An international team of researchers from North Carolina State University and CNRS have found that aye-ayes (Daubentonia madagascariensis) possess pseudothumbs — which consist of both a bony component and a dense cartilaginous extension — that may help them grip objects and branches as they move through trees.

The aye-aye is a type of lemur that is found only on the island of Madagascar.

The species is famed for its large eyes, big ears and bony finger used for probing.

“The aye-aye has the craziest hand of any primate,” said Dr. Adam Hartstone-Rose, a scientist in the Department of Biological Sciences at North Carolina State University.

“Their fingers have evolved to be extremely specialized — so specialized, in fact, that they aren’t much help when it comes to moving through trees. When you watch them move, it looks like a strange lemur walking on spiders.”

Dr. Hartstone-Rose and colleagues were studying the tendons that lead to the aye-aye’s unusual hands when they noticed that one of the tendons branched off toward a small structure on the wrist.

Using physical dissection techniques and digital imaging on seven individuals (six adults and one immature individual), the researchers found that the structure in question is composed of both bone and cartilage, and has musculature that allows it to move in 3D.

“Using these digital techniques allows us to visualize these structures in 3D, and to understand the organization of the muscles which provide movement to the digit,” said Dr. Edwin Dickinson, a postdoctoral researcher at North Carolina State University.

“The pseudothumb is definitely more than just a nub. It has both a bone and cartilaginous extension and three distinct muscles that move it,” Dr. Hartstone-Rose added.

“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.”

The aye-aye may have developed the pseudothumb to compensate for its other, overspecialized fingers.

“Other species, like the giant panda, have developed the same extra digit to aid in gripping because the standard bear paw is too generalized to allow the dexterity necessary for grasping,” Dr. Hartstone-Rose said.

“And moles and some extinct swimming reptiles have added extra digits to widen the hand for more efficient digging or swimming. In this case, the aye-aye’s hand is so specialized for foraging an extra digit for mobility became necessary.”

“Some other primate species have reduced digits to aid in locomotion. The aye-aye is the first primate to dial digits up in the hand rather than dial them down. And it’s amazing that it’s been there the whole time, in this strangest of all primates, but no one has noticed it until now.”

The discovery is reported in a paper in the American Journal of Physical Anthropology.

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Adam Hartstone-Rose et al. A primate with a panda’s thumb: the anatomy of the pseudothumb of Daubentonia madagascariensis. American Journal of Physical Anthropology, published online October 21, 2019; doi: 10.1002/ajpa.23936