Madagascars aye-aye lemur is an endearing aberration of an animal.

It has enormous ears, a bushy tail, mammary glands between its legs and white hairs that bristle when its agitated. Above a vampiric nose and mouth of incisors that never stop growing, candy-corn-colored eyes glow like full moons. Gangly fingers capped in curled claws punctuate its hands. The middle finger, a spindly stick, rotates on a ball-and-socket joint. Sometimes it folds over an even longer fourth finger.

If these werent enough to meet the quota for eccentric adaptations, scientists just found another a secret spare thumb. This pseudo thumb, as it is called, even has a fingerprint.

Weve studied the function of those weird, spindly middle fingers so long that nobody ever looked at this kind of lowly structure on the wrist, said Adam Hartstone-Rose, an anatomist at North Carolina State University who led the team that completed the research. But that structure has arguably more evolutionary significance than that weird, little spindly finger.

Other animals also have extra appendages. But the aye-ayes spare thumb, described on Monday in The American Journal of Physical Anthropology, may have developed under circumstances not yet observed in any other animals. The study demonstrates that theres still much more to learn about how different lineages of vertebrates evolved even by looking at anatomy we thought we understood.

A century of study and numerous X-rays overlooked the aye-ayes pseudo thumb. Dr. Hartstone-Rose and his team found it by accident when examining the aye-ayes forearm muscles. They followed a muscle that attaches to all primates hands for mobility, but were surprised to find tiny tendons also connecting it to the palm and a nubby bone between the thumb and wrist with a cartilaginous extension.

We realized that there were these different muscles that were controlling this whole thing, Dr. Hartstone-Rose said, allowing the structure to move away from and toward the other fingers, as well as into the palm, like a thumb.

They confirmed this structure existed in seven total aye-ayes, suggesting it was part of the misfit mammals anatomy. And the reason for it seems to be related to its peculiar foraging technique.

At night, aye-ayes hunt using echolocation. They tap branches with slender fingers, listening for voids in the wood. When a branch bounces back a particular hollow howl insect tunnels intersecting beneath the bark the lemur bites down and rips open the woody flesh. It inserts its spindly middle finger into the hole and swivels it around, fishing out grubs with its claws.

While these long, delicate fingers are perfect for tap foraging, theyre lousy at much else. If the animal tried using the middle finger for traveling, it would snap beneath the aye-ayes weight.

The aye-ayes hands became so specialized at this tap foraging that they lost the ability to grip, Dr. Hartstone-Rose said. They compensated, he thinks, with pseudo thumbs.

If the researchers confirm their hypothesis by studying aye-aye lemurs living at the Duke Lemur Center in Durham, N.C., the aye-aye will be the first animal described to have evolved an extra digit for dexterity because of a hyper-specialized trait.

In other cases, animals evolved additional digits to compensate when their fingers became less specialized, or to broaden their hands. For instance, the original thumb became just another finger for the bear lineage, but the Giant Panda developed a pseudo thumb because it helped with eating bamboo. And extra digits made moles better diggers.

But aye-ayes may not be alone for long: Were starting to look at some of the wrist structures in bats, Dr. Hartstone-Rose said. We think that they have some similar kinds of little grasping accessory digits that theyve added to their wrists.

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Add Pseudo Thumb to the Aye-Aye Lemurs Bizarre Anatomy - The New York Times