Affording larger brains: testing hypotheses of mammalian brain evolution on bats

Several major hypotheses have been proposed to explain how larger brains in mammals, such as those of humans, are afforded in energetic terms. To date, these have been largely tested on primates, with some cross-mammal analysis. We use morphological, ecological, and metabolic data for 313 species of bats to examine the allometry of brain mass and to test key predictions from three of these hypotheses: the direct metabolic constraint, expensive tissue, and maternal energy hypotheses. We confirm that megachiropteran bats (entirely fruit-eating) have larger brains for their body mass than microchiropteran bats (fruit-eating and non-fruit-eating) and fruit-eating species (Megachiroptera and Microchiroptera) have larger brains than non-fruit-eating species (Microchiroptera). Although our analyses demonstrate little or no support for any of the three hypotheses, we show that 95.9% of the variance in brain mass can be explained by the independent effects of gestation length and body mass. This indicates that among bats, the duration of maternal investment plays an important role in the adult brain mass finally obtained. These analyses serve to emphasis the crucial importance of testing the general applicability of macroevolutionary hypotheses (often developed in isolation in one clade) in multiple clades with different evolutionary histories.