Frogs in a Hurry: Morphological and Physiological Correlates of Inter-Site Movement Success in an Aquatic Invader

Movement is a key driver of population dynamics. Movement ability and propensity often vary among populations and individuals. These differences may be particularly strong in aquatic species, where the ability to move within a site is not necessarily correlated to the ability to move between sites. In periods of range expansion, these differences can lead to non-equilibrium dynamics, whereby more mobile phenotypes arrange themselves spatially. This can be even more pronounced when dispersal success is nonrandom with respect to a heritable trait, thus acting as an agent of selection. This process—dubbed spatial sorting—can be particularly pronounced in non-native species, often hastening the speed of invasion spread. However, before spatial sorting occurring, there must first be individual differences in traits that confer greater movement success. Recently, a high-density breeding and expanding population of the non-native pipid frog, Xenopus tropicalis, in west-central Florida, offering a great opportunity to test whether movement success is predicted by individual differences in morphology or locomotor capacity. To test this, we compared the morphology, maximal exertion capacity, and jumping performance of movers and residents. We found that relative to residents, movers had longer hindlimbs, wider ilia, and traveled for greater time intervals before reaching exhaustion. These results suggest functional morphological and physiological traits are important in determining inter-site movement success.