Both diet and sociality affect primate brain-size evolution

Increased brain size in humans and other primates is hypothesized to
confer cognitive benefits but brings costs associated with growing and
maintaining energetically expensive neural tissue. Previous studies have
argued that changes in either diet or levels of sociality led to shifts in
brain size, but results were equivocal. Here we test these hypotheses
using phylogenetic comparative methods designed to jointly account for and
estimate the effects of adaptation and phylogeny. Using the largest
current sample of primate brain and body sizes with observation error,
complemented by newly compiled diet and sociality data, we show that both
diet and sociality have influenced the evolution of brain size. Shifting
from simple to more complex levels of sociality resulted in relatively
larger brains, while shifting to a more folivorous diet led to relatively
smaller brains. While our results support the role of sociality, they
modify a range of ecological hypotheses centered on the importance of
frugivory and instead indicate that digestive costs associated with
increased folivory may have resulted in relatively smaller brains.