Variation in the duration and pattern of breast-feeding contributes significantly to inter-population differences in fertility. In this paper, measures of suckling frequency and intensity are used to compare the effects of breastfeeding practices on the duration of lactational amenorrhoea, and on the length of the birth interval in three prospective studies undertaken during the 1980s, among Quechua Indians of Peru, Turkana nomads of Kenya, and Gainj of Papua New Guinea.
In all three societies, lactation is prolonged well into the second year postpartum, and frequent, on-demand breast-feeding is the norm. However, the duration of lactational amenorrhoea and the length of birth intervals vary considerably. Breast-feeding patterns among Gainj and Turkana are similar, but Turkana women resume menses some 3 months earlier than do the Gainj. The average birth interval among the Gainj exceeds that of nomadic Turkana by over 15 months. Suckling activity decreases significantly with increasing age of nurslings among both Gainj and Quechua, but not among Turkana. Earlier resumption of menses among Turkana women may be linked to the unpredictable demands of the pastoral system, which increase day-to-day variation in the number of periods of on-demand breast-feeding, although not in suckling patterns. This effect is independent of the age of infants.
The short birth intervals of Turkana women, relative to those of the Gainj, may be related to early supplementation of Turkana nurslings with butterfat and animals' milk, which reduces energetic demands on lactating women at risk of negative energy balance.