Dr. Freund, along with Julia Fischer, who studies primate cognition at the German Primate Center in Goettingen, Germany, and their colleagues, wanted to know how age influenced the behavior of more than 100 Barbary macaque monkeys living in an enclosed 50-acre park in southern France. They studied how the monkeys, ranging in age from 4 to 29 (which is about 105 in human years, according to Dr. Fischer), responded to physical objects like novel toys and tubes baited with food; social interactions like grooming “friends” or fighting; and social information, like photos or calls of “friends” and “strangers.”

The researchers found that the monkeys’ interest in toys waned when they became reproductive. And around 20, (their “retirement age”) monkeys, like humans, had fewer social contacts and approached others less frequently. What surprised the researchers is that this apparent withdrawal wasn’t driven by a social tendency to avoid old monkeys: Younger monkeys still approached and groomed their elders. And it wasn’t that older monkeys just weren’t interested in anything: They still responded to photos of other monkeys and hissed at others during fights. “They are still very much tuned into what’s going on,” said Dr. Fischer. “But they don’t want to participate themselves.”

Dr. Freund said she sees the same behavior patterns in humans.

The dominant psychological theory to explain this in people is that we become more choosy with age in order to maximize the use of the time we have left with death in sight. While monkeys have excellent memories, there is no evidence that they are aware of their impending deaths. So if both humans and monkeys act similarly, perhaps this theory is just a way of rationalizing a natural behavior with biological roots, said Dr. Fischer.

Perhaps monkeys and humans just lose stamina with age, and maybe the monkeys are too tired to deal with relationships that are ambivalent or negative, she added. Or maybe, as the researchers are now trying to investigate, aging monkeys are less socially interactive because they tend to take fewer risks, which is what appears to happen in humans according to some research.