Digital morphology in the Chiroptera: the passive digital lock.

Most bats hang from their hindlimb digits for extended periods of time during a variety of activities. Many species possess digital locking mechanisms consisting of modified digital flexor tendons and tendon retinacula. When the digits are flexed, scales or tubercles on the tendon surface engage transverse ribs which invest the inner surfaces of the tendon retinacula. This locks the tendon in place and prevents the digit from extending. Theoretically, by locking the digits during hanging, energy normally spent maintaining contraction of the digital flexor muscles is conserved. In this study, digits from 10 species of bats from 5 families were examined by dissection and scanning electron microscopy. Variation in digital morphology was observed in species that exhibit ecological diversity. It is proposed that hibernation and body weight are important selection pressures for possession of the passive digital lock. In nonhibernating species the mechanism may be present, modified or absent. The passive digital lock is absent in the vampire bats, a group whose feeding habits require substantially altered forms of locomotion. It is hypothesized that the passive digital lock has helped enable many species of bats to occupy habitats inaccessible to most other animals and that species lacking it may exhibit alternative mechanisms that allow them to hang in an energy-efficient manner.