A study of free-ranging rhesus macaques found that those with more social partners had bigger brain areas involved in social decision-making and empathy
13 April 2022
Adult rhesus macaques with bigger social circles have enlarged brain regions associated with social decision-making and bonding, a study has found.
Primates, including rhesus macaques and people, live in large, complex social networks. The cognitive demands of navigating these networks is thought to have contributed to the relatively large brain size of primates, but less is known about the influence on internal brain structures.
Camille Testard at the University of Pennsylvania and her colleagues studied the relationship between the number of social partners and brain structure in 103 rhesus macaques (Macaca mulatta) living on Cayo Santiago island in Puerto Rico.
The monkeys, aged between 1 month and 25 years old, were supplied with food and water but otherwise left to their own devices so they could socialise freely.
The researchers counted how many social partners each adult monkey had – including friends and family members – by observing how many others they groomed or were groomed by over a three-month period. Some monkeys had dozens of social partners while others had none.
When the monkeys died, the researchers removed their brains and scanned them using MRI to measure the volumes of different brain structures.
The scans showed that two brain regions involved in social behaviour were larger in adults with more social partners: the mid-superior temporal sulcus and the ventral-dysgranular insula.
The mid-superior temporal sulcus has previously been shown to be involved in social decision-making, including deciding who to cooperate or compete with.
The ventral-dysgranular insula is thought to be involved in bonding and empathy. For example, one study that electrically stimulated this region in monkeys found that it caused them to make friendly lip-smacking gestures.
No differences in brain structures were found in infants with greater social contact, suggesting that these changes occur as the monkeys get older.
In people, there is also some evidence that having more friends changes brain structure. For example, one study found that the number of Facebook friends – which appeared to correlate with real-life friends – predicted grey matter density in brain regions associated with social behaviour.
“How many friends you have and who these friends are determine the rest of your social landscape and require sophisticated social cognition to maintain,” says Testard.
Social connectedness is increasingly recognised as a key driver of primates’ biological success. For example, male chimpanzees with larger friend networks have been found to sire more offspring, while adolescent humans with better social integration have lower blood pressure, levels of inflammation, waist circumference and body mass index.
“There is a vast literature on the importance of social relationships for primates – it is a very active area of research,” says Testard.
Journal reference: Science Advances, DOI: 10.1126/sciadv.abl5794
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