The functional and comparative anatomy of the forelimb muscles in the sun bear (Helarctos malayanus)

Abstract

The sun bear (Helarctos malayanus) is the smallest extant member of the Ursidae, inhabiting the tropical mainland forests of Southeast Asia and the islands of Sumatra and Borneo. This study provides morphological data for the forelimb of Helarctos, whose anatomy has been scarcely documented. The left and right forelimbs of a 16 year old female sun bear from the Reid Park Zoo (Tucson, AZ) were dissected and photodocumented. In addition, muscle origins and insertions were recorded and muscle maps produced, providing a valuable resource for those analyzing the postcranial remains of fossil ursids. The forelimb musculature was compared to that of other carnivores and analyzed for functional adaptations. Dissections revealed common ursid features, such as powerful flexors of the digits, elbow and carpal joints, as well as traits particular to the sun bear. For example, m. flexor carpi radialis has an extensive insertion onto the scapholunatum and metacarpals II and III, assisting in adduction of the carpal joint. Mm. flexor digitorum brevis manus and palmaris brevis were also observed, which may contribute to increased grasping ability of the manus. Taken together, these findings are likely adaptations to the climbing and feeding behaviors which are central to the arboreal lifestyle of Helarctos.