Brain areas that increase in size with social network size.

The number of individuals that a single person can keep close track of is generally taken to be roughly 150 ("Dunbar's number), which would be the size of a tightly knit social grouping. This estimate derives from a comparative analysis of primate neuroanatomy and behavior and has led to the corollary that the magnitude of the number is determined by the size of the neocortex. Sallet et al. have now made the interesting observation that the relationship between brain size and social group magnitude can be plastic, finding that that housing macaque monkeys in larger groups increases the amount of gray matter in several parts of the brain involved in social cognition. We know from many studies that requiring increased skill in perceptual or motor abilities correlates with increases in sensory and motor areas of the brain, so it makes sense that requiring exercise of "social muscles" increases the grey matter volume in temporal and frontal lobes areas that have been identified as potential contributors to social success in both humans and monkeys. Here is the abstract:

It has been suggested that variation in brain structure correlates with the sizes of individuals’ social networks. Whether variation in social network size causes variation in brain structure, however, is unknown. To address this question, we neuroimaged 23 monkeys that had been living in social groups set to different sizes. Subject comparison revealed that living in larger groups caused increases in gray matter in mid-superior temporal sulcus and rostral prefrontal cortex and increased coupling of activity in frontal and temporal cortex. Social network size, therefore, contributes to changes both in brain structure and function. The changes have potential implications for an animal’s success in a social context; gray matter differences in similar areas were also correlated with each animal’s dominance within its social network.