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Female sexual behaviour as an indicator of the timing of ovulation in crested black macaques (Macaca nigra)

Carina Saggau, BSc, Georg-August Universit├Ąt, Goettingen, Germany

From the data presented in this study, it can be concluded that sexual behaviour of female crested black macaques indicates the fertile phase within an ovarian cycle. Furthermore, males seem to be able to read these given cues and use them for their mating decisions. These findings indicate a change in female attractiveness in crested black macaques between the three cycle phases. Behaviourally indicating the fertile phase might for females be advantageous in the way that they are able to bias paternity to dominant males and therefore gain genetic benefits. Thus, females may play a major role concerning the reproductive success of both sexes through actively choosing their mating partners. For males it might be advantageous being able to time their activities to the best period for obtaining conception, because they are able to reduce costs and to maximize their chances of paternity. Males experience an increased chance of fathering offspring during consortships (Altmann et al. 1996) and being able to identify the fertile phases of female ovarian cycles increases male reproductive success. With the given information about females being fertile, males can time effort to appropriate times for fertilisation within an ovulation cycle. Thus, the ability of monopolising females in a given time might increase strongly.

As female crested black macaques have pronounced sexual swellings (Reed et al. 1997), it would be very important in future investigations to determine whether the fertile phase of an ovarian cycle generally lies within the defined maximum swelling stage. If the period of maximum swelling would greatly exceed the fertile phase and if the timing of ovulation would be variable within the maximum swelling period this would support the graded signal hypothesis (Nunn 1999), which suggests that swelling size only shows the probability of ovulation but does not pinpoint its precise time. In contrast to sexual swellings, changes in female sexual behaviour could be seen as a reliable indicator for the timing of the fertile phase as found out as well for long-tailed macaques (Engelhardt 2004). In the future it should also be determined if the consortship behaviour of females as well as the number of mating partners is related to the fertile phase. Mating with many males outside the fertile phase and mating with the alpha male during the fertile phase would support the graded signal hypothesis as well as my results concerning males having information about the precise time of ovulation.

In future studies it will also be important to determine if there is a relationship between sexual female behaviour and male dominance rank, because this could explain some of the apparent inconsistencies in the behavioural data (e.g. female grooming). Another factor that should be investigated in future is the presence of other potential indicators of the fertile phase like for example olfactory cues or changes in the acoustic structure of copulation calls. My results suggest that female crested black macaques might be able to manipulate male behaviour in the way that they use specific behavioural patterns to confuse paternity through polyandrous mating, but at the same time bias paternity to ordained males for gaining genetic and direct benefits. Together the results suggest that female choice should not be underestimated in crested black macaques, although I suggest that males may dominate females through strong sexual dimorphism.