Ecological and phenotypical convergences are a main aspect of evolutionary studies. The independent acquisition of slow arboreality in the two lineages of ‘tree sloths’ has been considered explanatory of their convergent low bone Cortical Compactness (CC). Accordingly, low CC can be expected in other therians with such ecology but this trait was never investigated in other slow arboreal taxa. We investigated humeral and femoral CC in a sample of 47 extinct and extant taxa, informing our statistical analysis with phylogeny. Together with ‘tree sloths’, other lineages which convergently evolved the slow arboreal lifestyle were included, namely ‘Lorisidae’, koalas, Palaeopropithecidae and Megaladapis. Moreover, closely related taxa with differing lifestyles, as anteaters, armadillos, galagids, wombats, ‘indriids’, and lemurids and extinct ‘ground sloths’ were sampled. ‘Tree sloths’ show the lowest CC in the whole sample and rather low CC was retrieved in palaeopropithecids (mainly Palaeopropithecus) and Megaladapis. Low CC in ‘tree sloths’, palaeopropithecids and Megaladapis could represent a convergent trait. However, ‘Lorisidae’ mirror the generalized mammalian condition of high CC. Koalas do not clearly differ from their close relatives (wombats) and unexpectedly yielded discrepant humeral and femoral patterns. Hence, slow arboreality in mammals is not consistently accompanied with low CC and no direct overall relationship between ecology and low CC can be established. All analyzed extinct sloths show high CC, which corroborate the hypothesis according to which the acquisition of low CC is a recent convergence event between the two genera of ‘tree sloths’.