Primates have some of the largest brain-to-body ratios in the animal kingdom. These intriguing observations have led some anthropologists to embrace the “social brain theory”, the notion that it was the challenge of managing increasingly complex social networks and interactions that mainly drove the development of larger brains, rather than the challenge of finding food. A new study adds further weight to this theory, finding that particularly extroverted rhesus macaques with large social networks tended to have enlarged regions of the brain linked to social decision-making and bonding.

For the macaques, friendship keeps bugs away and strengthens the brain

To gain a better grasp of how internal brain structures may have an impact on primates’ social abilities, researchers at the University of Pennsylvania closely monitored 103 rhesus macaques (Macaca mulatta) in their natural environment on Cayo Santiago Island in Puerto Rico. The Old World monkeys were supplied with some food and water by the researchers, but other than that they socialized freely like they would in the wild. They varied from one-month-old infants to 25-year-old elderly monkeys.

Rhesus macaques live in social groups typically composed of a few adult males and many more adult females and their offspring. The adult females and their offspring belong to several different matrilines (i.e. families related through the maternal lines) due to the way their social structure is organized. Males emigrate from their natal group at puberty to join a new group, whereas females stay in the group they were born into their entire lives.

Similar to people, the strength of social bonds between different individuals in a rhesus group is generally predicted by their kinship, sex, and age. The strength of social bonds can be measured by the amount of time spent in close proximity, physical contact, and grooming behavior. Social grooming is particularly important — it’s the main affiliative behavior used by rhesus macaques to establish and cement social relationships with one another.

Grooming serves a hygienic function, cleansing monkeys of parasites, but it is also relaxing to the recipient. If a macaque monkey requests grooming, usually by lip-smacking to encourage a peer to approach, and then usually lying down in front of the other, exposing the part that needs to be groomed, that’s a significant sign of social bonding.

Over a three-month-long period, the researchers counted how many monkeys were groomed or performed the grooming in order to establish how many social partners each adult monkey had. Besides the number of grooming partners, the researchers also looked at other social factors like physical distance from other monkeys, social status, connectedness to high-status individuals in the group, and ‘betweenness’ — the ability to act as a social bridge between disconnected parts of a group.

After they performed MRI scans on 35 juvenile and adult macaques to measure the volumes of different brain structures, the researchers, led by graduate student Camille Testard, found that the monkeys with the largest social networks had larger volumes in two key areas: the mid-superior temporal sulcus and the ventral-dysgranular insula.

These brain regions are well known to play important roles in social cognition in humans. For instance, activity in the mid-superior temporal sulcus is modulated by the predictability of others’ behaviors.

One of the most interesting findings was that macaque infants weren’t born with these enhancements in brain structure, but rather gained them with development. According to the researchers, these differences likely emerge as a result of patterns and interactions that individuals make during their socialization.

Although the study followed free-ranging macaque monkeys, the findings could have important implications for humans as well. Previously, studies found that the number of Facebook friends a person has — a proxy for real-life friends — can predict the density of gray matter in brain regions linked to social behavior.

There are also many parallels between macaque and human social hierarchies. Young macaques born of popular mothers tend to become popular themselves, helped by a supporting network of friends, whereas those born in a family with poor social links will tend to have a low social status when they become adults. It goes without saying that being born into a good family is a huge advantage for humans as well.

“Our findings demonstrate that the size of specific brain structures varies with the number of direct affiliative social connections and suggest that this relationship may arise during development. These results reinforce proposed links between social network size, biological success, and the expansion of specific brain circuits,” the authors concluded.

The findings appeared in the journal Science Advances.