Dispersal in most taxa is characterized by many individuals that stay close to their origin and large variation in the distances moved by those that leave. Despite the wide-ranging effects dispersal can have on population and evolutionary dynamics, we still do not have a clear understanding of how individual variation in dispersal distance is maintained in natural populations. We are testing for a role of natural selection in maintaining variation in dispersal distance and associated phenotypes in the stream salamander Gyrinophilus porphyriticus. Here, we assess the relationship between morphological traits and swimming performance, based on the hypothesis that swimming performance is a proximate mechanism driving variation in dispersal distance in this system. We built an in-stream trackway to test the swimming performance of 190 adult and larval salamanders captured during capture-mark-recapture surveys. Mean velocity, peak acceleration, and total swim duration were extracted from high-speed video footage of swimming trials. We used multiple regression analyses to test for a relationship between these performance variables and aspects of head, trunk and limb morphology.
Preliminary analyses of a subset of individuals show that salamanders with relatively longer, thinner trunks attained the highest mean velocities (F1,28 = 7.359, P = 0.011) and swam for a greater duration of time (F1,28 = 7.77, P = 0.001). The same trend was evident for peak acceleration, though marginally significant (F1, 28 = 3.684, P = 0.065). These results suggest that trunk morphology may represent a dispersal phenotype subject to selection. To test this prediction, we are collecting direct data on dispersal distance and fitness in the wild for the same individuals in the swimming trials (N = 190). Previous work in this system showed that dispersing individuals have higher fitness than residents. Mechanistically, thin-trunked individuals may have increased swimming performance and disperse farther in nature as a result of reduced hydrodynamic drag due to their slender shape. Conversely, if thick-trunked individuals disperse farther and have higher fitness, energy reserves may be more important for dispersal than overcoming the effects of drag.