Meeting Abstract
Intense physical competition between males for access to mating opportunities is widespread among mammals. In such agonistic encounters, males with a combination of morphological and behavioral characters that allow them to dominate an opponent 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 muscle mass and distribution influence dominance status. Greater muscle mass is associated with (1) larger muscle cross-sectional area, which provides an increased capacity for force production, and/or (2) longer muscle fascicles, which allow for a faster contraction. Therefore, individuals with relatively larger muscle mass may be capable of producing more power, permitting them to more easily manipulate an opponent. To understand which muscles are most important when mammals fight, we compared the mass of 10 major muscle groups in a set of 9 dominant and 20 non-dominant male mice. House mice are appropriate organisms to study social dominance because they possess a polygynous mating system where males compete physically to gain access to territory and, thus, access to females. As expected, extensor muscles of the forelimb and distal forelimb muscles were found to be larger in dominant males. However, our results were only partially consistent with our expectations. For example, muscles of the neck from the trapezius and sternocleidomastoid complexes did not differ based on dominance status. These results suggest that our current conceptualization of mammalian dominance behavior may be incomplete.
