Hind limb myology of the ringtail (Bassariscus astutus) and the myology of hind foot reversal

The ringtail (Bassariscus astutus) is a widely distributed small carnivorous mammal (Procyonidae) in Mexico and the southwestern United States. As in other procyonids, the ringtail is capable of rotating its hind foot to allow headfirst descent of vertical substrates. The osteological correlates of this process, termed hind foot reversal, are well documented, but potential myological correlates have never been investigated. We present the 1st detailed study of the muscular anatomy of the hind limb of B. astutus, including the 1st muscle maps of the pelvis and pes of any procyonid. Comparison of the hind limb myology of the ringtail with other arctoid carnivorans, including taxa incapable of hind foot reversal, indicates that the muscles responsible for the action of reversal do not differ significantly between nonreversing forms and taxa capable of partial or full reversal. This suggests that specific myological adaptations are not necessary to achieve hind foot reversal. However, increased development of the digital flexors, which maintain a grip while body mass is supported by the hind limb, may characterize taxa that make use of reversed postures. The hind limb myology of members of Procyonidae does not strongly support either morphological or molecular hypotheses of relationship, in part because relatively few differences among members of the family can be documented.