Primate sympatry shapes the evolution of their brain architecture

The main hypotheses on the evolution of animal cognition emphasise the role of conspecifics in affecting the socio-ecological environment shaping cognition. Yet, space is often simultaneously occupied by multiple species from the same ecological guild. These sympatric species can compete for food, which may thereby stimulate or hamper cognition. Considering brain size as a proxy for cognition, we tested whether species sympatry impacted the evolution of cognition in frugivorous primates. We first retraced the evolutionary history of sympatry between frugivorous primate lineages. We then fitted phylogenetic models of the evolution of the size of several brain areas in frugivorous primates, considering or not species sympatry. We found that the whole brain or brain areas used in immediate information processing were best fitted by models not considering sympatry. By contrast, models considering species sympatry best predicted the evolution of brain areas related to a long-term memory of interactions with the social or ecological environment, with a decrease of their size the higher the sympatry. We speculate that species sympatry, by generating intense food depletion, leads to an over-complexification of resource spatio-temporality that may counteract the benefits of high cognitive abilities, drives niche partitioning and specialisation, and thereby induces lower brain area sizes. In addition, we reported that primate species in sympatry diversify more slowly. This comparative study suggests that species sympatry significantly contributes to shaping primate cognition and diversification.