Abstract

<jats:p> Male-biased sexual size dimorphism in mammals is usually attributed to the success of large males in intrasexual combat for mates. However, mating success may be determined by contests that are not combative. In the mating chase of the yellow-pine chipmunk (Tamias amoenus), a mammalian species with female-biased sexual size dimorphism, fast males may have an advantage in acquiring matings with estrous females. However, the effects of intraspecific variation in body size on running speed are not obvious; heavy individuals may run more slowly than light individuals because excess mass can be a hindrance to locomotion, but individuals that are structurally large may run faster than small individuals because of longer stride length. We examined the effects of both body mass and structural size on running speed in male yellow-pine chipmunks using manipulated runs in which male chipmunks were chased over a known distance. Structurally large male chipmunks had faster running speeds than small males, potentially giving large males an advantage when chasing estrous females. However, small male chipmunks are known to be aggressively dominant over large males. This leads to a potential trade-off in male body size between two behavioural components of mating success (running speed and dominance) that may constrain the evolution of male body size, ultimately leading to female-biased sexual size dimorphism. </jats:p>