Mating Systems
Photo Courtesy of Dr. Pascal Ngoboka from Ngamba Chimp Sanctuary, Lake Victoria, Uganda
Estrus and Gestation
Puberty in both sexes occurs at about seven years of age. Females raised in captivity begin mating at eight to nine years and give birth for the first time at ten to eleven years. Wild chimpanzee females mature three to four years later. Females are not receptive for three to four years after giving birth, and then resume sexual activities for one to six months until conception. Gestation lasts 230 to 240 days. There is no regular breeding season, but females only mate during heat, which lasts two to three weeks or more and occurs every four to six weeks. During this time females have prominent swelling of the pink perineal skin (Pankopf, 2000).
Male Courtship Behaviors
Copulation is usually initiated by male courtship behaviors. He will display signals to the female that indicate his sexual arousal and get her attention. Such displays are always accompanied by a penile erection, and may include direct gaze, hand on branch, erect hair, branch shaking, an outstretched arm or arms towards the female, bipedal swagger, or sitting hunched. While the adult male is usually the one who initiates sexual encounters with females, adolescent females are known to sometimes solicit sex from males of any age (Goodall, 1986).
Copulation
Young females leave their natal groups and travel with adult males during their first estrus periods. During adolescents females have a period of sterility that lasts one to three years. At this time females will mate with males of her own community and males with neighboring groups as well. She makes a commitment to one community usually when she becomes pregnant for the first time. Males on the other hand always remain in their natal group. They compete with other resident males for mating rights, and they band together to protect female resources from other neighboring males (Goodall, 1986).
For the first week or more of estrus, females are mostly promiscuous and mate on an average of six times a day (Pankopf, 2000), but there are six different sexual situations a reproductively mature female may engage in. The first is during the early stages of tumescence, which is the stage right before ovulation in the female estrus cycle. During this phase she is usually ignored by adult males, but may be mated by infants, juveniles, and early adolescents. The second situation may occur once a female’s swelling reaches its maximum size. In this situation she is the center of attention in a party of males, and mating is usually promiscuous. She may copulate with every male in the party with the exception of an adult son or adult maternal sibling. The third situation can occur during any time during her period of swelling. Here an adult male may become possessive of her and follow her around incessantly. He will try and be the only one who copulates with her during this time. A fourth situation is similar to the third, except that the male is the alpha male. Here he can essentially monopolize the female and use aggression to more successively ward off any other males. A fifth type of situation is when another adult male tries to lead her away from the group. This is called a consortship, and it can occur during any phase of her cycle and even during anestrus. If he is successful in leading her away, they may stay together for up to three months. A final situation is when a female wanders towards or is drawn in by males of a neighboring community and is mated by them. She may either stay with them as an immigrant or return to her own community (Goodall, 1986).