ABSTRACT
The last two decades of the twentieth century have witnessed a dramatic growth i n the field of human reproductive ecology, resulting i n a better understanding of variation i n human fertility patterns related to changes i n ecological context. The first, and perhaps most significant, advance i n this area was the recognition of the suppressive effects of lactation on post partum fecundity. I t is now wel l established that lactation is the major determinant of the period of postpartum infecundity. As a result of this recognition, lactation plays a preponderant role i n any discussion of vari ation i n natural fertility patterns (Wood 1994). Many studies have shown the great variety of social and physical environments i n which lactation occurs, as wel l as the variability i n the response of female reproductive physiology. However, the proximate mechanisms underlying the phe nomenon of lactational infecundity are still not clear. The aim of this chap ter is to summarize the information available on lactation and postpartum fecundity and to present the current debate surrounding how the sup pressive effects of lactation are mediated. After a brief introduction to the costs and benefits of breastfeeding, we concentrate on the links between lactation and postpartum fecundity. A historical overview describes the context i n which the current ideas evolved and sets the stage for the pres entation of the two leading hypotheses aimed at identifying the proximate causes of lactational infecundity. Al though focused on mechanisms, these two hypotheses also have implications for the functional interpretation of lactational amenorrhea. We conclude w i t h the presentation of data from our research of lactating Toba women of northern Argentina as a case study that allows us to confront the predictions of the two competing hypotheses.
