Space and time: The two dimensions of Artiodactyla body mass evolution

Body mass has important physiological and ecological implications. The high degree of variability in herbivore mammals' body mass is linked to differences in their gut anatomy, which, in turn, has important effects on their feeding habits. We aimed to untangle the effects of Cope's and Bergmann's rule on the evolution of body size in even-toed hoofed mammals (Artiodactyla, Mammalia). By using variation partitioning techniques and direct sister species comparisons, we estimated the potential effects of latitude, time and phylogeny on body size variation in Cenozoic Artiodactyls. We tested whether species tend to be larger when living at higher latitudes as compared to their phylogenetically closest relative, whether closely-related species living in sympatry tend to diverge more in body size, and whether recent species tend to be larger than their older relatives. Our results showed that both latitudinal and temporal trends in Artiodactyla body size are evident throughout the Cenozoic, and became particularly intense during the coldest part of it. Artiodactyls tend to be larger than their close relatives when living at higher latitudes, in keeping with Bergmann's rule. Yet, this relationship is weakened by the strong tendency for sister species to diverge in body size when living nearby. We also found a clear tendency for late Artiodactyl species to be larger, and live for shorter, than their ancestors.Thus, there is strong evidence that the evolution of body size in even-toed hoofed mammals was influenced by climatic changes, and by an overall tendency to grow larger, mostly in coincidence with the vegetation shift from forested to grassland biomes which characterized the second half (Neogene) of the Cenozoic. Although Cope's and Bergmann's rule in mammals are often portrayed as having climate change as a common explanation, our data suggest they are mechanistically separate phenomena. © 2015 Elsevier B.V.