Locomotor characterization of the sub-fossil lemur Babakotia

Palaeopropithecids or “sloth lemurs” are a diverse clade of large-bodied Malagasy subfossil primates characterized by their suspensory positional behaviors. The most recently discovered genus of the family is Babakotia. This species has been described as more arboreal than Mesopropithecus but less than Palaeopropithecus. A remarkably complete skeleton of Babakotia is housed at the Division of Fossil Primates of the Duke University Primate Center. To further understand arboreal adaptations of Babakotia we compared within bone and between bones articular and cross-sectional diaphyseal proportions of limb long bones of this specimen to suspensory primates (hominoids and monkeys) and true sloths (Choloepus and Bradypus). Results show that Babakotia humeral articular to diaphyseal proportions are generally similar to those of true sloths than to those of apes. Babakotia femoral proportions are also similar to true sloth distributions, but are also similar to lesser apes in their proximal ends (i.e., relative femoral head surface area and superoinferior breadth). Distally, Babakotia is closer to great apes and lesser apes than to monkeys; unfortunately true sloth comparative data are missing for this comparison. Interlimb proportions show that Babakotia’s relative diaphyseal strengths are similar to the mean value for orangutans and in the range of true sloths. As for interlimb articular proportions, Babakotia overlaps with great apes for proximal articulations, but with true sloths for distal articulations. These results provide new evidence on the arboreal adaptations of Babakotia and further highlight similarities with both extant suspensory primates and non-primate slow arboreal climbers and hangers, i.e., true sloths.