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 named for their special milk-producing structures called mammary glands. A female mammal may have two (like humans) or as many as 18 mammary glands. Each gland is full of small spaces lined with milk-secreting cells that all drain into a nipple. Newborn mammals have an innate sucking reflex that stimulates the mother's mammary glands to release milk. Milk is a superfood for young mammals, containing a nourishing mix of fats, proteins, vitamins, minerals, and even immune-system chemicals. The first milk (colostrum) contains antibodies from the mother to protect against disease. As the offspring grows, milk changes to maintain the right balance of nutrients. Production of milk (lactation) makes most female mammals temporarily unable to get pregnant, resulting in natural gaps between offspring. The spacing of births benefits both the offspring and the mother in terms of survival. Mother mammals stop nursing (wean) their babies after several days or years, depending on the species and environmental conditions.