For young people, friends are often determined by proximity. Whoever is around is your friend. Whereever you are, that's the playground.

As humans get older, they become more selective in choosing whom they spend time with, as well as how they spend their time. According to a new study, the same goes for monkeys.

The findings, detailed in the journal Cell Biology, suggest human selectivity may be deeply ingrained in our evolutionary history.

"An important psychological theory suggests that humans become more socially selective when they know that their remaining life time is limited, such as in old age," Laura Almeling, a biologist at the

Leibniz Institute for Primate Research, explained in a news release. "We assume that monkeys are not aware of their own limited future time."

"Therefore, if they show similar motivational changes in old age, their selectivity cannot be attributed to their knowledge about a limited future time," Almeling continued. "Instead, we should entertain the possibility that similar physiological changes in aging monkeys and humans contribute to increased selectivity."

Researchers presented a group of Barbary macaques with a variety of novel animal toys, only one of which featured a food treat. Adolescent monkeys quickly grew out of their interest in the toys, preferring only the food-baited object once they matured.

Researchers also measured how monkeys' social interactions change as they age, using field observations and by measuring the responses of monkeys to pictures and sounds of friends and infants.

Although older monkeys remained engaged and interested in group dynamics and social interactions, they invest less time and energy in social relationships.

"With increasing age, the monkeys became more selective in their social interactions," Almeling said. "They had fewer 'friends' and invested less in social interactions. Interestingly, however, they were still interested in what was going on in their social world."

"Older females continued to respond particularly strongly to hearing a scream for help from their best friend," Almeling explained. "Older males still looked preferentially at pictures of the newborns", she says, noting that Barbary macaque males use infants as status symbols."

Almeling and her colleagues plan to conduct further tests to rule out the possibility that older monkeys avoid social interactions as they age due to the stress they cause.