimal territories: these competition experiments differed in their duration (3 days vs. 8 weeks), habitat size (0.42 vs. 30 m2), male-to-female sex ratio (4:1 vs. 5:8), and number of rounds (2 bouts producing two-time winners and two-time losers vs. a single round of competition). The total mass of the dissected muscle groups was greater in dominant males in only one of the two experiments. Using the combined data, we discovered that dominant mice have greater mass in the muscles of the upper forelimb (biceps and triceps) and the gluteus muscles than their non-dominant counterparts. Muscle mass did not differ significantly in the hamstrings, knee extensors, or ankle plantarflexors. Bone mass measurements from one set of mice partially corroborate these results, with dominant mice possessing more massive forelimb bones. These results are consistent with our current understanding of house mouse agonistic behavior, which involves grappling with the forelimbs, often while standing bipedally.
