When to Wean
Dettwyler, Katherine A., Natural History
Breast-feeding in humans is a biological process grounded in our mammalian ancestry. It is also an activity modified by social and cultural constraints, including a mother's everyday work schedule and a variety of beliefs about personal autonomy, the proper relationship between mother and child (or between mother and father), and infant health and nutrition. The same may be said of the termination of breast-feeding, or weaning. In the United States, children are commonly bottle-fed from birth or weaned within a few months. But in some societies, children as old as four or five years may still be nursed. The American Academy of Pediatrics currently advises breast-feeding for a minimum of one year (this may be revised upward), and the World Health Organization recommends two years or more. Amid conflicting advice, many wonder how long breastfeeding should last to provide an infant with optimal nutrition and health.
Nonhuman primates and other mammals give us some clues as to what the "natural" age of weaning would be if humans were less bound by cultural norms. Compared with most other orders of placental mammals, primates (including humans) have longer life spans and spend more time at each life stage, such as gestation, infant dependency, and puberty. Within the primate order itself, the trend in longevity increases from smaller-bodied, smaller-brained, often solitary prosimians through the larger-bodied, larger-brained, and usually social apes and humans. Gestation, for instance, is eighteen weeks in lemurs, twenty-four weeks in macaques, thirty-three weeks in chimpanzees, and thirty-eight weeks in humans. Studies of nonhuman primates offer a number of different means of estimating the natural time for human weaning. First, large-bodied primates wean their offspring some months after the young have quadrupled their birth weight. In modern humans, this weight milestone is passed at about two and a half to three years of age. Second, like many other mammals, primate offspring tend to be weaned when they have attained about one third of their adult weight; humans reach this level between four and seven years of age. Third, in all species studied so far, primates also wean their offspring at the time the first permanent molars erupt; this occurs at five and a half to six years in modern humans. Fourth, in chimpanzees and gorillas, breast-feeding usually lasts about six times the duration of gestation. On this basis, a human breast-feeding would be projected to continue for four and a half years. …