Research into changing tool use by wild chimpanzees suggests evolutionary origins of Alzheimer's disease and related disorders.
For decades, scientists assumed only humans experienced dementia. Now, new field observations suggest that wild chimpanzees-our closest primate relatives-may also show signs of cognitive decline with age.
In a study published in eLife, an international collaboration of researchers-including Dora Biro, the Beverly Petterson Bishop and Charles W. Bishop Professor of Brain and Cognitive Sciences at the University of Rochester-reports that aging chimpanzees may mirror humans in experiencing age-related declines in their ability to perform cognitively challenging tasks. The research offers clues to the origins of Alzheimer's disease and other cognitive disorders, and what these patterns reveal about human brain aging.
"Our work is a mixture of trying to understand chimpanzee cognition in its own right and then trying to see what that tells us about the evolution of our own minds," Biro says. "Getting these comparative data points between us and our non-human primate relatives can be really valuable for reconstructing evolutionary histories, whether it be anatomical, behavioral, or cognitive."
The behavioral changes that occur with aging have been widely studied in humans and captive primates, but scientists know little about how growing older affects apes in the wild, in part because surviving to advanced age is rare in natural habitats. For those that do survive, tracing their long-term behavior changes-including the use of tools-can yield fundamental insights.
"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 Elliot Howard-Spink, a postdoctoral researcher at the Max Planck Institute of Animal Behavior, and Biro's former PhD student at the University of Oxford.
"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," adds Howard-Spink.
Aging in the wild
To address this gap, Biro and her colleagues analyzed decades of video footage of chimpanzees in the Bossou forest in Guinea, West Africa. Scientists have observed this chimpanzee community since the 1970s, tracking their lives, behaviors, and social interactions. While there were originally around 20 members in the community, Biro notes that only four individuals remain today, largely due to anthropogenic impacts such as habitat loss and disease outbreaks.
Since 1988, researchers have maintained an "outdoor laboratory" in Bossou consisting of a clearing in the forest where stones and nuts are placed to study chimpanzee tool use. Here, the researchers have observed chimps demonstrating one of the most complex tool-use behaviors in the animal kingdom: nut cracking. The chimps use a stone as a hammer and another stone as an anvil to carefully crack hard nuts to access the kernels inside.
Cracking nuts is a culturally learned skill that takes individuals years to master. Long-term observations have allowed scientists to trace how young chimpanzees hone their skills over extended periods of practice. For instance, infants begin by playing with the stones and nuts separately before putting the objects together. They then gradually master the complex sequence of actions needed for nut cracking over several years.
A surprising shift in skill
Biro used to visit the field site every year, but after taking a break for several years, she returned with a colleague who had also been away. On their return trip, they were astonished to see that one of the older female chimpanzees, who used to be among the most skilled nutcrackers in the group, now seemed confused by the same tools she had mastered in the past. She frequently changed stones, misaligned the nuts she was attempting to crack, and faltered in performing tasks she had previously performed smoothly.
The researchers also observed broader behavioral changes in elderly chimpanzees, including reduced attendance at the outdoor nut-cracking site and longer processing times to perform tasks.
"It gave us the idea that there's a very interesting period at the end of life where we can potentially see how individuals decline, not just in their physical abilities, but also in cognitive abilities," Biro says.
This kind of decline is unusual to study in nature because most wild animals, including chimpanzees, fall prey to predators, disease, starvation, or human activity before reaching old age. But this community of chimpanzees is an exception. Living in a relatively peaceful region without rival chimpanzee groups and protected by local people who view them as a totem animal, several of the group's members have reached their 50s and 60s, which is a remarkable feat in the wild.
Clues to our evolutionary past
These rare observations open the door to answering bigger evolutionary questions. Chimpanzees and humans share a last common ancestor that lived approximately six to eight million years ago. By observing aging chimpanzees at Bossou, researchers can begin to determine whether conditions such as dementia existed in our shared evolutionary past or whether they emerged more recently in humans.
"We're trying to understand the evolution of these various human traits and whether or not they're unique," Biro says. "If they're unique, that suggests they're traits that evolved since our split from chimpanzees. "Comparisons such as this provide valuable data for reconstructing evolutionary history and the roots of diseases in humans. If dementia and cognitive decline appear in chimpanzees, it suggests the origins of Alzheimer's are deeper than previously thought, offering clues that could shape how we understand, prevent, and treat aging-related disorders in people.
Cognition across the lifespan
Observing aging chimpanzees also helps researchers understand the mental abilities required for complex behaviors. By tracking how older chimpanzees perform tasks such as nut cracking, scientists can identify the baseline memory, planning, coordination, and problem-solving skills necessary for these behaviors.
"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 behavior in natural settings," Biro says. "Future work should explore whether skill proficiency in early life and adulthood, as well as later-life practice, influences the maintenance of such behaviors over an individual's lifetime."
