For example, de Waal reported stable social relationships and very low rates of aggression among captive females in a stable group at Arnhem Zoo. On a day-to-day basis, competitive interactions among females are much less obvious. Breeding is not seasonal in chimpanzees, thus low rates of female reproduction produce highly skewed operational sex ratios of adult males to fertile females which result in intense male–male competition.Īs in many species, early studies of competition in chimpanzees emphasized the conspicuous and dramatic competitive behaviour of males. By contrast, male reproductive success depends most heavily on their access to fertile females. There is no direct paternal care in chimpanzees therefore, we expect female behaviour to be shaped critically by their success in accessing high-quality food resources, their maximization of foraging efficiency and their avoidance of mortality risk. Age at first reproduction and interbirth intervals in captive female chimpanzees are each reduced by 2–3 years, and there is increasing evidence from the field that differences in food availability influence reproductive rates. In chimpanzees, as in many other mammals, reproduction is accelerated by increased access to food. To maximize reproductive success females must therefore invest in their own mortality reduction as well as parental care. To date, the maximum observed number of surviving offspring produced by any female is seven (by Fifi at Gombe National Park, Tanzania ) and at most sites, average female reproduction is such that populations are at or below replacement levels. Females in the wild do not start reproducing until they are 13–15 years old they then produce one infant every 5–7 years and die at a maximum age of 50–60 years. The difference between the sexes in the resources that limit reproduction is particularly stark in chimpanzees, where the pace of female reproduction is among the slowest of any mammal. These patterns are compared to those in other species, including humans.īecause female mammals invest much more heavily than males in individual offspring through gestation and lactation, their reproductive success is usually limited by their ability to turn food resources into offspring, while male reproductive success depends more on their access to mates. The intensity of such aggression correlates with population density. Females are aggressive to immigrant females and even kill the newborn infants of community members. High rank correlates with high reproductive success, and high-ranking females win direct contests for food and gain preferential access to resource-rich sites. Although rates of aggression are low, females compete for space and access to food. To varying extents in different populations, females avoid direct competition by foraging alone or in small groups in distinct, but overlapping core areas within the community range to which they show high fidelity. Communities are fission–fusion societies in which individuals spend time alone or in fluid subgroups, whose size depends mostly on the abundance and distribution of food. Because males are philopatric and jointly defend the community range while most females transfer to new communities before breeding, adult females are typically surrounded by unrelated competitors. Chimpanzee communities contain multiple adult males, multiple adult females and their offspring. Therefore, their reproductive success depends critically on long-term access to high-quality food resources over a long lifespan. Female chimpanzees exhibit exceptionally slow rates of reproduction and raise their offspring without direct paternal care.
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