Social connections influence brain structure of rhesus macaques

What's the link between social life and brain structure? Researchers from the University of Pennsylvania, the Stem Cell and Brain Research Institute at Inserm, and elsewhere are now one step closer to understanding this connection for rhesus macaques.

In work published in Science Advances, the team found that for these nonhuman primates the number of social connections predicted the size of key nodes in parts of the brain responsible for social decision-making and empathy. Specifically, the researchers determined that, for macaques with more grooming partners, the mid–superior temporal sulcus (STS) and ventral-dysgranular insula grew larger. They found no such link between brain structure and other variables like social status.

"For the first time, we're able to relate the complexity of social lives of a group of living primates with brain structure," says Camille Testard, a fourth-year doctoral student in the Platt Labs at Penn and lead author of the paper.

Previous research on human social networks has hinted at this relationship, says Michael Platt, the James S. Riepe Penn Integrates Knowledge University Professor. "The literature, for example, ties variation in the size of the amygdala to the number of Facebook friends that you have. But it's hard to get granular data on human social interactions because we can't follow people around all day long," he says.

With the rhesus macaques living on Cayo Santiago, an island off the coast of Puerto Rico, however, it's a different story. Platt and colleagues have studied this group of free-ranging nonhuman primates for more than a decade. Part of that research has focused on grooming partners, which represent direct and important relationships for the macaques, as well as looking at the animals' broader social networks, representing individuals they interact with indirectly.

After Hurricane Maria hit the island, for example, the researchers examined whether the macaques grew or shrunk their social networks in the face of more limited resources. Testard, who joined the lab in 2018, led the analysis for that study, which found that the animals became more social and more accepting of one another, forming new relationships in addition to those they already had.

Building on that and on previous work from collaborator Jérôme Sallet of Inserm, Testard also designed the current study. Here, the team recorded the detailed interactions of a social group of 68 adult rhesus macaques on Cayo Santiago, then examined five factors: social status, number of grooming partners, physical distance with other monkeys, connectedness to popular monkeys in the network, and what the researchers called "betweenness," or the ability to act as a bridge between disconnected parts of the social network. They also collected brain scans for every individual in the social group, including 35 juvenile and infant macaques.

Analyzing the adult data, Testard and colleagues discovered that the more grooming partners individuals had, the larger their mid-STS and ventral-dysgranular insula were. "It was very interesting to find these regions, as their importance is known for social cognition in humans," Sallet says. "We also identified the mid-STS region in another study showing that activity in this region is modulated by the predictability of others' behaviors."