The Oxford University study found that monkey brains grew bigger with every cage-mate they acquired, showing that certain parts of the brain linked to processing social information expand in response to more complex social information.
"Interestingly, there are a couple of studies in humans by different research groups that show some correlation between brain size and the size of the social network, and we found some similarities in our studies," study author Jerome Sallet told LiveScience.
"Our study reinforces the idea that the human social network was built on something that was already there in the rhesus macaques."
For their study, published in the journal Science, the researchers studied 23 rhesus macaques living in different size groups in a research facility, having a dominance-based hierarchy (except the one monkey that was caged alone).
One's rank among male cage-mates is dependent upon social interactions, including the ability to make friends and form coalitions, which grants the monkey access to valued resources.
The researchers scanned the brains of the monkeys using magnetic resonance imaging to gauge the sizes of different brain regions and saw enlargements in gray matter in several areas of the brain associated with social interactions.
On average, they saw more than a five per cent increase in gray matter mass per extra cage-mate.
The boosted brain areas included the temporal cortex, inferior temporal gyrus, the rostral superior temporal gyrus and the temporal pole.
Based on what scientists know about these areas, increase in gray matter there could "reflect an increasing need to decode the significance of the facial expressions, gestures and vocalisations of a greater number of individuals and combinations of individuals as network size increased", the researchers said.
They then compared these brain scans with each male monkey's position within their dominance hierarchy. They saw several brain areas correlated to higher levels of dominance as well.
Specifically, the inferior temporal sulcus and the prefrontal cortex showed size increases with higher dominance rating. These analyses accounted for social network size.
These changes in brain size are an example of the brain's plasticity, or its ability to change over time. Past research has indicated that learning physical skills might be able to enlarge motor areas in the brain, but this hasn't been shown for social interactions.
Especially for the correlation to social standing, these brain areas were probably expanding to deal with storing extra information about greater numbers of dominant and submissive cage-mates, the researchers added.
