Reproductive behaviour of wild chimpanzees in the Gombe National Park, Tanzania.

(1) The reproductive behaviour of chimpanzees in the Gombe National Park, Tanzania, was studied for 16 months and additional information came from 19 years of demographic data collected by Dr Jane Goodall and members of the Gombe Stream Research Centre. (2) The mating system of the Gombe chimpanzees comprises 3 mating patterns: (a) opportunistic, non-competitive mating, when an oestrous female may be mated by all the males in the group; (b) possessiveness, when a male forms a special short-term relationship with an oestrous female and may prevent lower ranking males from copulating with her; and (c) consortships, when a male and a female leave the group and remain alone, actively avoiding other chimpanzees. While males took the initiative in possessive behaviour and consortships, females had to co-operate for a successful relationship to develop. (3) Data from 14 conceptions indicated that the majority of females (9) became pregnant while participating in the restrictive mating patterns, possessiveness and consorting. Adult males showed differential frequencies of involvement in the restrictive mating patterns. The following male characteristics showed significant, positive correlations with participation in the restrictive patterns: (a) the amount of time spent in the same group as oestrous females, (b) the proportion of that time spent grooming oestrous females, and (c) the frequency with which males shared food with females. Dominance ranks of the adult males showed no consistent correlation with involvement in the restrictive patterns, but it was clear that the most dominant male did gain an advantage. He was the only male able to monopolize oestrous females by showing possessiveness. (4) Developmental and demographic data from observations for 19 years show that female chimpanzees have a very limited reproductive potential. Females reached menarche at about 11 years of age but this was followed by a period of adolescent sterility of median length 26 months. Each birth, if the infant survived, was followed by a long period of lactational amenorrhoea of median duration 42 months. The mean inter-birth interval of the Gombe chimpanzees was 5 years 8 months. Thus, the theoretical lifetime reproductive potential of females is 5-6 offspring who survive to weaning. Data on achieved reproductive success showed that females had a median of 3 births in their lifetime, with only 2 offspring surviving to reproductive age.