Although a controversial issue in the not distant past, it is now well established that breastfeeding is the primary factor mediating post-partum subfecundity. This complex interaction between behavior and physiology is a central research focus in several fields sharing a common concern with the mechanisms that regulate human reproduction. Despite the demonstrated linkage between lactation and return of the first postpartum menses, many questions regarding the relationship between variation in breastfeeding behavior and variation in ovarian function remain unanswered. In particular, women of seemingly comparable breastfeeding magnitude display substantial heterogeneity in the duration of post-partum subfecundity. In addition, the hormonal mechanisms by which lactation regulates ovarian function remain unclear, and the debates regarding the contribution of factors other than breastfeeding to post-partum subfecundity continue unresolved. Part of the difficulty in addressing these questions derives from the use of disparate methodological approaches and a n absence of specificity in analytical units. In particular, maternal recall of breastfeeding behavior is subject to substantial bias yet is the data source for most analyses. In addition, although a growing body of evidence suggests that variation in breastfeeding structure may contribute to variation in ovarian function, most studies fail to distinguish or evaluate the different components of suckling behavior. Elucidating the bases of this variation amongst women and populations requires disentangling the roles of the environment, culture, and behavior in structuring suckling activity. Anthropologists in particular have a n opportunity to achieve significant insights into the ecology of human breastfeeding and reproduction. An impressive body of research has firmly established that breastfeeding is the major determinant of post-partum amenorrhea (e.g., Perez et al., 1971;Berman et al., 1972;van Ginneken, 1977;Wenlock, 1977;Konner and Worthman, 1980;Jain and Bongaarts, 1981;Bracher and Santow, 1982;Howie and McNeilly, 1982;Hennart et al., 1985;Rivera et al., 1985;Diaz, 1989;Glasier, 1989;Gray et al., 1990). These investigations were initially prompted by the hypothesis that variation in lactation might contribute to fertility differences among populations (Gioiosa, 1955;Henry, 1961;Tietze, 1961), a suggestion contrary to the then prevailing biomedical position, which dismissed any purported effects of nursing on conception.0 1994 Wiley-Liss, Inc. [Vol. 37, 1994 Investigations of lactation now occupy a central position in the study of human fecundity and fertility in several fields. Ecologists seek to understand how the human reproductive system has evolved and how women partition energy to the competing demands of production and reproduction. Of particular interest is the selective advantage that may exist in delaying effort in the production of more offspring (Frisch, 1978;Quandt, 1984;Blurton Jones, 1986;Ellison, 1990Ellison, , 1991Peacock, 1990 Pea...
