The common chimpanzee (Pan troglodytes) and its close relative, the bonobo (Pan paniscus) are the closest relatives to Homo sapiens alive today. Like humans and other primates, chimps are social animals, forming relatively stable but fluid communities, with males, females, adults and adolescents living in close proximity over extended periods. Compared with their human counterparts, female chimps tend to be more promiscuous and go longer between births; both male and female chimps employ a greater variety of reproductive strategies than humans do.
Chimpanzees reach puberty at roughly the same ages humans do. Male chimps become sexually mature between about 9 and 15 year old, while females experience their first menstrual periods at about the age of 10, with a menstrual cycle of about 36 days. Anogenital swelling in females signals their encroaching fertility. In contrast to humans, female chimps remain infertile for about two to four years after their first menstrual cycle, although they copulate with aplomb during this time; this latent period is probably an adaptation to ensure safety, as females have typically migrated to a new community at around this time.
Hierarchies and Competition
The males in chimpanzee communities are organized in a more-or-less linear manner from top to bottom, with an "alpha" male at the top. The alpha male is usually between the age of 20 and 26 and possesses unusual intelligence and physical prowess. Females have their own, somewhat more fluid hierarchy, and all females are subservient to all males. The males routinely patrol their communities' boundaries and will ferociously attack male interlopers from other communities they encounter there. The females considered most sexually appealing usually attract a following of multiple males, and the alpha among them sometimes tries to bar others from mating with her until he can take his own turn. Adolescent females can move around among various chimpanzee communities.
Strategies and Patterns
As in humans, there is no particular time of year in which females are more fertile, or in estrus. That said, within a community, a higher fraction of females are in estrus when the food supply is most abundant. Females will mate with a multitude of males when they are at peak fertility unless prevented from doing so by a dominant male. In fact, alpha males may prevent other males from mating even with females he is not interested in himself. Chimps also manifest consortship mating, in which a male and his partner leave a community for days to weeks, as well as extra-group mating, wherein females covertly mate with males outside the community.
Male chimpanzees, like other male mammals, exhibit patterns of behavior toward females that disarm females' resistance to mating. These behaviors may include physical force and qualify in human terms as rape or sexual assault, or they may be more subtle or indirect, as when males engage in activities that partition females from other males. Direct sexual coercion includes a male keeping an ovulating female to himself, which limits sperm competition. An indirect form of sexual coercion is males killing infant babies that he is fairly sure aren't his own. This may be an effort to spur the mother into becoming fertile again so that he can mate with her. Female chimpanzees also kill the babies of other chimp mothers.