With bulging eyes, spindly fingers, and bat-like ears, aye-ayes are arguably the world’s weirdest primate. New research now finds that these small lemurs are also world-class secret keepers.

Researchers at North Carolina State University were studying the alien-like anatomy of the aye-aye hand. Found only in the biodiverse nation of Madagascar, aye-ayes have five “creepy, spindly little fingers” that are unique to only their species. A particularly long and skinny middle finger has a ball-and-socket joint where the knuckle would be so that the primate can swivel it around to tap along wood, finding food by using echolocation. Once it hears a grub, it uses its bizarre finger to fish out the tasty treat.

"The aye-aye has the craziest hand of any primate," said evolutionary biologist and lead study author Adam Hartstone-Rose 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, it looks like a strange lemur walking on spiders."

To better understand the role of these unique fingers, scientists physically dissected the forearm musculature to observe the tendons, when they traced one that went into the hand and split into a “weird appendage” accessory digit, or a “secret sixth finger,” located in a small structure on the wrist. MRI data confirmed the existence of this “pseudothumb” in the left and right hands of male and female juveniles and adults, both raised in captivity and in the wild.

"The pseudothumb is definitely more than just a nub," said Hartstone-Rose. "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."

Consisting of both a bony and cartilaginous portion, the unique musculature allows the thumb to move in three directions in the same way that human thumbs rotate. Three distinct muscles allow for the aye-aye to exert almost a kilogram of force during contraction – all the better for snatching up tasty insect morsels.

But why such a bizarre adaptation? Researchers believe it could have evolved to compensate for the “overspecialization” of its other fingers. Such adaptations have been observed in other species like the panda, which has evolved a similarly specialized pseudothumb out of its wrist in order to grab bamboo.  

"Other species, like the panda bear, 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," said Hartstone-Rose. "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."

The study, published in the American Journal of Physical Anthropology, confirms the aye-aye is the first primate to our knowledge to have developed more digits in order to aid in locomotion, rather than less when they're not needed anymore.

“[I]t's amazing that it's been there the whole time, in this strangest of all primates, but no one has noticed it until now,” said Hartstone-Rose.