Abstract
DeCasien et al. reported that diet, rather than sociality, predicts relative brain size across primates, with frugivores having larger brains than folivores. Their interpretation moves the discussion from sociality towards ecology yet leaves the proposed energetic link biologically unspecified. The matter arising from this dietary framework is gut microbial metabolism, a plausible mechanistic link between dietary substrates and the energy and metabolites available to support neural development and maintenance. Microorganisms help determine how much energy and which metabolites a host obtains from a dietary substrate, while social contact helps transmit those microorganisms among hosts. Evidence for both processes was available when the article appeared. Because the study represented diet using broad feeding categories or percentage frugivory, it could not distinguish food intake from the nutrition realized by the host-microbe system. The results establish a dietary association, but they do not identify the mechanism through which diet could support the evolution of a larger brain.