Monkeys' social circles tend to grow smaller as they grow older, according to a recent study that researchers say could also shed some light on human behavior.

The study, published in Current Biology, studied the social behavior of more than 100 Barbary macaque monkeys living in an enclosed 50-acre park in southern France. The monkeys ranged in age from 4 to 29, the latter age being the equivalent of about 105 in human years.

Researchers found that monkeys of different ages reacted in different ways when presented with various stimuli, including toys, social interactions such as fighting or grooming one other, and social information such as photos and calls of "friends" and "strangers."

When monkeys reached a reproductive age, interested in toys decreased. And once they reached "retirement age" – around 20 years old – they had fewer social contacts and approached others less frequently.

The surprising thing about this behavior, said Julia Fischer, who studies primate cognition at the German Primate Center in Göttingen, Germany, is that it was not because the younger monkeys chose not to socialize with their elders. Rather, older monkeys were still frequently approached and groomed by their younger friends.

Furthermore, retirement-age monkeys' interest in social interactions didn't appear to decrease; 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. Fisher to the New York Times. "But they don’t want to participate themselves."

If this all sounds familiar, it's because humans tend to exhibit similar behavior patterns as they grow older.

The dominant existing psychological theory to explain this behavior in people is that as we grow aware of the limited time we think we have left to live, we become more picky about how we spend that time. There's no evidence, however, that monkeys have an awareness of death approaching. This suggests that perhaps the pattern actually has biological origins.

"Our behaviors that seem very much the result of our deliberation and choice might be more similar to our primate ancestors than we might think," said Alexandra Freund, a developmental psychologist at the University of Zurich who worked on the study, to the Times. "This clearly tells us that we, as humans, are not unique in the way we age socially but that there might be an evolutionary 'deep' root in this pattern."

There are several potential explanations for aging monkeys' social patterns that could also account for human behavior. It's possible that, like humans, older monkeys tend to take fewer risks, making them less socially active. Researchers are currently investigating this theory.

It's also possible, Dr. Fischer said, that both humans and monkeys simply grow too tired to deal with relationships that are ambivalent or negative.

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Going forward, the research could help scientists learn more about the biology behind human social behavior, study author Laura Almeling told Medical Daily.

"As we are living in an 'aging society' there is substantial interest in the consequences of aging, with a particular focus on age-associated diseases," Dr. Almeling said. "However, we also need to study the biological foundations of 'normal' aging processes to obtain a better understanding of the roots of the desires and preferences of aging humans."