The social brain hypothesis holds that increases in brain volume in primates had been driven primarily by the size of social groups they inhabit – the bigger the group, the higher the brain volume.
While there is some research to support it, things are rarely as one-dimensional as that. A new study out in the journal Nature Ecology and Evolution suggests that diet – in this case, a high-fruit one – may have been just as influential.
In the study, researchers analysed the brain volume and diets of over 140 primate species and found that those who relied on fruit as their staple food had up to 25 percent more brain tissue than those who feasted mainly on leaves, even after controlling for body size and species relatedness.
The obvious (although not necessarily correct) explanation for the finding is that foraging for fruit requires more cognitive flexibility than simply reaching out and grabbing some leaves from a nearby tree.
According to the researchers, while social complexity does correlate with brain volume, it isn’t always clear which came first – larger brains or larger and more intricate group formations – while diet seems to be a much more reliable cause.
Stephen Montgomery, a researcher studying brain and behavioural evolution at the University of Cambridge who was not involved in the study, claims that the two competing hypotheses are not necessarily mutually exclusive.
Given the diversity of primate behaviour and habitat, some species might have derived more benefit (in terms of brain size) from living in larger groups, while others reached a similar point by consuming a more energy-dense diet.
The creator of the social brain hypothesis, Robin Dunbar, concurs, while also pointing out that the authors of the new study only looked at total brain volume, although previous research has shown that social complexity is more predictive of the volume of the neocortex in particular.
“Diet, social structure, cognitive abilities – they’re likely to have co-evolved together in primates,” said one of the study lead authors Alex DeCasien.
Only further experimental and observational insight might elucidate the relative importance of – and the interplay between – the two competing, or rather complementary, lines of explanation.