Chimpanzee friendships are surprisingly human

Grey hair, wrinkles around the eyes — these are the physical hallmarks of aging. But in a new study, scientists are interested in less visible aspects of getting older.

As we age, we tend to have fewer — but closer — friends. This is a phenomenon scientists call socioemotional selectivity.

Also, the older we get, the less our appetite for drama in our social lives (although everyone probably has an anecdote of how this isn’t always the case).

In humans, scientists think these changes in our social behavior as we age is related to our inherent fear of death.

To better understand the evolutionary history of this shift, researchers investigated how it plays out in one of our closest living relatives: chimpanzees.

Scientists analyzed 78,000 hours of footage of chimpanzees in Uganda. The video was collected across two decades. They focused on 21 male chimpanzees, ranging in age from 15 years to 58 years in age.

In particular, the researchers wanted to answer two main questions:

1. Did these chimps' preference for close friends change as they aged?

2. Did these chimps prioritize positive behaviors over aggressive ones as they aged?

They found some striking similarities between these chimpanzees and their human counterparts. For example, they found mutual friendships were more common among older chimps compared to younger chimps.

Researchers also found that aggressive behavior decreased with age, while positive behaviors didn't tend to change.

Older chimpanzees were also more selective in their friends — choosing friends around their own age and sex — than younger chimps.

Scientists don't know why chimpanzees show such similar social behavior to humans. Other primates tend to show social withdrawal with old age, but scientists speculate the chimps' behaviors may be related to the flexibility of their social relationships.

“[S]ocial relationships are flexible, can occur outside of kinship, and last many years in long-lived humans and chimpanzees," write the authors. "Thus, strongly established relationships may be more reliable for older chimpanzees than for other primates."

The results are an important step towards understanding the social and emotional connections between humans and other animals.