Female Monkeys Purposefully Have Fewer Friends As They Age

The benefits of social interaction change with time.

Rhesus macaques on Cayo Santiago
Rhesus macaques on Cayo Santiago.

Lauren Brent

Female rhesus macaques get pickier about friendships as they get older, saving their energy for family members or monkeys they’ve known for a long time, a new study finds.

Having fewer friends is common as humans age, as people get more selective about who they spend time with it. Research suggests that other mammals also have smaller social groups when they are older.

“This pattern of narrowing social networks with age is common in humans,” said Lauren Brent, a professor from the University of Exeter’s Center for Research in Animal Behavior.

“Our study offers the most conclusive evidence to date that social selectivity is not unique to humans, and therefore might have deeper evolutionary underpinnings.”

For their work, an international team of researchers analyzed eight years of data on more than 200 macaques (Macaca mulatta) living on the island of Cayo Santiago, Puerto Rico. The monkeys are very social animals and this population has been very well-studied.

Researchers gathered information on grooming interactions and how physically close the monkeys were to one another to measure how socially connected they were. They specifically focused on females between 10 and 28 years old because they wanted to look at age-related changes in social interactions starting in prime adulthood.

Macaques are considered to be adults at 6 years old. The median life span for females in these populations is 18 years old, with a maximum age of about 30 years.

They noted who they interacted with and how the social lives of each monkey changed as they got older.

Changing Social Benefits

After ruling out other potential causes for changes in social behavior like the death of a partner, researchers found that females “actively reduce” their social networks as they age.

They found four different ways their social interactions changed. As they got older, female monkeys reduced the number of animals they interacted with and they were more likely to have relatives in their networks. They also emphasized relationships with stable and strongly connected partners.

Females also had fewer partners as they got older, suggesting that this was a decision made by the aging females. However, although they became choosier about partners, older females remained appealing to other monkeys. They continued to be approached by the same number of partners and received the same amount of grooming.

The findings were published in the journal Proceedings of the National Academy of Sciences.

“There are many possible reasons why macaques become more socially selective with age,” said researcher Erin Siracusa from the University of Exeter. “For example, the benefits of social interactions might change with time. Young macaques might benefit from a wide social group that can help them explore and find potential mates.”

But for older monkeys, it might be easier and safer—in terms of threats like disease transmission and conflict—to remain with family and old friends.

 Siracusa said, "New relationships also require more mental effort, so while we see no decline in time spent socializing, older macaques might save mental energy by shrinking their network."

View Article Sources
  1. Siracusa, Erin R., et al. “Within-Individual Changes Reveal Increasing Social Selectivity with Age in Rhesus Macaques.” Proceedings of the National Academy of Sciences, vol. 119, no. 49, 2022, doi:10.1073/pnas.2209180119

  2. "Female Monkeys ‘Actively Reduce’ Social Network As They Age." University of Exeter.