WING TIP ANATOMY AND AERODYNAMICS IN FLYING SQUIRRELS

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 thi s styliform cartilage is extended has been misunderstood, and the sign ificance of the upturned wing tip in flying squirrels has not been app reciated. To understand this mechanism, we examined the wrist anatomy of flying squirrels and compared it with wrists of tree squirrels to d etermine homologies. We found that the wing tip was extended by the ab ductor of the thumb on the opposite side of the wrist from the stylifo rm cartilage. This muscle acts by means of a ligament between the falc iform bone and the styliform cartilage. In tree squirrels, the homolog ue of the styliform cartilage is the previously undescribed hypothenar cartilage which supports the hypothenar pad of the hand. The stylifor m-falciform ligament is derived from superficial fibers of the transca rpal ligament. Wing tips of flying squirrels are held at an upward ang le to the rest of the wing and form airfoils. We propose that they ser ve the function of reducing induced drag, similar to the winglets of m odern aircraft.