Evolutionary biologists had long assumed brain size increased in mammals in a more or less consistent fashion, with each species gaining roughly the same boost relative to body size over time. However, Oxford researchers Dr. Susanne Shultz and Professor Robin Dunbar have revealed in their new study that the truth is a bit more complicated, and it's actually the sociability of a given species that determines brain size. Unsurprisingly, primate brains grew the most over time, followed by horses, dolphins, camels, and dogs. All these creatures tend to live in stable social groups. Solitary mammals, including everything from cats to rhinos, showed much slower brain size growth over the same evolutionary period.