The convergent evolution of mammalian reliance on a 'keystone vegetation structure' in Madagascar and Australia
Tree hollow inhabitance is a trait that has evolved multiple times over the course of mammalian evolution, but the selective pressures for this trait are not well-understood, especially in the tropics. In Australia, there are 35 species of tree hollow-inhabiting marsupials (75% of the continent’s arboreal marsupials). The adaptive radiation of Australian tree hollow inhabiting mammals is hypothesized to have been driven by thermoregulatory demands and constrained by tree hollow abundance and body size. Because Australia exhibits disparate temperature extremes and the highest tree hollow abundance globally, the continent can support a high diversity of tree hollow inhabiting species. However, tree hollow inhabitance has not been investigated similarly in the tropics, and there is a paucity of data on tree hollow abundance in low-latitude forests. My investigation of tree hollow inhabitance in Madagascar identified 36 species of tree hollow-inhabiting lemurs, comparable to the mammalian tree hollow inhabitant species richness of Australia. I hypothesized that tree hollow inhabitance within Madagascar was driven by thermoregulatory demands and constrained by tree hollow abundance and body size. I tested this hypothesis by comparing the selective pressures and constraints acting on lemuriform tree hollow inhabitants, lemuriform non-tree hollow inhabitants, and tree hollow inhabitant marsupials in Australia. Because data on tree hollow abundance are lacking in Madagascar, I used a proxy for tree hollow abundance: mean yearly rainfall, which positively correlates with global tree hollow abundance. Within Madagascar, tree hollow inhabitant sites did not differ in thermoregulatory demands or tree hollow abundance, but non-tree hollow inhabiting lemuriformes were significantly larger than tree hollow inhabiting lemuriformes. Furthermore, tree hollow inhabitant sites in Madagascar had significantly higher rainfall levels (suggesting high tree hollow abundance) than Australian tree hollow inhabitant sites, but Australian tree hollow inhabitant sites had significantly higher temperature variability (suggesting higher thermoregulatory demands) than tree hollow inhabitant sites in Madagascar. Therefore, the species richness of mammalian tree hollow inhabitants in both regions may be due to different selective pressures. Because nearly 80% of tree hollow inhabiting lemuriformes are critically endangered, endangered, or vulnerable, understanding the selective pressures and constraints for inhabitance of this keystone vegetation structure is critical to conserving tree hollow inhabitant species threatened with extinction.