Mating Systems
(Polygyny)
Image courtesy of Paul Massicot
Females start to breed around the age of 9 and they menstruate about every 28 days. Gorillas can mate in any season. Females have a gestation period of 8 to 9 months and will typically produce only 3 to 6 offspring during her lifetime. After reproduction, a female will not start ovulating for several years. This shows that they have a very slow reproduction rate. Males do not mature sexually until their teens. Mating behavior is started by the female during estrous. Females will approach the male indicating that they are ready to reproduce. One of the main goals of the male is to breed with many females to ensure good reproductive success. Due to the slow reproduction rates of the females and since nearly half of the infants die before reaching maturity because of disease and infanticide, the reproductive success of the males is relatively low (http://www.everwonder.com/david/gorillas/reproduction /index.html).
Mountain gorillas are polygnous and have been classified as a one-male mating system (Robbins, 1999). There are two different types of polygyny, female defense polygyny and resource defense polygyny. Mountain gorillas exhibit female defense polygyny, which is when females live in social groups but not in a range that can be defended by a male. The males will instead follow the female groups and defend them. The relationships within a group are not based on resource defense since gorillas’ home ranges overlap extensively and food resources are in abundance. Polygyny has evolved in mountain gorillas since there are so few mature males in each group. Polygyny is more beneficial for males since they can gain access to more than one female, which will lead to a higher reproductive success. However, it does give females the chance to mate with high quality males. Polygyny also heightens competition between the females since they are vying for time with the dominant male. There is also high competition between males for reproductive opportunities. According to a study by West-Eberhard in 1981 (as cited in Watts, 1985), a gorilla’s relative competitive success will usually influence their reproductive success. The main reasons that females will accept polygyny is for protection of their young against other males since infants are vulnerable to infanticide and there are very few males compared to females.
Copulations among mountain gorillas was seen to occur, although it was not very common. Harcourt and Stewart (as cited in Maple & Hoff, 1982) observed 98 copulations during 2.5 years of research in the Virunga Volcanoes and all of them were initiated by the females. Copulations generally took place during the travel-feed periods of the day. Harcourt and Stewart (as cited in Maple & Hoff, 1982), argued that the reproductive success of males depended on the maintenance of exclusive rights to mature females and the only way to achieve this was to form a permanent bond with every female in the group.