Human lactation and the structure of infant feeding

Much human behaviour has characteristics that we share with other species among the primates, the order to which Homo sapiens belongs. But we share the way we feed our young much more widely with other species of mammals, the class to which the primates belong. Indeed, the defining characteristic of this class is the feeding of their young on milk, and the class is named for the mammary glands that make this possible. Because infant mammals are initially fed exclusively on milk, it contains a large number of different nutritional constituents. The largest is water. It also contains energy-yielding components, which include a sugar, lactose, and fats, which provide a particularly dense source of energy, and it contains protein, in the form of casein and whey protein. These constituents of milk provide for the initial growth and energy expenditure of infants after birth. A range of immunological constituents help protect the infant against disease.

Feeding their young on milk has been a very successful adaptation for mammals; indeed, Caroline Pond has argued that it has been a major determinant of their ecological role (Pond, 1977; 1983). Although some reptiles guard their young as a form of parental care, reptiles do not bring food to the nest. The young must forage for themselves, and reptiles must therefore reproduce in environments that provide a supply of food suitable for infants as well as for adults.