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Another non-human primate has entered the Stone Age 鈥� the fourth type known to have done so. One population of white-faced capuchins living in Panama routinely use stones to smash open nuts and shellfish.

Other nearby populations don鈥檛 make use of stone tools, which might suggest that primates 鈥� perhaps including our ancestors 鈥� stumble into the stone age by chance.

Chimpanzees in west Africa, macaques in Thailand and several species of tufted, strongly built capuchin monkeys living in South America use stone tools to access food. Brendan Barrett at the Smithsonian Tropical Research Institute in Anc贸n, Panama, and his colleagues have now discovered that a species of non-tufted, slender-bodied capuchin monkey also uses stone tools.

The tufted and non-tufted capuchins are estimated to have split from each other about 6.2 million years ago, says Barrett. 鈥淭hat鈥檚 a similar divergence time between our lineage and the last common ancestor of chimpanzees and bonobos,鈥� he says.

In other words, he says, the non-tufted capuchins are the fourth distinct type of non-human primate known to use stone tools on a regular basis.

Welcome to the Stone Age

It鈥檚 an exciting discovery, according to both Dorothy Fragaszy at the University of Georgia in Athens and Patr铆cia Izar at the University of Sao Paulo, Brazil. 鈥淚t reinforces our suspicions that we have interesting things to discover about even well-studied species by looking at populations in new places,鈥� says Fragaszy.

It was in 2004 that Barrett鈥檚 colleague, independent researcher Alicia Ib谩帽ez, first noticed that white-faced capuchins in Panama鈥檚 Coiba National Park use stone tools. In March 2017 Barrett and his co-workers followed up on the observation, placing camera traps across three maritime islands in the park.

They discovered that male capuchins in one corner of Jicar贸n island use stone tools to crack open coconuts, crabs and snails. Capuchins elsewhere on Jicaro虂n do not use stone tools, and neither do the capuchins on the other islands.

Barrett thinks several factors might have encouraged the Jicaro虂n capuchins to experiment with stone tools. There are no ground-based predators on the island, so the monkeys can afford to spend more time on the ground with their attention focused on tool use. There are also relatively few easily accessible sources of food on the island, which makes it worthwhile for the capuchins to use stones to crack open tough nuts and shells.

But that leaves a mystery. Capuchins elsewhere on Jicaro虂n and on the other islands experience those conditions too, but they don鈥檛 seem to use stone tools. 鈥淲e were surprised that this behaviour appears to be geographically localised,鈥� says Barrett.

There might be a simple explanation, says Michael Haslam at the University of Oxford. 鈥淭here must be a strong, perhaps over-riding, element of chance in stone tool adoption in primates,鈥� he says.

Perhaps it takes a single hyper-intelligent individual to make the leap and begin using stone tools, with others then copying the idea. 鈥淕ood innovations are pretty rare, but if they are adaptive they can take off,鈥� says Barrett.

But that still doesn鈥檛 really explain why other capuchins living on Jicaro虂n don鈥檛 use stone tools, he says, because individuals often migrate between groups and so useful innovations should spread. He hopes that closer study of the monkeys in the years ahead will help solve that puzzle.

Reference: biorxiv, DOI: