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| Three males groom together in a chain — Likizo (a younger male) grooms Big Brown (an older male), who grooms Lanjo (another younger male). Photo: John Lower Source: Havard Gazette |
Despite the denial of Creationists, humans are very like their closest relatives, the chimpanzees in so many way, and not just physical, physiological and genetic. Our psychology is similar too, as this latest finding shows.
A team of psychologists and primatologists from the Harvard Department of Human Evolutionary Biology has shown that chimpanzees, Pan troglodytes, like humans, tend to be increasing selective in their friendships as they age. Their findings are published today in Science.
It had been thought that this tendency was unique to humans and stemmed from our sense of our own mortality (itself believed to be a uniquely human trait). However, detailed observation has shown that aging chimpanzees also show the same selectivity in their friendship, tending to favour those they know and trust and with whom there has been little history of animosity.
Abstract
Humans prioritize close, positive relationships during aging, and socioemotional selectivity theory proposes that this shift causally depends on capacities for thinking about personal future time horizons. To examine this theory, we tested for key elements of human social aging in longitudinal data on wild chimpanzees. Aging male chimpanzees have more mutual friendships characterized by high, equitable investment, whereas younger males have more one-sided relationships. Older males are more likely to be alone, but they also socialize more with important social partners. Further, males show a relative shift from more agonistic interactions to more positive, affiliative interactions over their life span. Our findings indicate that social selectivity can emerge in the absence of complex future-oriented cognition, and they provide an evolutionary context for patterns of social aging in humans.
Rosati, Alexandra G.; Hagberg, Lindsey; Enigk, Drew K.; Otali, Emily; Emery Thompson, Melissa; Muller, Martin N.; Wrangham, Richard W.; Machanda, Zarin P.
Social selectivity in aging wild chimpanzees
Science 2020 370(6515), pp. 473-476. DOI: 10.1126/science.aaz9129
© 2020 The authors.
Published by The American Association for the Advancement of Science
Reprinted with kind permission under licence #4934750793198
Juan Siliezar, writing in the Harvard Gazette, explains the significance of this research:
When humans age, they tend to favor small circles of meaningful, established friendships rather than seek new ones, and to lean toward positive relationships rather than ones that bring tension or conflict. These behaviors were thought to be unique to humans but it turns out chimpanzees, one of our closest living relatives, have these traits, too. Understanding why can help scientists gain a better picture of what healthy aging should look like and what triggers this social change.
[...]
The study draws on 78,000 hours of observations, made between 1995 and 2016. It looked at the social interactions of 21 male chimpanzees between 15 and 58 years old in the Kibale National Park in Uganda. It shows what’s believed to be the first evidence of nonhuman animals deliberately selecting who they socialize with during aging.
The researchers looked only at male chimpanzees because they show stronger social bonds and have more frequent social interactions than female chimps. Analyzing a trove of data, the researchers saw that the chimpanzees displayed much of the same behavior as aging humans exhibit.
Kakama and Makoku grooming together; these males are long-term mutual friends and show a high level of tolerance.Video: Ronan Donovan
The older chimpanzees they studied, for instance, had on average more mutual friendships while younger chimps had more one-sided relationships. Mutual friendships are characterized by behavior such as reciprocated grooming whereas in lopsided friendships grooming isn’t always returned.
Older males were also more likely to spend more time alone and showed a preference for interacting with — and grooming — chimps they deemed to be more important social partners, like other aging chimps or their mutual friends. And like older humans looking for some peace and quiet, the chimpanzees showed a shift from negative to more positive interactions as they reached their twilight years. The preference is known as a positivity bias.
“The really cool thing is that we found that chimpanzees are showing these patterns that mirror those of humans,” said Alexandra Rosati ’05, an assistant professor of psychology and anthropology at the University of Michigan and one of the paper’s lead authors.
A behaviour which was thought to be based on a uniquely human knowledge of our own mortality is also observed in chimpanzees. If the assumed cause of this behaviour in humans is correct, it suggests chimpanzees also have the same awareness and yet another assumption about the uniqueness of humans is refuted. This in turn suggests a characteristic shared by our common ancestor which would then also be present in our hominid and hominin ancestors, the Australopithecines and archaic humans.
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