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amount of effort merely lugging them about during the day.Richard Lee has calculated that over a four-year period of dependency a Bushman mother will carry her child a total of 4,900 miles on collecting expeditions and campsite moves. No Bushman woman wants to be burdened with two or three infants at a time as she travels that distance.
    The best method of population control available to stone age hunter-collectors was to prolong the span of years during which a mother nursed her infant. Recent studies of menstrual cycles carried out by Rose Frisch and Janet McArthur have shed light on the physiological mechanism responsible for lowering the fertility of lactating women. After giving birth, a fertile woman will not resume ovulation until the percentage of her body weight that consists of fat has passed a critical threshold. This threshold (about 20–25 percent) represents the point at which a woman’s body has stored enough reserve energy in the form of fat to accommodate the demands of a growing fetus. The average energy cost of a normal pregnancy is 27,000 calories—just about the amount of energy that must be stored before a woman can conceive. A nursing infant drains about 1,000 extra calories from its mother per day, making it difficult for her to accumulate the necessary fatty reserve. As long as the infant is dependent on its mother’s milk, there is little likelihood that ovulation will resume. Bushman mothers, by prolonging lactation, appear to be able to delay the possibility of pregnancy for more than four years. The same mechanism appears to be responsible for delaying menarche—the onset of menstruation. The higher the ratio of body fat to body weight, the earlier the age of menarche. In well-nourished modern populations menarche has been pushed forward to about twelve years of age, whereas in populations chronically on the edge of caloric deficits it maytake eighteen or more years for a girl to build up the necessary fat reserves.
    What I find so intriguing about this discovery is that it links low fertility with diets that are high in proteins and low in carbohydrates. On the one hand, if a woman is to nurse a child successfully for three or four years she must have a high protein intake to sustain her health, body vigor, and the flow of milk. On the other hand, if she consumes too many carbohydrates she will begin to put on weight, which will trigger the resumption of ovulation. A demographic study carried out by J. K. Van Ginneken indicates that nursing women in underdeveloped countries, where the diet consists mostly of starchy grains and root crops, cannot expect to extend the interval between births beyond eighteen months. Yet nursing Bushman women, whose diet is rich in animal and plant proteins and who lack starchy staples, as I have said, manage to keep from getting pregnant four or more years after each birth. This relationship suggests that during good times hunter-collectors could rely on prolonged lactation as their principal defense against overpopulation. Conversely, a decline in the quality of the food supply would tend to bring about an increase in population. This in turn would mean either that the rate of abortion and infanticide would have to be accelerated or that still more drastic cuts in the protein ration would be needed.
    I am not suggesting that the entire defense against overpopulation among our stone age ancestors rested with the lactation method. Among the Bushmen of Botswana the present rate of population growth is .5 percent per annum. This amounts to a doubling every 139 years. Had this rate been sustained for only the last 10,000 years of the old stone age, by 10,000 B.C . thepopulation of the earth would have reached 604,463,000,000,000,000,000,000.
    Suppose the fertile span were from sixteen years of age to forty-two. Without prolonged nursing, a woman might experience as many as twelve pregnancies. With the lactation method, the number of pregnancies comes down to six.