ABSTRACT
Ape mothers, like most mammalian females, invest more i n parental effort than ape fathers do. This investment, i n the form of internal gestation, lactation, and infant transport, reduces the mother's capacity to invest i n subsequent reproduction. Theory therefore suggests that access to envi ronmental resources such as food should be the primary constraint on female reproductive success, and an important determinant of female reproductive t iming (Emlen and Oring 1977; Trivers 1972; Wrangham 1980). For example, females should conceive only when the resources nec essary for successful pregnancy and parturit ion are available (Ellison 1990; Wasser and Barash 1983). I n line w i t h this expectation, i n humans (reviewed i n Ellison et al. 1993) and other great apes (Bentley 1999; Knott 1999, this volume) ovarian function is extremely sensitive to fluctuations in both energy balance and activity. This means that females have a lower probability of conception during periods when a successful reproductive outcome is less likely.
