However, infanticide is certainly not universal in traditional societies and is less common than infant death due to “benign neglect.” (That euphemism means that an infant is not actively killed but instead dies through neglect, e.g., due to the mother stopping nursing, or nursing the infant less often, or rarely cleaning or washing the infant.) For example, when Allan Holmberg lived among a group of Siriono Indians in Bolivia, he found that infanticide and abortion were unknown. Even though 15% of Siriono children were born with club feet, and only one out of five of those children survived to adulthood and raised a family, those children received normal love and feeding.
Weaning and birth interval
In the U.S. the proportion of infants who were nursed at all by their mothers, and the age at which those nursed infants were weaned, decreased through much of the 20th century. For example, by the 1970s only 5% of American children were being nursed at the age of six months. In contrast, among hunter-gatherers not in contact with farmers and without access to farmed foods, infants are nursed far beyond six months, because the only suitable infant food available to them is mother’s milk: they have no access to cow’s milk, baby formula, or soft food replacements. The age of weaning averaged over seven hunter-gatherer groups is about three years old, an age at which children finally become capable of fully nourishing themselves by chewing enough firm food. While some solid pre-chewed foods may be introduced around the age of six months, a hunter-gatherer child may not be fully weaned off its mother’s milk until the mother is pregnant with the next child. Individual !Kung children continue to nurse beyond the age of four if a next sibling has not yet been born. Studies show that, the older the age of a !Kung child when it is weaned, the more likely is the child to survive to adulthood. But in settled agricultural populations and among hunter-gatherers trading with farmers, those weaning ages and birth intervals of two and a half to four years for nomadic hunter-gatherers decrease to an average age of two years, because farmers do have livestock milk and soft cereal gruels onto which to wean a small child. For instance, when the !Kung themselves settle down to become farmers, as has been happening increasingly in recent decades, their birth interval quickly drops from three and a half years to the two years typical of farmers.
The ultimate evolutionary causes and the proximate physiological mechanisms responsible for those long birth intervals of nomadic hunter-gatherers have been the subject of much discussion. It appears that the ultimate reasons are two-fold. First, a mother without access to cow’s milk or cereal gruel, and hence likely to nurse a child until the age of three or more years, cannot produce enough milk to nurse both a newborn and a not-yet-weaned older child. If she tried, one of those children would be likely to starve for lack of milk.
The other reason is that only when a child is around four years old or more does it become capable of walking fast enough to keep up with its parents when they are shifting camp. Younger children have to be carried then. While walking, a 90-pound !Kung woman has to carry an under-four-year-old child of up to 28 pounds, a load of wild vegetables weighing from 15 to 40 pounds or more, and several pounds of water, plus utensils. That’s already a large burden, and it would be even heavier if a younger infant were added to the load. We thus have a second ultimate evolutionary factor contributing to the rapid decrease in birth interval when nomadic hunter-gatherers settle down to become farmers: most farmers live in permanent villages and don’t face the problem of having to carry all children less than four years old whenever they shift camp.
That late weaning age means that, for a hunter-gatherer mother, much physical and emotional energy goes into the rearing of one child. Western observers have the impression that a !Kung child’s very close relationship with the mother, and the exclusive attention that it enjoys for several years without younger siblings, provide an emotional security in childhood that translates itself into the emotional security of !Kung adults. But when a hunter-gatherer child finally does become weaned, the result can be traumatic. Within a short time, the child receives much less maternal attention, becomes hungry without mother’s milk, has to cede to the next infant its sleeping place at night next to the mother, and may be increasingly expected to enter the adult world. !Kung children being weaned are miserable and have tantrums. !Kung who survive to become old adults still look back on weaning 70 years earlier as a painful experience. In camps of Piraha Indians at night, one often hears children screaming, almost always because they are being weaned. Nevertheless, while traditional societies do wean at a later age than do modern Americans, the specific patterns vary among societies. For example, Bofi and Aka Pygmy children wean gradually rather than abruptly, tantrums are rare, and weaning is often initiated by the child rather than by the mother.
On-demand nursing
Those two ultimate causes responsible for the long birth intervals of hunter-gatherers leave open the question of the proximate physiological mechanism ensuring that there are not two children less than several years old to be cared for simultaneously. One mechanism is the resort to neglect or (less often) infanticide, as we have already mentioned: if a hunter-gatherer mother becomes pregnant when her previous child is still less than two and a half years old, then she may neglect or even kill the newborn, knowing that she cannot take care of it as well as of the older child. The other proximate factor is that physiological mechanisms operating in a mother nursing according to the on-demand schedule of frequent feedings characteristic of hunter-gatherer babies (as opposed to the infrequent nursing bouts set for the convenience of the mother in Western society) make it less likely that a nursing mother will become pregnant, even if she resumes sex while nursing.
In hunter-gatherer groups in which nursing has been specifically studied, it is often “on demand.” That is, the infant has constant access to the mother’s breast, is held in contact with the mother during the day, sleeps next to the mother at night, and can nurse at any time it wants, whether or not the mother is awake. For example, measurements among the !Kung have shown that an infant nurses on the average four times per hour during the day, 2 minutes per nursing bout, with an average interval of only 14 minutes between bouts. The mother wakes to nurse the infant at least twice a night, and the infant nurses without waking the mother several times per night. This constant opportunity for nursing on demand usually continues for at least three years of the !Kung child’s life. In contrast, many or most mothers in modern societies schedule nursing at times when the mother’s activities permit it. The organization of a mother’s work, whether the work is a job outside the house or domestic work within the house, often involves mother-child separations of several hours. The result is many fewer daily nursing bouts than the dozens of bouts for a hunter-gatherer mother, longer individual bouts, and much longer intervals between bouts.
That high nursing frequency of hunter-gatherer mothers has physiological consequences. As mentioned above, nursing hunter-gatherer mothers usually do not conceive for several years after a child’s birth, even if the mother resumes sexual activity. Evidently, something about traditional on-demand nursing acts as a contraceptive. One hypothesis is termed “lactational amenorrhea”: suckling releases maternal hormones that not only stimulate the secretion of milk but that may also inhibit ovulation (a woman’s release of eggs). But that inhibition of ovulation requires a constant regime of frequent nursing; a few bouts of nursing per day do not suffice. The other hypothesis is termed the “critical-fat hypothesis”: ovulation requires that the mother’s fat levels exceed a certain critical threshold. In a nursing woman from a traditional society without abundant food, the high energy costs of milk production keep the mother’s fat level below that critical value. Thus, sexually active nursing mothers in modern Western industrial societies, unlike their hunter-gatherer counterparts, may still conceive (to their surprise) for either or both of two reasons: their nursing frequency is much too low for hormonally induced lactational amenorrhea; and they are sufficiently well nourished that their body fat levels remain above the critical threshold for ovulation, even despite the caloric expenditure of lactation. Many educated Western mothers have heard of lactational amenorrhea, but fewer have heard that it is effective only at high nursing frequencies. A friend of mine who recently, to her dismay, conceived again only a few months after the birth of her previous child thereby joined the long list of modern women exclaiming, “But I thought that I couldn’t conceive while I was nursing!”