Some chimpanzees are better at cracking nuts than others

Photo credit: Tetsuro Matsuzawa

Doctoral research by Dr Sophie Berdugo suggests some chimpanzees may have greater cognitive or motor abilities than others, a finding that may have important implications for understanding early human evolution.

Some chimpanzees are consistently more efficient at using stone tools to crack nuts than others, according to research published in Nature Human Behaviour. The findings are based on an analysis of 25 years of video footage of almost 4,000 periods of nut-cracking from wild chimpanzees in Bossou, Guinea.

Chimpanzees in West Africa are known to use stones to crack open nuts, a behaviour often considered the most complex form of tool use in non-human animals. Being more efficient at cracking nuts with stones has direct evolutionary benefits, as individuals can eat more food while using less energy. Although there is evidence to suggest differences in tool use between individuals in some species — such as in oyster-cracking by Burmese long-tailed macaques — this research has relied on data collected over short time spans, which may not reflect consistent differences.

Sophie Berdugo and colleagues analysed video footage of 3,882 observations of 21 wild chimpanzees cracking nuts in Bossou (totalling over 800 hours of footage) between 1992–2017 to determine whether some are more efficient than others at the task, on the basis of five different measures of efficiency. These included the length of a bout of nut-cracking (bout duration), strikes per nut, success rate, the number of times a strike displaced the nut (displacement rate), and tool switch rate.

They found that there are reliable individual-level differences in efficiency across four of the five measures (for example, some chimpanzees take twice as long to access the enclosed nutritious kernel than others of the same age and sex), except for tool switch rate (the number of times the chimpanzees changed their tools while cracking nuts). Each individual’s efficiency was also found to improve up to 11-years-old, the age of adulthood in Bossou chimps, suggesting that chimpanzees continue to hone their skills after first learning the behaviour. There were no differences between males and females. This contradicts previous short-term research finding a female bias in chimpanzee tool use, highlighting the importance of longitudinal research for studies of long-lived species.

The findings suggest that some chimpanzees may have greater cognitive or motor abilities than others. Sophie Berdugo’s next study from her DPhil, now available as a pre-print on BioRxiv, establishes the causes of this individual variation in a subset of 10 of these chimpanzees.

Lead author of the study, Dr Sophie Berdugo of the School of Anthropology and Museum Ethnography at the University of Oxford, said:

This study of the Bossou chimpanzees, which is a population on the cusp of extinction, is essential for preserving the community’s behaviours and cultural heritage. But our findings also have important evolutionary implications. The fact that we found meaningful and consistent individual variation in the stone tool efficiency in the Bossou chimps - who use stone tools in a way we think early humans would have used them - means that the percussive damage on the tools may vary too. Our research shines a light on the potential variation in the hominin archaeological record, and how palaeolithic archaeologists can interpret it.

Read the full article here: Reliable long-term individual variation in wild chimpanzee technological efficiency | Nature Human Behaviour