Harvard Researchers Discover Social Selectivity in Non-Human Animals Increases with Age

A team of psychologists and primatologists affiliated with Harvard’s department of Human Evolutionary Biology published a study Oct. 23 suggesting that social selectivity among non-human animals increases with age.

The study, published in Science, used more than 20 years of data collected by the Kibale Chimpanzee Project in Uganda to analyze the social interactions of male chimpanzees between the ages of 15 and 58.

The authors found that older chimpanzees were more selective in their friendships than their youthful counterparts. Older chimpanzees were also more likely to have “mutual relationships” reciprocated by both parties than younger chimpanzees.

Anthropology professor Richard W. Wrangham said he and his co-authors relied on measures including grooming patterns, physical closeness, and time spent together to determine whether chimpanzees had a mutual relationship.

“In any species that grooms, it is being shown repeatedly that the act of grooming is a very good measure of affiliate relationships or friendships,” Wrangham said.

He also said the close evolutionary link between humans and chimpanzees makes the results directly relevant to human psychology. Wrangham founded the Kibale Chimpanzee Project in 1987 with the goal of learning more about human behavior and ecology through the study of chimps.

“We actually know more about chimpanzees because we can follow them about the place and get regular data — from day to day, from hour to hour — in a way you can’t do with humans,” Wrangham said. “It would be too intrusive.”

“It’s an extraordinary irony, difficult as chimpanzees are to observe, we can be more systematic in our observations of them than in our observations of humans,” he said.

Alexandra G. Rosati ’05 — a professor at the University of Michigan who co-authored the study — said its results could provide “a new model for thinking about what it means to successfully age.”

Likewise, Wrangham said the results could offer humans a baseline to understand the “normal and characteristic” features of aging, like a “contracting” social circle.

“If we understand the typical course of aging and its effects on social behavior, then we can advise people when something is going wrong,” he said.