DURHAM, N.C. — In chimpanzee communities, strong social ties can be a matter of life and death not just for the adults who form them, but for their kids, too.
A new federally-funded study of wild eastern chimpanzees (Pan troglodytes schweinfurthii) from Gombe National Park shows that female chimpanzees who were more socially integrated with other females in the year before giving birth were more likely to raise surviving offspring.
The findings, published online on June 18 in iScience , show that these survival benefits hold for females even in the absence of close kin. Contrary to many other species, chimpanzee females are the dispersing sex, usually leaving their maternal group at sexual maturity and establishing themselves in a new social group away from their kin.
"In species where females live in groups with their sisters and mothers, it's less surprising that female sociality is beneficial," said Joseph Feldblum , assistant research professor of evolutionary anthropology at Duke and lead author on the study. "But female chimps don't usually have that. They are also less gregarious than males, so the fact that forming strong social connections still matters is striking."
To test the connection between friendships and offspring survival, researchers analyzed more than three decades of behavioral data from 37 mothers and their 110 offspring. They focused on association and grooming — how often females spent time near each other or engaged in social grooming behavior — in the year before birth, to avoid including social behavior from the post-birth period, during which it would be difficult to establish the causal relationship between infant loss and social behavior.
Females who were more socially connected had a considerably better chance of raising their babies through to their first year — the period of highest infant mortality. A female with a sociality score twice the community average had a 95% chance her infant would survive the first year. One who was halfway below average saw that chance drop to 75%. The effect persisted through age five, which is roughly the age of weaning.
The research team then tested whether having close female kin in the group — like a sister or mother — accounted for the survival benefit. The answer was "no." They also tested if the key was having bonds with males, who could potentially offer protection. The answer was also "no." What mattered the most was having social connections with other females, regardless of kinship.
"That tells us it's not just about being born into a supportive family," said Feldblum. "These are primarily social relationships with non-kin."
The authors noted that the mechanism of the survival benefit remains unresolved, although there are several possibilities. Social females might receive less harassment from other females, more help defending food patches or protecting their young, or their offspring could be less likely to be killed by another group member. Social connections might also have helped these females stay in better condition — maybe better fed and less stressed — through pregnancy, giving their offspring a better chance from the get-go.
And it's not just about the year before birth. Social females stayed social after their babies were born — a sign of stable relationships, not short-term alliances. "Our results don't prove causation, but they point to the value of being surrounded by others who support you, or at least tolerate you," said Feldblum.
"We humans are remarkably collaborative and cooperative. We cooperate at scales that are pretty much unique in the animal kingdom," he said. "Human females who don't have access to kin — for example because they moved to a new city or village — are still able to form strong bonds that can benefit them. Studying these social dynamics in chimpanzees can help us understand how we evolved to be the social, cooperative species we are today."
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REFERENCE: Joseph T. Feldblum, Kara K. Walker, Margaret A. Stanton, Elizabeth V. Lonsdorf, Deus C. Mjungu, Carson M. Murray, Anne E. Pusey, Socially integrated female chimpanzees have lower offspring mortality, iScience, 2025, 112863, https://doi.org/10.1016/j.isci.2025.112863 .
FUNDING: Data collection was supported by the Jane Goodall Institute, construction of the long-term database was supported by grants from the NSF (DBS-9021946, SBR-9319909, BCS-0452315, IOS-1052693, IOS-1457260), the Harris Steel Group, the Windibrow Foundation, the University of Minnesota, and Duke University.
