Primates have really big brains relative to their body size if you compare them to other animals, even other mammals. The sheet of cells that forms the outer layer of mammal brains (the neocortex) is greatly enlarged. This neocortex is the newest part of the brain to evolve, and beneath it is a primitive brain similar to a reptile's brain (the medulla). As the center for memory, creative thinking, spatial reasoning, language, and conscious thought, the neocortex is what makes primates smart. A human being has the highest ratio of neocortex to medulla of any animal. Much of primate intelligence comes to play in social and learning behavior. Scientists believe the extreme development of the neocortex may be an adaptation for the advanced cooperation (e.g. for hunting) that takes place in primate societies. Sustained social interactions require that primates remember a lot of information about each other.

All mammals have hair, outgrowths of the skin, during at least some part of their development. While it evolved to insulate animals in cold climates, mammal hair has various functions. Long, stiff hairs (vibrissae, or whiskers) are found in all mammals but humans. Whether on the face, legs, or tail, vibrissae extend an animal's sense of touch. Some mammals, such as porcupines, have enlarged hairs that function as defensive spines. Defense is also achieved by raising individual hairs to make an animal look larger (such as an angry cat). Muscles in the skin make the hair stand up or lie down. Raising and lowering hair changes how much air is trapped under it, which also makes it more or less insulating. Mammals in cold climates tend to have an extra layer of insulating fur, or extra fat under their fur. Mammals in climates that change seasonally shed and replace their hair to create a thicker winter coat and a thinner summer coat.