Human evolution has been marked by two major milestones: an increase in brain size and the development of social groups. Many scientists have assumed that to solve sophisticated social cognitive problems, brain size and social skills need to go hand in hand.

New research led by anthropologist Dr Evan MacLean from Duke University now shows that, among lemurs, the size of an individual's social networks is the best predictor of how well it can perform cognitive tasks, rather than its brain size. The new study, published in the June issue of PLoS ONE, suggests that cognition can evolve without an increase in brain size. But when it comes to social networks, the bigger the better.

Dr MacLean and his colleagues tested several species of lemurs for their cognitive abilities. In each species there was a variety of social group sizes, but they all had relatively small brains when compared to humans and other primates.

First, the lemurs had to steal food from two humans who were perceived as competitors for the food. Each time they had to pilfer food, the lemurs had to deal with either a human experimenter who faced the lemur, or one who did not. This way, the researchers knew they were testing a lemur's ability to read social cues (the human face) while performing a task (stealing food). This experiment was a test for social cognition. Lemurs that belonged to larger social groups performed better, and the species that performed the best, the ringed-tail lemur, also formed the largest social groups (they usually live in groups of 15).

In another experiment, the lemurs were forced to find alternative ways to grab food in a transparent container, usually by reaching around the container. This was a purely cognitive task, where no social skills were required. In contrast to the first experiment, performance of this cognitive task did not correlate with the lemurs' social group size.

The scientists concluded that the size of the lemurs' social group, and not brain size, or some 'special' ability unique to a species, determines how well they solve social cognition problems.

"These data provide the first demonstration of a link between group size and experimental measures of social cognitive skills in animals," the researchers wrote in the study. "Being socially savvy doesn't make you brainy in every domain. Our data suggest that for lemurs, living in social networks favored the evolution of social intelligence without changing over cognitive abilities."

The data also shows that, since changes in social cognitive abilities don't necessarily depend on brain size, we can't assume that larger brains automatically translate into superior social cognition. One of the hallmarks of sophisticated social interaction is flexibility - using a multitude of social tools to respond to a large number of situations. It could be that, as this study suggests, lessons learned from interacting with many peers are more important than excelling at one particular task.