Comparative studies indicate that species differences in mammalian brain size relate to body size, ecology, and life-history traits. Previous analyses failed to show intrasexual or behavioral patterns of brain size in mammals. Across the terrestrial Carnivora, I find to the contrary. Differences in female, but not male, brain size associate with a fundamental ecological and evolutionary characteristic of female behavior. Other factors equal, females that provide the sole parental care have larger brains than those of biparental or communal species. For females, more parental investment accompanies larger brains. Future comparative studies of mammalian brain size must recognize that some patterns arise independently in the two sexes.