Wing tip anatomy and aerodynamics in flying squirrels

Abstract

Flying squirrels are the only gliding mammals that extend the wing tip by means of a cartilage at the wrist. However, the means by which this styliform cartilage is extended has been misunderstood, and the significance of the upturned wing tip in flying squirrels has not been appreciated. To understand this mechanism, we examined the wrist anatomy of flying squirrels and compared it with wrists of tree squirrels to determine homologies. We found that the wing tip was extended by the abductor of the thumb on the opposite side of the wrist from the styliform cartilage. This muscle acts by means of a ligament between the falciform bone and the styliform cartilage. In tree squirrels, the homologue of the styliform cartilage is the previously undescribed hypothenar cartilage which supports the hypothenar pad of the hand. The styliform-falciform ligament is derived from superficial fibers of the transcarpal ligament. Wing tips of flying squirrels are held at an upward angle to the rest of the wing and form airfoils. We propose that they serve the function of reducing induced drag, similar to the winglets of modern aircraft.