Investigating the morphology, locomotory performance and macroecology of a sub-Saharan African frog radiation (Anura: Pyxicephalidae)

The phenotypic diversity between closely related species is often attributed to the process of natural selection. This process retains heritable traits within a population which increases the survival and reproductive output of an organism. Most animals rely on locomotion for basic and vital tasks, and thus require effective movement within the environment they occupy. Selection on morphological traits can result in adaptations that influence aspects of locomotion to better suit their requirements. Locomotion, in addition to other traits such as body size, is hypothesised to also have an effect on distributional patterns, and could drive large-scale macroecological patterns. Understanding how interspecific differences in morphology relate to functional, adaptive and distribution patterns can provide clues to the evolutionary and macroecological processes that drive them.

In this study I compare interspecific differences in morphology and locomotor performance of the Pyxicephalidae, a recent frog radiation within sub-Saharan Africa that has given rise to remarkable morphological and ecological diversity. I hypothesise that morphology will affect locomotor performance in accordance with simple biomechanical predictions and findings from studies on other anurans. Furthermore, I categorise the habitat and ecology of each species into different frog ecotypes. I hypothesise that species differences in both morphology and locomotor traits correspond to changes in the ecotype. Additionally I determine the geographic range size of pyxicephalid species to investigate whether interspecific differences in morphological and reproductive traits can explain differences in range sizes. By using climatic and topographic variables, I model species distribution and niche breadth to account for habitat difference between species to test hypotheses of colonisation and dispersal. I hypothesise that the selected morphological and reproductive traits will correlate with species range size and that these correlations persist in either colonisation ability or niche breadth.

I captured five to ten individuals for each of 25 species of pyxicephalid frogs from their natural habitat across South Africa. Swimming and jumping performance was tested by filming frogs at 120 and 240 fps respectively, terrestrial and aquatic endurance was assessed by chasing frogs around a circular track for 20 and 15 minutes respectively and grip performance was measured by placing frogs on a non-stick surface that was rotated until the frog lost traction. Species ecotype was categorised using field observation from available literature as well as expert experience. Specimens were measured and dissected from museums to obtain morphological and reproductive traits. Range size was calculated using a minimum convex polygon for distribution data obtained from assessable databases and institutions. MaxEnt was used to model habitat suitability with Worldclim and topographic predictors. Colonisation Index was derived from habitat suitability to quantify the ability of a species to occupy nearby suitable habitats. In addition, niche breadth was calculated with the Outlier Mean Index (OMI) analysis, using the same predictor variables, but constraining the geographic extent to South African and species therein.

Species morphology had a significant influence on locomotor performance, which confirmed similar functional relationships found for other frog clades. Body size explained the greatest variation in performance across species, while hindlimb length was positively related to burst performance but negatively with persistent locomotion. Furthermore, I find support that separate selective optima between burrowing, terrestrial and semi-aquatic ecotypes have acted on overall morphology, but not on locomotor performance. However, specific tests between traits that have been linked to ecotypes in other anurans revealed that semi-aquatic and semi-arboreal ecotypes have specialised morphological adaptations, but only semi-aquatic ecotypes showed support for a performance optimum. Species geographic range size was positively correlated with body size and relative clutch size, but not relative head width or hindlimb length. The Colonisation Index was not robust for comparing species from different environments and range extents. Species niche breadth was not explained by either body size or relative clutch size, but by relative hindlimb length, suggesting that these former traits do not affect range size by increasing species ability to colonise and occupy a broader range of environmental conditions.

In summary, species body size and reproductive output are indirectly linked to range size patterns, but these patterns appear to be the result of an indirect association with abundant habitats or the ability to disperse and colonise within suitable habitat. The morphological diversity of the Pyxicephalidae has functional significance for locomotor performance, and some of these traits do represent ecotype adaptations. However, the limited evidence presented in this study does not support the 'exceptional' adaptive diversity indicative of an adaptive radiation within the Pyxicephalidae.