Pseudothumb

Pseudothumb, often called a false thumb, is a morphological adaptation in some mammals in which the radial sesamoid bone at the wrist enlarges to form a projection that functions as an opposable digit. It is not a true thumb, which involves a metacarpal and proximal phalanx; the pseudothumb is a modified bone that, together with surrounding soft tissues, allows enhanced grasping.

The best-known examples are in the giant panda (Ailuropoda melanoleuca) and the aye-aye (Daubentonia madagascariensis). In the panda, the radial sesamoid expands into a pad-like structure that facilitates gripping bamboo culms; in the aye-aye, a similarly enlarged radial bone aids in handling small objects during foraging. In both cases, the structure is a case of convergent evolution, evolving independently to improve manipulation of specific foods.

Functionally, the pseudothumb increases the animal’s ability to manipulate objects and feed efficiently. It acts as a specialized platform that aids gripping, but it is not a true opposable digit and does not replicate the full range of motion of a normal thumb.

In scientific literature, the term pseudothumb is used to describe this type of adaptation, particularly in discussions of bamboo-feeding pandas and foraging-related grasping in aye-ayes. The concept is also relevant to studies of convergent evolution and the diversification of skeletal structures in mammals.