Sexual behavior of adult male chimpanzees of the Mahale Mountains national park, Tanzania

Abstract

This study, based on 687 hr of focal observations, aims to describe overall patterns of the sexual behavior of the adult male chimpanzees of the Mahale Mountains, to compare the results with previous reports, and to explain the variations between studies. Genital inspection of cycling females by adult males was eight times as frequent as that of lactating females, and twice as frequent as that of pregnant females. Inspection of the genitals of cycling females increased dramatically 7–10 days before the onset of maximal swelling and gradually decreased as the day of ovulation approached. Adult males likely obtained information on the attractivity of females by inspecting their genitals. Mating was usually initiated by male courtship and followed by pelvic thrusts in a dorsoventral posture, performed on, rather than above, the ground, which continued for 7 s. on average, and was typically followed by female squeaking and darting from the male, or by the male grooming the female. Higher-ranking males mated with females in the peri-ovulatory period more frequently than did lower-ranking males. In particular, two alpha males mated with such females more often than did any other adult males. A male who interfered with a mating pair was dominant over the mating male in other agonistic contexts. The duration of intromission was correlated with neither dominance rank nor age. However, when an adult male declined in rank from alpha in 1991 to third in 1992, he showed a significantly shorter duration of intromission. This indicates that for a particular male, the alpha rank guaranteed longer duration of intromission. Allies of alpha males tended to mate with peri-ovulatory females more frequently than expected from their low dominance ranks. The number of mating partners was not correlated with male dominance rank, but was sometimes negatively correlated with male age. Females were significantly more likely to emit a copulatory squeak when mating with younger, rather than older, adult males. Male dominance rank and the rate of female copulatory squeaking were not correlated. Weaning infants regularly interfered with their mothers' mating. Occasionally, unrelated adolescent males and rarely females pushed themselves in between copulating adults. Female choice was indicated when they performed a “penis erection check” or took the initiative in courtship, or on the other hand showed strong reluctance to mate with particular males. Young adult males more often received erection checks than did prime males, while none of the three old adult males did. Courtship initiated by estrous females was not directed to two of the oldest males, the exception of which was the alpha male. The oldest males, except for the alpha, were consistently avoided by many estrous females, both young and old. In response to female reluctance, males behaved violently, however, this was not effective, because other more dominant males came to rescue the female. Neither courtship nor mating was seen between mature sons and their mothers, nor between brothers and sisters.