Brief Communication: Seasonality of diet composition is related to brain size in New World Monkeys.

New World monkeys exhibit a more pronounced variability in encephalization than other primate taxa. In this comparative study, we tested two current hypotheses on brain size evolution, the Expensive Brain hypothesis and the Cognitive Buffer hypothesis, in a sample of 21 platyrrhine species. A high degree of habitat seasonality may impose an energetic constraint on brain size evolution if it leads to a high variation in caloric intake over time, as predicted by the Expensive Brain Hypothesis. However, simultaneously it may also provide the opportunity to reap the fitness benefits of increased cognitive abilities, which enable the exploitation of high-quality food resources even during periods of scarcity, as predicted by the Cognitive Buffer hypothesis. By examining the effects of both habitat seasonality and the variation in monthly diet composition across species, we found support for both hypotheses, confirming previous results for catarrhine primates and lemurs. These findings are in accordance with an energetic and ecological view of brain size evolution.