Is it easier to live in a small town than in a big city? Living in a big city requires frequent interactions with many different individuals and responding appropriately to a huge variety of social situations, while living in a small town requires fewer types of interactions, with fewer individuals. The complexity of living in big groups has led to the ‘social brain hypothesis’, which suggests that as group size increases, brain size should increase to meet these social demands. However, the ‘task specialization hypothesis’ is a counter theory, which suggests that as social group size increases individuals start to perform increasingly specific tasks. According to this theory, someone living in a small town is more likely to be a jack-of-all-trades, while a big-city dweller will be more likely to have a highly specialized job. Thus, the ‘task specialization hypothesis’ suggests that as social group size increases, the brain should become increasingly specialized, rather than becoming larger overall.

Sabrina Amador-Vargas, from the University of Texas at Austin and the Smithsonian Tropical Research Institute, with colleagues from both institutions and the University of Arizona, decided to investigate the ‘social brain hypothesis’ versus the ‘task specialization hypothesis’ in acacia ants (Pseudomyrmex spinicola). These ants live in social groups with a single queen and a host of workers, and have a symbiotic relationship with acacia trees. Each social group lives within a tree and workers chase off animals that try to feed on the acacia leaves. In return, the trees provide the ants with ‘Beltian bodies’ – small packets of food that the ants find irresistible.

In wild ant colonies in Panamá, the researchers estimated colony size by marking and recapturing worker ants, and counting the number of entrance holes on the host tree. The scientists then assessed task specialization by counting how many of the worker ants performed the same task day after day. They also observed how workers responded to simulated foraging or defensive tasks. The researchers predicted that as colony size increased, worker ants would be less likely to switch jobs and more likely to exclusively perform either foraging tasks on the leaves or defensive tasks on the trunks. They further predicted that as social group size increased, the foraging ants would be more likely to ignore an intruder, while the defensive ants would be more likely to ignore Beltian bodies. After conducting behavioural observations, the researchers collected worker ants and measured the volumes of different brain areas.

They found that as colony size increased, task specialization also increased. In larger social groups, worker ants were more likely to perform exclusively either defensive or foraging tasks, rather than switching between jobs. Importantly, the scientists showed that specific brain regions related to learning and memory became larger in foraging workers as social group increased, but smaller in defensive workers as social group increased. Thus, these results support the ‘task specialization hypothesis’, where ants living in small groups are more general jacks-of-all-trades and ants from big groups become more specialized for a specific job. The social brain isn't necessarily a bigger brain, but it is a unique brain that is built one increasingly specialized task at a time.

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Specialization and group size: brain and behavioural correlates of colony size in ants lacking morphological castes
Proc. R. Soc. B