A comparative analysis of brain size in relation to foraging ecology and phylogeny in the Chiroptera

Variations in total brain mass and in the mass of three brain regions (main olfactory bulb, hippocampus, auditory nuclei) were examined using a data set for 63 species of bats (Chiroptera). Using both conventional and phylogenetically based analysis of covariance (log body mass as covariate), we tested several hypotheses that relate total brain mass or the size of the components to variation in foraging ecology, categorized as phytophagous, gleaner, and aerial insectivore. In some analyses, the category phytophagous was split into phytophagous pteropodid and phytophagous phyllostomid to examine differences between two distinct clades of bats. Because the Megachiroptera orient primarily by vision and olfaction, whereas all other bats rely on laryngeal echolocation to locate their prey, we hypothesized that the former would differ in size of the main olfactory bulb, as compared with all other bats. This hypothesis was supported by our analyses. Our more general prediction was that insectivorous bats, which rely heavily on echolocation for the pursuit and capture of their prey, would have larger auditory nuclei than do phytophagous species. This, too, was supported. We also compared phytophagous (fruit or nectar consuming) bats in two families, the Pteropodidae and the Phyllostomidae. We hypothesized that the phyllostomids, which use echolocation while foraging, would have larger auditory nuclei. Although statistical power is low in phylogenetically informed comparisons of the two clades, we did find weak evidence in support of this hypothesis. We conclude that bat brains show evidence of adaptation to foraging ecology.