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.

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.