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Aye-Ayes Have Been Hiding a Secret Sixth Finger This Whole Time

Aye-ayes are among the strangest primates on Earth, with their oversized ears, spider-like hands, and — as new research shows — a previously undetected sixth finger.

Five fingers per hand tend to be the norm in The animal kingdom, but scientists have documented several cases in which organisms feature a sixth finger, an anatomical quirk known as an "accessory digit." An extra digit among giant pandas, for example, improves their grip, while for moles and some extinct reptiles , the bonus extremity allows for more powerful digging through dirt and more efficient swimming.

Aye-ayes feature a specialized middle finger, which they use to poke grubs out of trees.
Image : David Haring / Duke Lemur Center

New research published this week in the American Journal of Physical Anthropology describes a previously unknown sixth finger in aye-ayes, making it is the first primate known to have an accessory digit.

As in giant pandas, the aye-aye's extra digital compensates for an otherwise missing thumb, hence its designation as a "pseudothumb." And because both aye-ayes and pandas use the pseudothumb to improve their grasping abilities, the feature is considered an example of convergent evolution in which the same trait emerges spontaneously in unrelated species.

The pseudothumb is also a striking example of contingent evolution, in which a species evolves an adaptive trait to compensate for a highly specialized pre-existing trait that, while beneficial, is also very constraining. Giant pandas, for example, have claws well-suited for walking but not grasping, and the pseudothumb emerged as a compensatory response. The pseudothumb in aye-ayes, however, came about because of its highly specialized hand — especially its middle finger — which it uses to hunt and capture grubs. The authors of the new study, led by evolutionary biologist Adam Hartstone-Rose of North Carolina State University, say the pseudothumb makes the aye-aye's hand more dextrous and capable of grasping, allowing the animals to hold onto branches as they move through trees

The sixth finger is hardly the only thing about aye-ayes that makes them weird. Found exclusively in Madagascar, the world's largest nocturnal primate. Aye-ayes have oversized ears, incisors in a perpetual state of regrowth, and exceptionally thin and elongated middle fingers. Fascinatingly, aye-ayes fill an ecological niche normally occupied by woodpeckers. After using echolocation to find grubs inside of trees, the aye-ayes use their incisors to poke through the trunk, snatching the grubs with their long middle fingers.

Digital scan showing the bone and cartilage of the sixth digit.
Image : A. Hartstone-Rose et al., 2019

Hartstone-Rose and NC State post-doctoral researcher Edwin Dickinson were studying aye-aye arm tendons when they stumbled upon the previously unknown psuedothumb. Using a 3D digital imaging technique, the scientists noticed that one of the tendons branched off towards a tiny anatomical structure on the wrist. Exploring further, the researchers found that the structure consisted of bone and cartilage supported by three distinct muscles.

This particular configuration, according to the new research, allows aye-ayes to move the extra digit in multiple directions, not unlike the human thumb. And with this crafty pseudothumb, the aye-ayes are capable of exerting nearly one kilogram (2.2 pounds) of force when making contracting hand movements.

"The pseudothumb is definitely more than just a hub," Hartstone-Rose said in a press release. “It has both a bone and cartilaginous extension and three distinct muscles that move it. The pseudothumb can wriggle in space and exert an amount of force equivalent to almost half the aye-aye's body weight. So it would be quite useful for gripping. ”

A very cool result, but future researchers could bolster these findings by studying living aye-ayes and watching the sixth finger in action. Sadly, the International Union for Conservation of Nature lists aye-ayes as an endangered specie s, its primary threats being the encroachment of agriculture, logging, and hunting. Importantly, the aye-ayes used in this study — six adults and one juvenile — were already dead, so "no animals were sacrificed for the purposes of this study," according to the paper.

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