The ecology of birth seasonality among agriculturalists in central Africa

Abstract

The Lese are subsistence farmers living in the Ituri Forest of north-east Zaïre. They exhibit significant birth seasonality, with lowest frequencies of conception when food production is least, nutritional status is low and ovarian function, as measured by salivary steroid hormone levels, is reduced. Efe pygmy foragers, who live in the same geographical area but are less dependent on cultivated foods and have a more flexible life style, do not exhibit frequent fluctuations in nutritional status nor significant birth seasonality. These findings support a model of birth seasonality relating climatic variables to variation in fertility through a causal chain linking rainfall to food production to energy balance to ovarian function to fertility. The model, which emphasises an ecological approach to the study of human reproduction, should have broad applicability since seasonality of food production and energy balance is widespread geographically and across a wide variety of economies and cultures.

PIP: Researchers compared 1980-87 data on rainfall, garden size, nutritional status, ovarian function, and births among the Lese subsistence farmers and the nomadic Efe pygmies who lived in the Ituri Forest in northeast Zaire to analyze the ecology of human birth seasonality. Natural fertility for the Lese was 2.35 and 2.56 for the Efe. Rainfall patterns determined the size of Lese gardens (p.05) which in turn determined nutritional status for both the Lese and the Efe (p.01), but particularly the Lese. Negative changes in female nutritional status diminished ovarian function. Specifically, women with lower indices of weight for height and who had lost 2 kg during the study had lower peak progesterone levels and ovulatory frequency than women who lost =or 2 kg. Menstrual cycles lengthened and duration of menstrual flow decreased with a decline in nutritional status. Conceptions were significantly reduced in May-July for the Lese (p=.012). For both Efe and Lese women, conceptions were lowest in May and highest in September. Lese women experienced considerably fewer conceptions during the periods with poor food availability than during other months (p=.002). The corresponding p value was almost significant for Efe women (.55). Thus Efe women experience less fluctuations in nutritional status than did Lese women, but variability in the seasonal pattern of rainfall did determine variation in fertility. The causal chain was rainfall - determined food production which determined energy balance which determined ovarian function which determined fertility. This model can be applied to any population in any geographic region. It integrates knowledge of behavior and cultural practices with the biology of human reproduction.