Cenozoic migrations, extinctions and the latitudinal gradient in body size of New World mammals

Background/Question/Methods

That large scale diversity patterns are imprinted by phenomena occurred along the evolutionary history of the taxa is not a novel idea. Historical facts such as palaeoclimatic changes, palaeogeographical modifications and subsequent biotic exchanges would have contributed to the shifting and shaping of the geographical distributions of present day biotas. In addition, macroevolutionary events summarized by phylogenies inform about the extent to which diversification patterns or phylogenetic relatedness might influence diversity gradients. Here we focus on how biotic exchanges of mammals during the Late Cenozoic have contributed to current geographic body size patterns. We explore differences in the environmental correlates and phylogenetic patterning of body size between the groups of mammals participating and not participating in past biotic exchanges. Because of well known megafaunal extinctions by the end of the Pleistocene, we also analyze the extent to which current latitudinal gradients in body size of mammals have responded to differential extirpation of extinct clades.

Results/Conclusions

Both the association of body size with environmental predictors and its phylogenetic signal were stronger for groups that immigrated into North or South America than for indigenous groups. We illustrate how the influences of Cenozoic migrations have increased average body size in the northernmost Nearctic while they have decreased the average size of mammalian assemblages in southernmost Neotropics and in the Andes. Consideration of extinctions revealed that contemporary latitudinal gradients in body size are strongly correlated with body size gradients of extinct clades of megamammals for all indigenous/non-indigenous groups (all r > 0.70). Even after inclusion of extinct clades in phylogenetic analyses body size of immigrated mammals still showed stronger signal than indigenous groups. Differences between mammals that participated in the biotic exchanges and those that did not participate can be interpreted based on the length of time that clades have had to diversify and occupy niche space. Moreover, we identify a role for historical events such as Cenozoic migrations on configuring contemporary mammal body size patterns.