Don’t eat all the shellfish Mark MacEwen / NaturePL

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.

Luncz says the macaques might deplete the prey on the islands. Afterwards, they will stop using stone tools and even forget how.

“Tool use, a socially learned behaviour, has always been viewed as this positive thing that opens up resources,” she says. “But by over-harvesting they’re putting their technology knowledge at risk.”

What’s more, Dominy thinks the study might help us better understand modern humans’ exit from Africa over 70,000 years ago. One idea is that our ancestors didn’t travel overland, but instead followed the Asian coastline, relying on shellfish for food.

“Over time, we see a reduction in shell size in the archaeological record, which suggests a systematic use of shellfish,” Dominy says. But nobody was sure whether size reduction was really due to large-scale human predation, or to changing ocean conditions. “This paper is the first to offer compelling evidence in support of the former,” he says.

This article appeared in print under the headline “Tool-using monkeys suck shellfish dry”

We corrected the implications of the effect that monkey tool use has on shellfish size for explanations of reducing shellfish size elsewhere