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Wild monkeys on a Thai island began using stone tools during the covid-19 pandemic, when travel restrictions meant they were no longer being fed by tourists.

Several other groups of primates use stone tools, but we haven鈥檛 been able to study the origins of this behaviour before, says at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who wasn鈥檛 involved in the study. 鈥淲e can observe it and we get to see it, but seldom do we get the chance to see it emerge as a new behaviour in a population.鈥�

The monkeys in question are common long鈥恡ailed macaques (Macaca fascicularis fascicularis). Hundreds of them live on Koh Ped, an island of just a quarter of a square kilometre in the Gulf of Thailand close to the city of Pattaya. Tourists visiting the city often take a boat to the island and feed the monkeys mangoes, cucumbers and nuts.

Researchers led by at Chulalongkorn University in Bangkok, Thailand, have been visiting the Koh Ped macaques for a decade. They never saw them use stone tools before 2020.

Then came the covid-19 pandemic. Tourists weren鈥檛 allowed to visit due to travel restrictions, and nor could the researchers. When the team finally returned in July 2022, they were surprised to see two adult male monkeys using pebbles to break open oyster shells.

On a follow-up visit in March 2023, they looked systematically for the tool-using behaviour and saw 17 monkeys using stones to crack open oysters. They did so in a clumsy way: the monkey would lift a rock in both hands to about shoulder height, then throw it at the oyster bed.

鈥淲hen we think about the broader diversity of primates, stone tool use is really rare,鈥� says Reeves. Previously, five populations were known to use stone tools: chimpanzees, bearded capuchins, yellow-breasted capuchins, white-faced capuchins and Burmese long-tailed macaques (M. fascicularis aurea) 鈥� which are closely related to the Koh Ped macaques.

In fact, a 2018 experiment on captive M. fascicularis fascicularis found that , despite being given training. The implication was that they didn鈥檛 have the capacity to do so. However, the new data on wild macaques suggests the issue was incentive, says Reeves: the wild macaques had experienced two years of food shortages, giving them a strong motivation to innovate.

A key unanswered question is whether a single macaque invented the behaviour and was then copied by others, or if multiple macaques came up with it independently. Malaivijitnond鈥檚 team noted that the monkeys they saw using stone tools were typically foraging alone, which hints that they weren鈥檛 copying. Reeves says it would be good to follow up and see when different individuals start using tools and who they associate with, to see if social learning is involved or if it is solely about individual creativity.

With the removal of covid-19 restrictions, tourists are once again visiting Koh Ped and feeding the macaques. Malaivijitnold鈥檚 team says this might cause the monkeys to stop using the stone tools, depriving them of the opportunity to study the behaviour further.

鈥淚t鈥檒l be interesting to see if it actually goes away,鈥� says Reeves. The macaques might decide stone tools are too helpful to stop using. 鈥淛ust because you鈥檙e getting the food you used to get, doesn鈥檛 mean you would abandon this entirely, because now you have this new food in your repertoire.鈥�