The world’s weirdest primate just got weirder.
Researchers from North Carolina State University have discovered an extra finger on the outlandish-looking aye-aye.
Small “pseudothumbs,” complete with their own fingerprints, may help the animals grip branches as they move through trees.
This is the first accessory digit ever found in a primate.
The endangered lemurs (Daubentonia madagascariensis) are native to remote parts of Madagascar. Born weighing just a few ounces, adults reach up to 5 lbs. and live as long as 20 years.
The world’s largest nocturnal primate, their distinctive look combines various skin-crawling features: rodent-like teeth, bushy tails, over-exaggeratedly piercing eyes, and skeletal fingers with an extra-long middle finger and hooked claws.
“The aye-aye has the craziest hand of any primate,” lead study author Adam Hartstone-Rose, associate professor of biological sciences at NC State, said in a statement. “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,” he explained, “it looks like a strange lemur walking on spiders.”
Hartstone-Rose and NC State post-doctoral researcher Edwin Dickinson made the discovery while studying the aye-aye’s unusual tendons—one of which branched off toward a small structure on the wrist.
“The pseudothumb is definitely more than just a nub,” Hartstone-Rose said.
Composed of bone and cartilage with three distinct muscles, it 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,” according to the researchers.
This sixth appendage, evident in male and female aye-ayes ranging in age from juvenile to adult, may have developed to compensate for its other, overspecialized fingers.
Other species such as the panda bear, mole, and some reptiles have also evolved an extra digit for more efficient gripping, digging, or swimming.
“In this case, the aye-aye’s hand is so specialized for foraging an extra digit for mobility became necessary,” Hartstone-Rose said.
“The aye-aye is the first primate to dial digits up in the hand rather than dial them down,” he continued. “And it’s amazing that it’s been there the whole time, in this strangest of all primates, but no one noticed it until now.”
Full study results were published in the American Journal of Physical Anthroplogy.
With only 24 residing in zoos across the US, and an unknown number in the wild, aye-ayes are among the world’s rarest—and hardest to see—animals.
But thanks to the recent birth of female Tonks, three of the nocturnal lemurs—considered by some as “so ugly they’re cute” (I’m firmly in the “they’re just plain frightful” camp)—call Denver Zoo their home.
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