Evolution of bone cortical compactness in slow arboreal mammals

Abstract

Lifestyle convergences and related phenotypes are a major topic in evolutionary biology. Low bone cortical compactness (CC), shared by the two genera of tree sloths, has been linked to their convergently evolved slow arboreal ecology. The proposed relationship of low CC with tree sloths lifestyle suggests a potential convergent acquisition of this trait in other slow arboreal mammals. Tree sloths, Lorisidae, Palaeopropithecidae, Megaladapis and koalas were included in a phylogenetically informed CC analysis of the humerus and femur, as well as closely related but ecologically distinct taxa. We found that tree sloths are unparalleled by any analysed clade, in featuring an extremely low CC. A tendency for low CC was however found in palaeopropithecids (especially Palaeopropithecus) and Megaladapis. On the other hand, low CC was not found in Lorisidae. Koalas, although deviating from the compact structure generally observed in mammals, are not clearly distinct from their sister taxon (wombats) and show humeral CC that is higher than femoral CC. Multiple factors seem to influence CC, preventing the recognition of a simple relationship between slow arborealism and low CC. Highly compact cortical bone in extinct sloths confirms that low CC in tree sloths likely represents a recent convergence.