Intersexual differences in body shape and locomotor performance in the aquatic frog, Xenopus tropicalis

Dimorphisms between the sexes are common in vertebrates and may reflect the divergent selective pressures operating on each sex. For example, in species where males do not show territory defense or pronounced malemale combat, females are typically larger than males as fecundity selection will favor large female body size. This is often the case in frogs where malemale competition is limited to calling behavior. Yet, whether differences in reproductive strategies between the two sexes are reflected in shape dimorphisms, and how those differences in shape may affect relevant whole-organism performance traits, remains poorly studied. Here we use Xenopus tropicalis frogs to test for intersexual differences in body size, body shape and locomotor performance traits. Our results show that females are larger than males, but that males have relatively longer limbs and heads than females. In absolute terms, males and females perform equally well at different locomotor tasks (burst performance and maximal exertion capacity). Yet, for a given body size, males have a higher exertion capacity than females. Increased exertion capacity in males is likely the consequence of their relatively longer limbs and may reflect selection on locomotor capacity in males to compensate for their smaller absolute body size.