Brain size in primates is better predicted by their diet than by how complex their social lives are, reports a paper published online this week in Nature Ecology & Evolution. The largest analysis of its kind to date, the study calls into question the current hypothesis for explaining why humans and certain groups of primates evolved larger brains than most other animals.

Previous studies investigating brain size evolution in primates have found a correlation between the average number of group members in which a species is typically found, and the size of their brain relative to body size. However, when different measures of social complexity are considered (for example, whether a species is monogamous or not), results have been inconsistent, and have not explored other potential driving factors in the environment.

Alex DeCasien and colleagues compile the largest dataset assembled so far on non-human primate brain sizes from more than 140 different species, and explore the relationship between brain size and multiple measures of sociality (group size, social system and mating behaviours) and feeding habits. They find no link between brain size and any measure of sociality, but do find that diet has a much stronger predictive power.

After taking into account each species' evolutionary relatedness and their relative body sizes, the authors find that fruit-eating primates have around 25% more brain tissue than plant-eating species. Although their analysis is not able to discern why eating fruit should lead to the evolution of larger brains, they suggest this is likely driven by a combination of cognitive demands (related to recall of fruit locations and manual extraction of fruit flesh) and energetic rewards (related to the consumption of energy-rich fruit compared with energy-poor plants).

In an accompanying News & Views, Chris Venditti writes: “I feel confident that their study will refocus and reinvigorate research seeking to explain cognitive complexity in primates and other mammals. But many questions remain.”