Phylogenetic patterns of skeletal morphometrics and pelvic traits in relation to locomotor mode in frogs

Frogs are one of the most speciose groups of vertebrate tetrapods (> 6200sp) with a diverse array of locomotor behaviours. Despite the impressive diversity in frog locomotor behaviours, there remains a paucity of information on the relationship between skeletal variation and locomotor mode in frogs and the evolutionary patterns in which these relationships are framed across the frog phylogeny. Our current understanding of the evolution of frog locomotion shows that hopping transitioned into jumping within the Neobatrachia where a variety of pelvic/hindlimb length patterns and locomotor niches have appeared, but this has yet to be studied over a broad taxonomic sample of frogs. Although limb length remains as the primary predictor of leaping performance, pelvic and sacral morphometrics have not been quantified in relation to limb proportions, body size and locomotor mode and previous studies have not sampled more than 24 families. We present a large-scale phylogenetic comparison of skeletal morphometrics in relation to locomotor mode in 188 genera from 37 families. Osteological variation in limb/pelvic girdle morphometrics and pelvic traits that are posited to be associated with locomotor mode were analysed to identify which aspects of the frog skeleton are the best descriptors of locomotor mode. Our results, contrary to previous work, reveal that the greatest axis of variation in frogs is represented by the shape of the sacrum with two pelvic morphologies evident in qualitative and quantitative ancestral trait reconstructions. Limb morphology was not significantly different across most locomotor modes, but we identified several outliers in hindlimb phylomorphospace. Patterns of sacral evolution together with hindlimb length outliers reveal how the general bauplan of this successful group of vertebrate tetrapods is constrained, has radiated and has converged on certain phenotypes to fill an array of locomotor modes.