Myological and Osteological Correlates of Hindfoot Reversal in the Kinkajou (Potos flavus)

Abstract

The kinkajou (Potos flavus) is a South and Central American procyonid that can plantarflex and invert its hindfoot around multiple joints in order to fully reverse the hindfoot 180°. However, the myological and osteological adaptions that facilitate this behavior have not been quantified metrically. Osteological correlates of hindfoot reversal have been described in the past, but recent advances in data collection allow for a more comprehensive evaluation of shape differences. Osteological features of the astragalus (talus), calcaneus, and distal tibia of P. flavus were collected and compared to other carnivoran species, including species that are full, partial, and non-reversers of the hindfoot. Hindfoot reversing procyonids were found to possess a relatively smaller medial malleousl, a wider sustentaculum tali, narrower calcaneal tuberosity, and a relatively longer astragalus. In general, the dimensions of the astragalus tend to discriminate hindfoot reversing taxa from other locomotor categories more reliably than the dimensions of the calcaneus or distal tibia. A detailed dissection, description, and documentation of the hind limb muscles in P. flavus was undertaken, and we present the first comprehensive review of its muscular anatomy utilizing muscle maps of the pelvis, femur, tibia, and fibula. Potos flavus has well-developed plantarflexor and invertor muscles, traits which have been hypothesized to characterize species that perform hindfoot reversal. This finding contrasts with other procyonid hindfoot reversing species, such as Bassariscus astutus, which appear to lack muscular adaptations to hindfoot reversal. Muscle semimembranosus, a muscle that facilitates hind limb suspension, was also enlarged. Overall, the hind limb musculature and hindfoot osteology of the kinkajou are consistent with proposed adaptations to hindfoot reversal.