The Hindlimb and Foot
The hindlimb of bats is no longer a weight-bearing appendage, such as that of the human leg. The upper leg bone (femur) has been rotated 180 degrees from it’s normal position in terrestrial animals, which causes the knee to bend in the opposite direction to that of a human. This facilitates variability while in flight. Since bats are no longer walking around on their feet, the strength required for compression, is not required. Bats do hang from their legs, but the strength for tension, is considerably different. Imagine supporting a kilogram of weight on the top of a vertically placed strand of dried spaghetti, versus hanging the same kilogram of weight FROM the strand of spagehetti!
The tendons in the legs and feet of bats are arranged in such a way that suspension of the bat causes the toes and claws to grip the foothold. This is important so that the bat can go to sleep, or even hibernate for months at a time without releasing it’s grip. The bat must actually be awake in order to release it’s grip.
Another structure unique to bats, is the calcar. It is a cartilaginous structure that articulates with the heel bone. It’s function is to support the trailing edge of the uropatagium during flight. In fishing bat species, it is very long and blade like, and in this species, it’s function is to hold the uropatagium out of the water. Recent discoveries show that the Hairy legged vampire (Diphylla ecaudata) use the calcar as an opposable thumb, while climbing trees or holding onto items!
