Proportional variation and scaling in the hindlimbs of hopping mammals, including convergent evolution in argyrolagids and jerboas

Abstract

Bipedal hopping is a mode of locomotion seen today in four rodent lineages and one clade of marsupials. The Argyrolagidae, marsupials from the Oligocene to Pliocene of South America, have also been considered to be hoppers. These lineages all convergently evolved similar general morphologies, with elongated hindlimbs, reduced forelimbs, and elongated tails, and their similarities and variations may be informative in understanding the evolution of hopping in mammals. This study uses principal components analysis and log-log regressions to investigate variation in the hindlimb proportions of these hopping mammals and how this relates to body mass. We find that the distribution of hopping mammal masses is bimodal, divided at roughly 500 g. These two domains among hopping mammals may reflect optimisation for different forms of hopping locomotion; species under 500 g tend to have more elongated metatarsals relative to the rest of their hindlimbs, perhaps to facilitate rapid vertical jumps for predator evasion, a behaviour not seen in larger hoppers. Despite this bimodal distribution in body mass, hindlimb proportions cluster more by clade than mass, with some similarities among clades being especially noteworthy. The jerboas (Dipodidae, Rodentia) and Argyrolagidae share a particularly extreme degree of metatarsal elongation. The drivers of this convergence are unclear, but we hypothesise that the elongation may be related to the reduction/fusion of metatarsals in these groups, or a greater reliance on bipedality at slow speeds, as jerboas are known to utilise multiple bipedal gaits in addition to hopping.