Most primates live in social groups. While group size ranges from a few to over 100 individuals (depending on species), the composition of a primate group is pretty stable. The hub of the group tends to be females with offspring. Some groups have just one male, the harem leader, who monopolizes females by chasing other males away from a territory. Other groups have multiple males, and may range over areas too large to defend. Regardless, dominance hierarchies are often established in primate groups. The strongest individuals get better access to food or other resources. Primates are constantly picking through each other's fur (grooming). While grooming removes parasites and cleans fur, it may also help maintain social hierarchies. Scientists have found that in some primate societies, subordinate females will groom dominant females in exchange for extra food.

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.