Tool-using monkeys suck shellfish dry

Humans aren’t the only primate to have pushed their prey towards extinction. Monkeys have also over-exploited animals for food.

Long-tailed macaques forage for shellfish on islands off Thailand, then crack them open with stone tools. They target the largest rock oysters, bludgeoning them with stone hammers, and pry open the meatiest snail and crab shells with the flattened edges of their tools.

These macaques are one of three primates that use stone tools, alongside chimpanzees in Africa and bearded capuchins in South America. “Stone tools open up an opportunity for foods they otherwise wouldn’t even be able to harvest,” says Lydia Luncz at the University of Oxford..

Luncz wanted to investigate the impact of the monkeys’ shellfish snacking on the prey themselves. Her team followed 18 macaques on their daily foraging routes along the shores of Koram and NomSao, two neighbouring islands off eastern Thailand, recording their tool selection and use. On Koram – the more densely populated island, home to 80 macaques compared with NomSao’s nine – Luncz’s group saw not only smaller oysters and snails, but also fewer of each species. Multiple prey species were less abundant on Koram than NomSao, with four times as many tropical periwinkles on NomSao as on Koram (eLife, doi.org/cc7d).

“It’s been shown that systematic predation causes prey of smaller size,” says Nathaniel Dominy at Dartmouth College in Hanover, New Hampshire. The oysters on Koram were about 70 per cent smaller than their counterparts on NomSao, and the periwinkles were less than half the size. A single tool-using monkey on Koram can eat over 40 shellfish a day, so Luncz’s group thinks this predation pressure is driving these shellfish changes.

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