Introduction

It has often been noted that female primates tend to have extended mating periods in their ovarian cycles, tend to mate polyandrously and also tend to mate during pregnancy (cf. Hrdy 1979; Hrdy & Whitten 1987). Since females in species vulnerable to infanticide show these features to a greater extent, this behavior was interpreted as serving to confuse paternity (cf. van Schaik et al. 1999; van Noordwijk & van Schaik, Chapter 14). The extent to which such mating tactics succeed in confusing paternity depends on the outcome of an “arms race” between males and females concerning the amount of information on paternity available to males. In order to examine more closely the claim that sexual behavior in primates serves at least in part to reduce infanticide risk, we must examine the physiological basis for paternity confusion, as well as for its complement, paternity concentration. Since ovarian cycles vary considerably in detail among taxa (e.g., Short 1984), we limit this examination to primates, the best-known order in this respect. We ask therefore how the ovarian cycles of female primates are organized in relation to the need for strategies to reduce infanticide risk. Two features are examined in particular which we will argue are related to the benefits to females of unpredictability in the timing of ovulation: (1) the large variance in the length of the preovulatory or follicular phase both within and between individuals, and (2) interspecific variation in the mean length of the follicular phase.