Macaques choose stone tools based on own size and strength

Wednesday, 21 March 2018

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Macaques appear to select stone tools to crack open oil palm nuts based on the size and strength of their own body, rather than the optimum weight and size of the stone, to make the process more efficient, new research involving Oxford Brookes University has found.

A research team including academics from Oxford Brookes University, UCL, the University of Oxford, Chulalongkorn University (Bangkok, Thailand) and Nanyang Technological University (Singapore), have for the first time, described the stone tools used by wild long-tailed macaque monkeys in southern Thailand to crack open nuts.

The research team say this in an important step forward in the emerging field of primate archaeology, as it increases our understanding of how different non-human primates use stone tools in the wild, and may allow future insights into how early human ancestors developed and used stone tools.

Dr Magdalena Svensson, MSc Primate Conservation Laboratory Technician at Oxford Brookes University and co-author of the paper said: “Tool use is one of the main hallmark achievements of human evolution.

“By analysing the stone tools used by macaques, we increase our understanding of the origins of tool usage. While tool usage has been seen in chimpanzees, who are more closely related to humans, the presence of this behaviour in a more basal Old World primate such as the macaque, provides further insight into this integral part of human evolution."

While tool usage has been seen in chimpanzees, who are more closely related to humans, the presence of this behaviour in a more basal Old World primate such as the macaque, provides further insight into this integral part of human evolution.

Dr Magdalena Svensson, MSc Primate Conservation Laboratory Technician, Oxford Brookes University

Dr Tomos Proffitt, UCL Archaeology and study lead said: “The stone tools we’ve uncovered show a high degree of battering and crushing on most surfaces. Interestingly, the stone tools are smaller than those used by wild chimpanzees for processing the same nut, and are smaller than those used by wild macaques to process another encased plant food, sea almonds.

“These new findings should help us to understand and compare skill levels between macaques and chimpanzees during tool use, and to assess differences in tool selection and mode of use."

To date, only chimpanzees in Western Africa have been observed opening oil palm nuts using a stone hammer and anvil, a behaviour some have suggested may be similar to the one that lead humans to develop stone tool technology nearly 3.3 million years ago. 

Last year, however, a team led by Dr Lydia Luncz of Oxford University, recorded wild long-tailed macaque monkeys on Yao Noi Island in Ao Phang Nga National Park using stone hammers and anvils to crack open hard shelled oil palm nuts in a similar way to that observed with chimpanzees.

Oil palm nuts are not native to the islands, but were first introduced by humans in the past few decades. Traditionally macaques focused on oysters by the sea for food intake, but when humans left, the macaques moved inland demonstrating a new adaptive behaviour.

This new follow-up study presents the stone tools used by these macaques and characterises the damage produced by this behaviour. By applying archaeological techniques used in the study of primates and tool use, the researchers can study primate behaviour even when direct observation of the monkeys is impossible.

The paper has been published online in the Royal Society Open Science.

The research was funded by the European Research Council starting grant, a British Academy Fellowship, Leverhulme Research Grant and a National Geographic Waitt Grant.

Image: A male long tailed macaque cracking an oil palm nut. Credit: Dr Lydia Luncz, the University of Oxford.