Meeting Abstract
Head first descent from trees is a common attribute of most arboreal mammals, allowing an animal to see and evaluate potential dangers on the ground during descent. In order to engage their claws and muscles during head first descent, arboreal mammals typically depend on hind foot reversal (HFR), in which the extended hind limb plantarflexes and supinates such that the plantar surface can contact the support while the digits point skyward. While arboreality appears to be the ancestral condition for sciurids, the ankle modifications that permit HFR are a derived trait seen in modern tree squirrels that are specialized for arboreal locomotion. Although the mechanism of hind foot reversal in sciurids has been noted in the literature, no detailed measures of HFR have been reported. In this study, we quantified HFR in 5 sciurid species that varied in their level of arboreality. Specifically, we examined two arboreal species (Gray squirrel, Sciurus carolinensis; Fox squirrel, Sciurus niger), one semi-arboreal species (Eastern chipmunk, Tamias striatus) and two terrestrial species (Utah prairie dog, Cynomys parvidens; Marmot, Marmota monax). Prairie dogs had significantly lower plantarflexion and supination compared to other species, while the highly arboreal gray and fox squirrels were significantly higher in both measures. Chipmunks and marmots were similar to arboreal species with respect to plantarflexion but were intermediate in value with respect to supination. While marmots are classically considered terrestrial, they can climb trees and these results suggest they should more correctly be classified as semi-arboreal. The combination of significant plantarflexion coupled with intermediate supination may not permit full HFR, but the overall increased foot mobility may be a requirement to efficiently navigate both on the ground and in the trees.
