Aye-ayes are surely most famous for the extraordinary, elongated, bony middle fingers that they use to winkle grubs from wood. But that’s not all these nocturnal lemurs use them for, according to new research – they also double as the perfect nose-picking tools.

“I was working on the evolution of grasping ability in primates, so I was studying lots of species eating and moving,” says Anne-Claire Fabre of Bern Natural History Museum, Switzerland. “I was recording this aye-aye eating and all of a sudden it stopped and started picking its nose and eating its snot.”

The behaviour was all the more startling for the fact that the aye-aye was capable of pushing the entire finger – all 10cm of it – into its nostrils, a feat that might make even human children’s eyes water.

Anne-Claire Fabre and an aye-aye called Merlin. © David Haring
Anne-Claire Fabre and an aye-aye called Merlin. © David Haring

“It was pretty impressive,” says Fabre. “I started wondering where exactly the finger was going.”

Scans of the aye-aye’s skulls revealed that the finger passes all the way through the nasal cavity and down into the throat.

Her research, published in the Journal of Zoology, also documents evidence of nine other primate species picking and eating their nasal mucous, which she found by trawling through online videos.

The challenge now is to work out why they do it. One possible answer is: because they can. “The aye-aye’s finger is shaped like it is because of its specialised feeding behaviour,” says Fabre. “It puts it in its nose because it fits in its nose.”

But it’s also possible that rhinotillexis – to give it its technical name – has a biological function. Fabre doubts the mucous, being 95 per cent water, provides much in the way of nutritional benefit. She is intrigued by the idea that it plays a role in priming the immune system via the ingestion of microbes trapped in the mucous. There is also some evidence that people who pick their nose require fewer dental cavities.

“I am not a nose-picking specialist,” says Fabre. “But in the future it would be really fun to look at the precise composition of nasal mucous.”

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Main image: aye-ayes could give human children a run for their money when it comes to nose-picking abilities © Thorsten Negro/Getty