This lemur has long been known for being pretty peculiar in the hand department. It is endowed with extraordinarily elongated, bony middle fingers, which it uses to winkle out grubs from tree trunks and branches.
But it just got weirder. An arboreal primate’s hands must be proficient at other tasks, too – not least, clambering through the canopy – and so to this end, the aye-aye also sports an extra ‘finger’ on each hand.
We have only just noticed this digit, because it is rather small. It’s formed from one of the wrist bones and functions like an opposable thumb, allowing the lemur to wrap its hand around a branch.
No other primate has this kind of false thumb, but similar structures are found in giant pandas, which use them to grasp bamboo, and in moles, where they increase the surface area of the hand – all the better for shovelling soil with.
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Main image: Aye-aye on a palm frond, Masoala Peninsula, Madagascar. © Thorsten Negro/Getty