In the aye-aye lemur of Madagascar, it’s an extension of the “hitchhike muscle,” attached to the radial sesamoid. FromNational Geographic:
The bone is also topped, the team reported in a study published October 21 in the American Journal of Physical Anthropology, with an extension of cartilage. Further investigation revealed two other muscles are connected to the radial sesamoid, which allow the bone to move in a gripping motion. Hartstone-Rose and colleagues have named this a “pseudo-thumb,” and suggest that it functions as sixth digit to help the arboreal animals hold onto tree limbs.
Douglas Main, “This bizarre primate has a newly discovered digit” at National Geographic
It sounds like an optional accessory:
But despite all the aye-aye’s bizarre features, their hands are perhaps their strangest attribute. The four fingers are primary thumb are long and spindly. “It kind of looks like a cat walking on spiders,” Hartstone-Rose says…
Hartstone-Rose says pseudo-thumbs are known from a few different animals. All bears used to have these digits, but most living species have lost them as they plodded around on the ground. The giant panda is the only bear that still has a pseudo-thumb, used for gripping the bamboo they feed on. Some rodents also developed pseudo-thumbs for similar reasons, to grasp twigs and grass.
A few species of extinct aquatic reptiles also had pseudo-thumbs to allow them to widen their flippers and improve their swimming efficiency. Some moles also have a pseudo-thumb to allow them to dig better,
Joshua Rapp Learn, “Extra Thumb Discovered on Aye-Aye Lemurs, Giving These Primates Six Fingers” at Smithsonian.com
Will we soon see a big internet Darwin movement for the aye-aye’s finger, alongside the panda’s thumb? Mmmm, a little late for that, perhaps.