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.

Mammals are able to make enough internal body heat to keep their bodies at a relatively constant, high temperature. Endothermy (endo= inside; thermic= heat) makes mammals less dependent on outside temperatures, freeing them to be active in a wide range of conditions. Mammals may be active at night (nocturnal) or in the day (diurnal), in climates as extreme as the cold poles or the hot tropics. Adaptations to warm up or cool off, such as shivering and sweating, allow mammals to maintain their preferred body temperatures in the face of these extremes. Mammals also regulate body temperature with behavior: elephants flapping ears to cool off, humans wearing coats to warm up, dogs panting to cool down. The hairs making up mammal fur are raised or lowered to provide more or less insulation as needed.