ÌÇÐÄÊÓƵ

Past their prime? Tool use declines with age in wild chimpanzees

New research led by University of Oxford researchers has found that old age likely impacts the habitual tool-use behaviors of some wild chimpanzees—although the extent to which different individuals are affected appears to be highly variable. The findings have been today in the journal eLife.

The wild chimpanzees showed reduced participation and performance in their tool-use behaviors as they grew older, according to the long-term video observations used in the new study. This provides 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.

The findings also indicate that chimpanzees mirror human beings in how the aging process affects their ability to carry out dexterous and cognitively-challenging daily tasks. Similar to humans, the effects of old-age varied considerably between individuals, with some chimpanzees struggling to use tools, while others maintained excellent performance into later life.

Wild chimpanzees use tools during some of the most cognitively and physically demanding foraging behaviors observed in non-human animals. While the behavioral 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 elderly wild apes. This includes how the capacity to address technical, real-world tasks changes as 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, animals' tool-use behaviors could be vulnerable to decline with old age," says Dr. Elliot Howard-Spink, who led this work while at the Department of Biology, University of Oxford, and is now a postdoctoral researcher at the Max Planck Institute of Animal Behavior, Germany.

"Until now, there has been no systematic study of how old age influences the technological behaviors 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 behavior of five chimpanzees, four female and one male who, across a 17-year period, were estimated to age from 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 gradually withdraw from nut-cracking behaviors. 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 behaviors, similarly to the variable patterns of behavioral and cognitive aging observed in human populations.

"Nut cracking is a rare behavior in wild chimpanzee populations, but where present, forms part of the shared cultural repertoires, which chimpanzees learn from one another," adds Dr. Elliot Howard-Spink. "By identifying effects of old-age in chimpanzees' tool-use behaviors, we can start to build a tentative picture of what the earliest hominins' technological lives may have looked like—a question which, thus far, has mostly overlooked periods of old age."

"Since this was an observational study, we cannot say for sure yet what precisely is leading to these changes in tool-use with aging—an area for future research," added Dora Biro, one of the senior authors on the paper, and a Professor of Biology at the University of Oxford at the time of the study. "Another ongoing question is whether the level of skill attained in early life, and the amount of practice, determines how well these behaviors are maintained over an individual's lifetime."