The locomotor behaviour of subfossil Malagasy sloth-lemurs (Strepsirrhini: Indriidae) and koala-lemurs (Strepsirrhini: Megaladapidae): new insights from limb trabecular bone

Abstract The locomotion of Malagasy Quaternary subfossil lemurs, including palaeopropithecines (‘sloth-lemurs’) and megaladapids (‘koala-lemurs’), has been investigated on abundant postcranial remains. Proposed strategies include some that lack living primate parallels, such as sloth-like suspensory arboreality in palaeopropithecines, although the degree of suspensory behaviour in palaeopropithecines or locomotor diversity in koala-lemurs is poorly understood. Unlike the external morphology, internal bone structure in these taxa is largely unexplored. We compared the humeral and femoral trabecular architecture of sloth- and koala-lemurs with several extant mammals, studying spherical/hemispherical trabecular samples extracted from high-resolution scans. After defining locomotor categories from quantitative data, we tested links between trabecular parameters and locomotor modes through exploratory and multivariate analyses, accounting for body size and phylogeny. In extant mammals, only femoral trabecular traits, particularly the degree of anisotropy and bone volume fraction, were significantly associated with locomotion, distinguishing suspensory and bridging arboreal taxa from others. Using this model, we inferred suspensory adaptations in palaeopropithecines, especially Palaeopropithecus, confirming earlier reconstructions, but also in Megaladapis edwardsi, a striking result that would place it alongside extant orangutans as the largest mammals known to adopt such habits. This work highlights the potential of internal bone structure for reconstructing primate locomotor evolution.