Aye-aye is the first primate with a sixth finger
One of the world’s weirdest animals just got a little bit weirder. The aye-aye – a nocturnal, Madagascan lemur with satellitedish ears and dinnerplate eyes – is now the first known primate to have a sixth finger.
This tiny extra digit – called a ‘pseudothumb’ – is a structure on each wrist made of bone and cartilage. It is believed to have evolved to help the lemur grip branches as it climbs through the trees.
“The aye-aye has the craziest hand of any primate,” said study leader Dr Adam Hartstone-Rose, a biologist and anatomist at North Carolina State University.
Most famously, the lemur’s hand sports an elongated middle finger, which it uses to tap against trees to find grubs, locating hollow areas by listening for the echoes.
Now, Hartstone-Rose and his team have discovered an extra structure, which they noticed when studying the tendons in the aye-aye’s hand.
To examine the structure in more detail, they dissected six aye-aye specimens and used digital imaging to visualise the pseudothumb in 3D, finding that it is attached to three distinct muscles.
“The pseudothumb can wriggle in space and exert an amount of force equivalent to almost half the aye-aye’s body weight,” said Hartstone-Rose. “So it would be quite useful for gripping.
“Other species, like the panda bear, have developed the same extra digit to aid in gripping, because the standard bear paw is too generalised to allow the dexterity necessary for grasping.”
The researchers believe that the aye-aye, on the other hand, developed this digit to compensate for its other, overspecialised fingers.
“Some other primate species have reduced digits to aid in locomotion,” said Hartstone-Rose. “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.”