Aye-ayes just got weirder with the discovery of a tiny, sixth ‘finger’

An extra digit, a pseudothumb, may help the primates grip objects

aye-aye

Aye-ayes’ long, slender fingers are highly specialized for a hunting technique called tap foraging. Now researchers have discovered the primates have a nubby sixth digit, too.

David Haring/Duke Lemur Center

Aye-ayes just got even more unusual. The tiny lemurs of Madagascar, known for their large cartoonish ears and continuously growing incisor teeth, also have a sixth “finger” on each hand.

The extra digit — a nubby little “pseudothumb” made of bone and cartilage — can move in three directions and carries its own distinctive fingerprint, researchers report October 21 in the American Journal of Physical Anthropology.

“It’s more than just a nub. It actually has a lot of function to it,” says study coauthor Adam Hartstone-Rose, a comparative anatomist at North Carolina State University in Raleigh. The pseudothumb, which is manipulated by three muscles, may help aye-ayes (Daubentonia madagascariensis) grip objects or branches.

aye-aye pseudothumb
Aye-ayes have a sixth digit, a pseudothumb (shown in the white box at center and enlarged in the inset) made of bone and cartilage that has three muscles that move it and a distinctive fingerprint.A. Hartstone-Rose

It’s the first time that a pseudothumb has been discovered on any primate, although some people are born with extra fingers (SN: 6/12/19). Other species also have pseudothumbs, including giant pandas, which use their sixth digits to grasp bamboo stalks (SN: 1/31/19). Giant pandas may have acquired that extra digit after the rest of their fingers became less specialized so that the bears could better walk. That’s not the case with aye-ayes, though, the scientists think.

Instead, the little lemurs’ hands may have become too specialized, with thin, elongated fingers, including an especially long third digit that has a ball-and-socket joint. That finger, in particular, is used in a hunting technique called tap foraging, where the animals tap the finger on dead and rotting wood and use echolocation to find bugs hiding inside. Then the primates bite the wood, puncturing a hole, and again use their long third finger for fishing out bugs and grubs found inside.

“Their fingers became so long and spindly that they were no longer good at finger stuff, like grasping,” Hartstone-Rose suggests. The pseudothumb may compensate for the aye-ayes’ other, overspecialized fingers, he and colleagues say.

This interpretation is plausible, says John Hutchinson, an evolutionary biologist at the Royal Veterinary College at the University of London, who was not involved in the work. But he notes that, in general, scientists “don’t know much about what false digits do in most species.”

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