In a huge study of female rhesus macaques, a team of researchers led by the University of Pennsylvania’s Michael Platt found those with many close female relatives have a higher life expectancy.

However, the effect fades with age, suggesting older females learn how to “navigate the social landscape” and have less need for social ties.

“In the prime of life, when there is a lot of competition over resources,” Platt said, “having friends, having relatives is really critical. The effort that these females put into building and maintaining a relationship has an impact on their lives and tends to make them healthier.”

Platt teamed on the study, published in the journal Proceedings of the Royal Society B, with lead author Lauren Brent of the University of Exeter and Angelina Ruiz-Lambides of the University of Puerto Rico.

​​​​​​​Platt is the James S. Riepe University Professor with appointments in the Department of Neuroscience in the Perelman School of Medicine, Department of Psychology in the School of Arts & Sciences and Department of Marketing in the Wharton School. 

“Our study,” Brent said, “supports the idea that social ties promote survival. This adds to a small but growing body of research that helps to explain why animals are social.”

The researchers relied on a dataset spanning 21 years and including 910 adult female rhesus macaques from the Caribbean Primate Research Center in Puerto Rico. It’s the largest dataset of its kind on non-human animals, including information on family groupings, genetics, behavior, dominance rank and more.

The researchers used the number of female relatives as a proxy for social ties, as females stay with their family group as they age. They found that, for each extra female relative present in her social group, a prime-aged (6-17 years) female macaque’s chance of dying in one year was reduced by 2.3 percent.

This relationship between number of social ties and survival disappeared for females older than 18.

“Older females are kind of winding down their careers,” Platt said. “The factors that would make social support so important, which we think of as competition and the stress that comes with competition, the stress that comes with raising babies, just don’t affect older females very much.”

“Older females,” Brent said, “behave differently from their younger counterparts. Macaques spend a lot of time interacting with one another. Being groomed helps rid them of parasites, while being aggressive helps establish their place in the social order.

“Each macaque would like to get a lot of grooming and give a lot of aggression, without spending much energy grooming others and without being the target of aggression.”

The study found that this is precisely what older females do: behave aggressively and spend a lot of time being groomed by others without offering much grooming in return or becoming the target of aggression themselves.

“The experience and social skills females gain with age could mean they no longer need to rely on help from their friends to get by,” Brent said.

Like these macaques, social interactions are central to humans as well, making this population of monkeys a useful stand-in for drawing conclusions about human sociality. 

“This is,” Platt said, “probably the most comprehensive dataset of the biology and behavior of a natural population. We can observe and measure aspects of their social lives that it would be very difficult to do in a human population.”

Platt and colleagues would like to follow up on this work with examinations of the power of social ties in male macaques, who generally leave the family group upon maturing. They also plan to dig deeper into how social bonds affect an animal’s biology, drawing upon genetic and microbiome data that the researchers have been collecting from the macaques.

The study was supported by the National Institutes of Health (grants MH096875 and MH089484) and by a Leverhulme Early Career Fellowship. The Caribbean Primate Research Center is supported by the National Center for Research Resources (grant 2P40OD012217) and the Office of Research Infrastructure Programs of the NIH.