The aye-aye, a type of lemur, has long been known for its bizarre looks. As Wired puts it, the species has “the tail of a squirrel, the ears of a bat, and a perpetual look like it just realized it left the oven on.” (See confirmation here.) But new evidence suggests that the “endearing aberration” of a primate is even weirder than previously thought. A new study published last month reveals that aye-ayes have a never-before-noticed “spindly” sixth digit on their wrist. The finger is called a “pseudo thumb,” and it even has a fingerprint.
The researchers who discovered the thumb think that it evolved so aye-ayes could grip tree branches. Their other longer, thinner fingers are designed for tap foraging—the tapping method aye-ayes use to find insect tunnels in tree bark—and could snap under their body weight. The species made up for this, the researchers argue, with pseudo thumbs.
You can read more about aye-ayes and their atypical anatomy here!