Aye-aye the first primate with a sixth finger © David Haring/Duke Lemur Center

Aye-aye the first primate with a sixth finger

This tiny extra digit – called a ‘pseudothumb’ – believed to have evolved to help the lemur grip branches as it climbs through the trees.

One of the world’s weirdest animals just got a little bit weirder. The aye-aye – a nocturnal, Madagascan lemur with satellite-dish ears and dinner-plate eyes – is now the first known primate to have a sixth finger.

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This tiny extra digit – called a ‘pseudothumb’ – is a structure on each wrist made of bone and cartilage. It’s 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 Dr Adam Hartstone-Rose, a biologist and anatomist at North Carolina State University who led the work.

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’s 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.

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The researchers believe that the aye-aye, on the other hand, developed this digit to compensate for its other, overspecialised fingers.

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“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.”

What is an aye-aye?

Lemurs exist only on the island of Madagascar. Most of these primates are furry, cuddly-looking creatures, except one: the aye-aye.

The aye-aye possesses rodent-like teeth that never stop growing, piercing eyes that allow it to forage at night and a middle finger so long and bony that it almost looks like a spider’s leg. Incredibly, the aye-aye has woodpeckers to thank for this latter adaptation.

Woodpeckers never made it to Madagascar, which meant the aye-aye could fill the niche for eating wood-boring grubs. The animal taps its elongated middle finger against tree stumps to locate grubs, listening for the tell-tale echoes of hollow areas, which indicate the presence of food. This hunting technique makes the aye-aye the only known primate to echolocate its prey: hence its extraordinarily sensitive, bat-like ears.

The aye-aye is to lemurs what Stephen King’s Pennywise is to clowns, at least according to local Malagasy legend. One belief is that this creature creeps into the houses of villagers at night and uses its elongated finger to slit the throats of sleeping children. The truth – if you’re a grub – is just as grisly.

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