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.

Thanks to their ability to produce milk, female mammals are able to feed their offspring for weeks, months, or even years after they are born. This sets the stage for the extended childhoods of many mammal offspring. With food and shelter taken care of (by the mother in most species), a remarkable amount of learning occurs for mammal offspring before they become independent. Elephants tend to their young for as long as 10 years, and humans for longer still. Young mammals may learn about food, shelter, migration routes, social structures, or other keys to survival from their parents. While extended childhoods support learning, they are costly, especially for the female. Lactating females must consume enough calories to feed their offspring and maintain themselves. Offspring that stay around after weaning from their mother's milk may still require protection and food. Human offspring are the most extreme case of investment, with parents usually providing intensive support until age 18 and continuing support beyond.