Examining the morphological, ecological, and environmental predictors of brain evolution in the order chiroptera

Brains evolve in conjunction with an array of morphological, ecological and environmental factors, ranging from diet to habitat. Although the details vary, most theories relating to brain evolution suggest that increasing variation, complexity or unpredictability in a species’ environment favours a larger brain or an increase in the size of a specific brain region. In this phylogenetic comparative study, we investigated the relationship between ecology and brain size across 204 bat species (Order Chiroptera) using variables related to diet, habitat, climate, and range. Our findings suggest that different evolutionary patterns exist between the two major groups of bats, Microchiroptera and Pteropodidae. Diet itself did not have an influence on the whole brain size of bats from either clade. However, frugivorous species of Microchiroptera have larger olfactory bulbs and smaller medullas than insectivorous species. In addition to the constituents of the primary diet, we found that diet breadth contributes to brain evolution: frugivorous Microchiropterans with narrower diets (i.e., lower diet breadth) had larger olfactory bulbs. Among Pteropodid species, those with narrower diets had larger hippocampi than those with broader diets. The size of some brain regions was associated with variables other than diet; for example, within Pteropodidae, medulla size was impacted by whether species lived on islands. This study highlights how different brain regions are under different selective pressures, which is consistent with the mosaic theory of brain evolution. We also found evidence supporting the cognitive buffer hypothesis, as temperature variability and island-dwelling seemed to have influenced the evolution of certain brain regions. General patterns of brain evolution are difficult to elucidate even within a single order, but overall, the dominant result seems to be that diet has had the largest impact on the individual brain regions, particularly within the Order Chiroptera.