In an effort to maximise their reproductive output, males and females may cooperate when it is to their mutual advantage. However, they may also control, manipulate, betray, and even attack each other when they have conflicting interests. Chimpanzees maintain a promiscuous mating system, for which females develop a huge swelling of sexual skin and males large scrota. These morphological features make sperm competition the characteristic feature of chimpanzee sexual activity. Males are expected to mate with as many females as possible, while females are expected to be more choosey because reproductive output over their lifetime is more limited. A powerful constraint on female chimpanzees is the need to consider males’ tendencies toward infanticide.
Which sex takes the initiative in mating?
‘Copulation is almost always preceded by a male courtship display that signals the sexual arousal of the male and attracts the attention of the female’, said Goodall on the Gombe chimpanzees (Goodall 1986). However, our findings show that males take the lead only 46–80 per cent of the time, with females otherwise taking the lead (Nishida 1997a, 1980). Furthermore, the data from Yukio Takahata’s 1981 study (Takahata et al. 1996) reveal that females in M-group take the initiative in terms of ‘approach’, ‘approach within 3 m of the other’, and ‘leaving’ in addition to ‘solicitation of copulation’ (courtship). Why do such remarkable differences exist across populations?