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Monkeys living on an island have learned to use a startling variety of tools and techniques to obtain the juicy innards of different foods – and to floss their teeth afterwards.

The Nicobar long-tailed macaque () is only found on three islands in the eastern Indian Ocean. One of them is Great Nicobar Island.

To find out about the macaques’ eating habits, at the Sálim Ali Centre for Ornithology and Natural History in Coimbatore, India, and colleagues followed 20 around a small coastal village on the island.

Will use anything

Many of the macaques’ favoured foods are thorny, slimy, hairy or mucky. To get rid of these inedible coatings, the macaques either wash the foods in puddles or wrap them in leaves and rub them clean. They also wrap leaves around certain foods to make them easier to hold. Trash like paper, cloth or plastic is also used for wrapping and wiping foods.

The macaques eat coconuts too, plucking them from the tree by twisting them around or using their teeth to cut them off. If it is tender, the macaques de-husk the coconut using their teeth, holding it down with their feet and hands, in order to get to the water and juicy bits inside.

If the coconut is ripe, however, they also have to crack its shell. To do so, they take it to a hard surface like a rock or concrete, and pound it.

It’s not just tool use. The macaques were seen beating bushes with their hands to disturb insects hiding within, catching those that fly out or drop to the ground.

Clean those teeth

After eating, adult and sub-adult macaques clean their teeth – they were seen holding a fine fibre between their teeth and pulling at it.

The macaques used a range of materials as dental floss: a tree needle, a bird feather, a blade of grass, a coconut fibre, a nylon thread and a metal wire. Some modify the threads before use, for instance by tearing them apart.

Nine of the 20 macaques were seen “flossing”. They did so after eating various foods in different habitats, says Kumara.

The Nicobar long-tailed macaques are the third monkeys seen flossing their teeth. use , while Thailand’s use .

Handy monkeys

“Wrapping irritating items, cleaning by rubbing, and flossing teeth with thin fibres have been described in other populations of macaques,” says primatologist at the University of Georgia, Athens. “The newest element, to me, is ‘bush-beating’ to flush insect prey.”

Macaques adapt well to human-dominated landscapes, where they tend to manipulate objects more, says primatologist at the Nanyang Technological University, Singapore. “They are the king of generalists… about as adaptable as we are.”

Tool use on its own doesn’t take much intelligence, says Gumert. “However, the modification of tools does show planning and foresight – something that someone who has ever observed macaques in any detail would never have doubted.”