Babies of social chimpanzee mothers are more likely to survive

In the wild, social lives shape survival. For female chimpanzees, bonds with others aren’t just emotional glue. They are critical for keeping babies alive.

A new study of wild eastern chimpanzees in Gombe National Park finds that social integration predicts whether a newborn survives, even when no close kin is around.

Chimpanzees with strong social bonds

Female chimps are often the ones who leave their birth group. That means most do not live near their mothers or sisters. Still, those who manage to build friendships with other females raise more babies to weaning age.

“In species where females live in groups with their sisters and mothers, it’s less surprising that female sociality is beneficial,” said Joseph Feldblum, lead author and research professor at Duke.

“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.”

Babies of social chimpanzees

The team analyzed 37 years of detailed records from 37 mothers and 110 offspring. Using a composite sociality index (CSI), they tracked how much time females spent associating or grooming others before giving birth.

Their best model showed that higher CSI scores, especially with other females, greatly increased infant survival to age one and even to age five. The effect held even after excluding mothers who died shortly after giving birth.

A female with a CSI score twice the average had a 95 percent chance of her baby surviving the first year. For those below average, this dropped to 75 percent.

Among females without any close kin, the trend was stronger. A single unit rise in sociality increased survival odds by over 17 times.

Kinship and male alliances

The researchers ran extra tests to see if the benefit was just from living with relatives. It was not. They also checked if relationships with males helped. That also came up short.

“That tells us it’s not just about being born into a supportive family,” said Feldblum. “These are primarily social relationships with non kin.”

Even when female kin were present, they had no direct effect on offspring survival. Instead, they raised the chance that a mother had strong friendships. But it was those friendships, not the family ties, that made the difference.

Social ties remain stable over time

The benefits of bonding were not a one time fluke. Females who were social before birth stayed social after birth too. Their social worlds remained steady.

“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.

This was not the case for bonds with males. Social connections between male and female chimpanzees were less stable and showed no link to baby survival.

Social bonds and chimpanzee babies

How do these friendships help? The exact mechanism remains uncertain. Social bonds might protect chimpanzee mothers from aggression. They could mean better access to food. Friendships might even prevent infanticide.

Earlier studies in this same group of chimps estimated that infanticide by other females may cause 3.5 percent to 19 percent of deaths in the first year. Social mothers may face less of that risk.

More research is needed, but one thing is clear. Socially connected females stay in better shape, which could boost prenatal health. Unlike rank or dominance, which did not correlate with survival during this study window, sociality kept its value.

Social chimps mirror humans

Though females often move away from family, they form lasting ties with unrelated females. The Gombe chimps mirror women in human patrilocal societies, who build networks after moving for marriage.

“We humans are remarkably collaborative and cooperative. We cooperate at scales that are pretty much unique in the animal kingdom,” said Feldblum.

“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.”

This study adds to the growing body of evidence that friendship matters. In baboons, dolphins, and horses, social bonds also improve reproductive success. In chimps too, strong bonds give babies a better chance at life.

Studying these dynamics in our closest relatives helps us understand our own nature. Like chimps, human survival does not just depend on strength or status. It depends on others.

The study is published in the journal iScience.

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