To understand this, we again draw from the work of Nils Bergman, an MD from South Africa who has also studied animal biology and behavior (Bergman, 2001). Dr. Bergman explains the four distinct ways mammals care for their young. The differences in their milks and in the maturity of their newborns tell us which type of care is right for each. Let's see where the human infant belongs.
Cache mammals. These include the deer and the rabbit. Cache mammals are mature at birth. Their mothers hide their young in a safe place and return to them every twelve hours. Consistent with this behavior, the milk of cache animals is high in protein and fat. It sustains the young animals for a long time, because the babies are fed infrequently.
Follow mammals. The giraffe and cow are follow mammals and like others of this group, are also mature at birth and can follow their mothers wherever they go. Since the baby can be near the mother throughout the day and feed often, the milk of the follow mammal is lower in protein and fat than that of a cache mammal.
Nest mammals. These include the dog and the cat. Next mammals are less mature than cache or follow mammals at birth. They need the nest for warmth and remain with other young from the litter. The mother returns to feed her young several times a day. The milk of nest mammals has less protein and fat than cache mammals. But it has more than follow mammals, who feed more frequently.
Carry mammals. This group includes the apes and marsupials, such as the kangaroo. The carry animals are the most immature at birth, need the warmth of the mother's body, and are carried constantly. Their milk has low levels of fat and protein, and they are fed often around the clock. Humans are most definitely carry mammals. Human milk has the lowest fat and protein of all mammalian milks. That, and our immaturity at birth, means human infants need to feed often and are meant to be carried and held.