The environment is a major driver of shape and size variation in widespread extant xenarthrans

Xenarthra is one of the most peculiar mammalian groups to have originated in South America, where most of its evolutionary history has taken place. Xenarthrans have experienced significant climatic changes and also geographical isolation for most of the Cenozoic. Thus, the cranial morphology of xenarthrans may reflect the different pressures that the group is currently undergoing. Our objective is to identify the drivers of phenotypic variation of the skull (both shape and size) in widespread representatives of the superorder Xenarthra throughout South America. We tested the influence of allometry and neutral and niche processes on three extant species, using a geometric morphometric approach: the three-toed sloth Bradypus variegatus, the lesser anteater Tamandua tetradactyla, and the armadillo Dasypus novemcinctus. Our results show that allometry is present, but has a weak effect on shape variation. Geography has a greater impact than allometry in explaining the size variation of T. tetradactyla and the shape of B. variegatus which is, interestingly, the only studied species that follows Bergmann's rule. Most importantly, the environment drives most of the variation in the shape and size of the three species studied. The responses of B. variegatus and T. tetradactyla to geographical space are congruent with their low mobility and more limited dispersal. The environment mainly affected B. variegatus (e.g., following Bergmann's rule), probably because of its particularly low metabolism, and D. novemcinctus, due to its high dispersal capacity.