EVOLUTION OF BONE CORTICAL COMPACTNESS IN SLOW ARBOREAL MAMMALS

Convergent evolution is a fundamental topic in the study of present and past livings’ diversity. The two genera of ‘tree sloths’ convergently evolved their slow arboreal ecology and they were previously found to feature low bone Cortical Compactness (CC). This phenotype was proposed to reflect the convergent lifestyle. Here, the possibility of low CC in other slow arboreal mammals is, thus, considered. Since never investigated, we analyzed CC in the humerus and the femur of a wide taxonomic sample while taking into account phylogenetic relatedness in our statistical analyses. Beside ‘tree sloths’, the sampling includes other slow arboreal mammals (extant: ‘Lorisidae’, koalas; extinct: Palaeopropithecidae and Megaladapidae) and close relatives with different ecologies (extant: anteaters, armadillos, galagids, wombats, ‘indriids’, and lemurids; extinct: sloths). We observe that CC in ‘tree sloths’ is the lowest in the analyzed clades and that a lower CC trend exists in palaeopropithecids (especially Palaeopropithecus) and megaladapids. We propose low CC as a convergent trait shared by palaeopropithecids, megaladapids and ‘tree sloths’. However, no low CC tendency was retrieved in ‘Lorisidae’. Moreover, koalas, despite with an evidently different compact bone structure in respect to the generalized mammalian condition, show differences between the humerus and the femur, not coherent with a systemic bone adaptation. Thus, no clear general relationships were found between slow arborealism and low CC and a likely multifactorial explanation is invoked. Finally, high CC in extinct sloths suggests the recent convergent evolution of low CC in the two lineages of ‘tree sloths’.