The Bizarre Aye-Aye Isn’t Giving Us the Finger After All

If it seems too good to be true, the old cliché goes, it probably is. And it doesn’t get much gooder than the bizarre hand of the aye-aye, a specialized lemur that uses a hyper-elongated middle finger to tap along hollow tree branches, listens for grubs within, gnaws a hole in the wood, and reaches that middle finger inside to fish out the food. It would seem, then, that the aye-aye (so named because of its cries) wanders the forests of Madagascar giving the world the highly elongated finger.

But now, a discovery that really ruins that gag: While exploring the anatomy of the aye-aye’s forearm and hand, a group of researchers discovered the critter has tiny pseudothumbs that likely help it grip branches. Technically speaking, then, the aye-aye has six digits on each hand, so it has no middle finger. Thus in one discovery, the aye-aye grows more remarkable yet less joke-worthy.

Found only on Madagascar, the aye-aye’s got the tail of a squirrel, the ears of a bat, and a perpetual look like it just realized it left the oven on. Surely its most bizarre adaptation, though, is that exceptional finger. Ambling along dead branches, it rapidly taps the wood, cocking its giant ears to pinpoint insect larvae squirming within. Target acquired, it tears at the wood with rodent-like teeth that are both continually growing, like a beaver’s, and so strong that in captivity aye-ayes have been known to chew through cinder blocks. Once it has torn open a hole, the hunter reaches in with that long, thin finger, which actually swivels in a ball-and-socket joint, like a human’s shoulder. At the end of the finger is a hooked nail that snags the grub and drags it out.

Photograph: David Haring/Duke Lemur Center

Problem, though: Having such a long and delicate finger isn’t conducive to getting a good grip on branches as the aye-aye forages. The aye-aye, the researchers reckon, evolved a pseudothumb to help it locomote without plummeting out of trees. In that way it’s like the panda, which also evolved a pseudothumb to help it grasp bamboo, a sort of pad situated below the row of five other fingers.

But how on Earth have researchers just now figured out the aye-aye has pseudothumbs, when the species has been known to science since the 18th century? In fairness, the digit is tiny, and indeed these scientists found it only by accident. They were exploring the forearm and hand anatomy of a specimen, specifically the tendon that in humans operates the thumb. In this aye-aye specimen, most of the tendon went to the base of its thumb, but part of the tendon spilt and headed through a wrist bone that we don’t have, called a radial sesamoid.

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