Old age takes its toll on tool use in wild chimpanzees

Wild chimpanzees show reduced participation and performance in their tool-use behaviours as they experience old age, according to long-term video observations. 

The study, published today in eLife, indicates that chimpanzees mirror human beings in how the aging process affects their ability to carry out dexterous and cognitively-challenging daily tasks. Much like in humans, the effects of old age varied considerably between individuals, with some chimpanzees struggling to use tools, whilst others maintained excellent performance into later life. The editors say the authors provide solid evidence that old age leads to gradual withdrawal from tool use, and is a contributing factor to lower efficiency in chimps' stone tool selection and use.

Wild chimpanzees use tools during some of the most cognitively and physically demanding foraging behaviours observed in non-human animals. While the behavioural changes that occur with aging have been widely studied in humans and some captive primates, exceptionally little is known about how growing older affects the lives of wild apes. This includes how the capacity to address technical, real-world tasks changes as wild chimpanzees become progressively older. 

“Tool use is uncommon among animals, possibly because it requires a suite of physical and cognitive abilities, such as planning, fine motor coordination, understanding causal relationships, and identifying physical properties of objects in the environment. Given many of these faculties can be impacted by aging, wild animals’ tool-use behaviours could be vulnerable to decline with old age” says lead author Dr Elliot Howard-Spink, a postdoctoral researcher at the Max Planck Institute of Animal Behavior, Konstanz, Germany (formerly working at the Department of Biology, University of Oxford). “Until now, there has been no systematic study of how old age influences the technological behaviours of wild animals, likely due to a lack of long-term data.”

To address this gap, the researchers used long-term data collected from almost two decades of study of chimpanzees in the Bossou forest, Guinea. Wild chimpanzees at the site are known for using stones as hammers and anvils to crack open oil palm nuts – a population tradition that requires selecting appropriate tools and executing a coordinated strike. The chimpanzees were systematically studied using an ‘outdoor laboratory’ in a clearing in the forest, where nuts and stone tools were made available for use by wild chimpanzees over many decades. The researchers monitored the behaviour of five chimpanzees, four female and one male who, across a 17-year period, were estimated to age from between approximately 39–44 years old, to 56–61 years of age.

Elderly chimpanzees showed a significant decline in their attendance at the outdoor laboratory over successive field seasons, unlike younger adults, suggesting that elderly chimpanzees began to withdraw from nut cracking behaviours. For some elderly individuals, this decline in attendance was accompanied by a marked reduction in time spent interacting with nuts and stone tools when at the outdoor laboratory, further suggesting a broader pattern of age-related resignation from nut-cracking activity.

Elderly chimpanzees also showed changes in their efficiency when selecting tools (with some taking noticeably longer in later years), and when using tools to crack open nuts. Nut-cracking efficiency declined across several metrics, including increases in the time taken and the number of actions required to process each nut. Interestingly, the authors noted stark differences in the  magnitude of changes across individuals, with some chimpanzees showing pronounced reductions in efficiency, and others remaining relatively stable throughout old age. These results point towards significant individual differences in how old age influences wild chimpanzee behaviours, similarly to the variable patterns of behavioural aging observed in human populations. 

“We first spotted an elderly female (Yo) seemingly experiencing difficulties during nut cracking whilst observing chimpanzees in 2012. Yo, formerly a keen and efficient tool-user, was taking a very long time to crack each nut, and was changing the stone tools she was using frequently” says co-author Prof. Susana Carvalho, Group Leader at CIBIO, Portugal, and an Associate Director at Gorongosa National Park, Mozambique. “It is incredible to have the long-term data confirming those observations, and to permit investigation of how tool use is performed during old age for other chimpanzees in this unique community (the only known site in Africa where chimpanzees use two movable stones to crack nuts).”

“Just as with human technical skills, chimpanzee tool use is culturally learned through social learning and individual practice” says co-author Dr Katarina Almeida-Warren, Researcher at the School of Anthropology and Museum Ethnography, University of Oxford. “By studying the same individual chimpanzees over nearly two decades, we’ve been able to see how chimpanzees’ cultural skills change with old age, and highlight the variation in how extreme aging affects behaviour.”

“At Bossou, nut cracking is an important foraging skill observed in nearly all individuals who have been studied in the population” says co-senior author Prof Dora Biro, Professor in the Department of Brain and Cognitive Sciences, University of Rochester, New York, US. “As ours was an observational study, we can’t yet identify the specific reasons why aging affects nut cracking, but our results raise important questions about how aging influences apes’ cognition and behaviour in natural settings. Future work should explore whether skill proficiency in early life and adulthood, as well as later-life practice, influence the maintenance of such behaviours over an individual’s lifetime.”

“This research was only possible through the unique video archive produced by decades of continuous observation of chimpanzees at Bossou, capturing both individual aging and group-level dynamics in stone-tool use” says Prof Tetsuro Matsuzawa, founding director of the Bossou Archive. “The Bossou archive offers a baseline to inform conservation strategies across wild chimpanzee populations, including accounting for behavioural changes over lifetimes.”

This research was led by researchers at the University of Oxford (UK), and involved collaboration across international research institutions, including the UK (University of St. Andrews & Queen Mary University of London), Portugal (University of Algarve and CIBIO-BIOPOLIS), Germany (Max Planck Institute of Animal Behavior), Switzerland (University of Geneva), USA (University of Rochester), China (Northwest University, Xi'an), Japan (Chubu Gakuin University), and Mozambique (Gorongosa National Park).

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