Western Gorilla Life History
Gorillas are known among the great apes for their fast life history patterns. In zoos it is common for female gorillas to give birth before reaching 10 years. Our knowledge of the physical maturation of wild gorillas is mainly based on studies of mountain gorillas carried out at the Karisoke research station; gorillas in that population show very similar life history milestones compared to captive western gorillas. This is in strong contrast to chimpanzees or bonobos in which captive animals mature much faster than those in the wild, because food provisioning under captive conditions accelerates physical maturation.
Maturation rates and life-history parameters are seen as evolutionary adaptations to different ecological and social conditions. In environments with poorer and unstable or unpredictable food availability, maturation is expected to take longer. This strategy helps to reduce the risks of starvation (due to intraspecific feeding competition) by spreading the metabolic needs for juvenile growth. Under this hypothesis more folivorous animals are expected to show rapid growth rates in the earlier stages of ontogeny and cease growth earlier than nonfolivorous/frugivorous species.
Gorillas are an interesting genus to investigate in order to ascertain whether the more frugivorous western gorillas mature more slowly because they occur in very different habitats: the seasonal lowland forest with many fruit trees at one extreme and the high-altitude montane rainforest with its dense herb availability at the other. So far the long-term data required to investigate whether western gorillas have a different maturation pattern compared to mountain gorillas has been lacking.
Recent analysis from the long-term data at Mbeli Bai indicates that indeed western gorillas are weaned at a later age compared with mountain gorillas and indicate a slower physical maturation for immatures. Western gorillas in the Mbeli Bai population were weaned (last time seen suckling) at an average age of 4 years and 9 months, which is 16 months later than in mountain gorillas. One female with known age, born during the study in April 1995, had her first baby at an age of 11 years and 4 months. Another female that we have continuously monitored since her birth is currently 11 years and 1 month old (October 2009) and has not yet given birth. Hence we assume that age at first parturition is later than that in mountain gorillas (average age at first parturition: 10 years 3 months; range 8 years 8 months to 12 years 10 months). We also assigned photographs to common life history classes and these results further support the finding that western gorillas in our population and probably elsewhere have slower maturation compared to mountain gorillas (but see Todd 2008 in Gorilla Journal 36 for a short interbirth interval of 3 years 10 months).
For example, we consider males to be fully grown at an age of 18 years compared to 15 years (or even earlier) in mountain gorillas. The development of the first secondary sexual characteristics (e.g. longer arm hairs of males) is not obvious before the age of 11 years at Mbeli Bai. We have therefore proposed new age boundaries for life history classes in western gorillas, which can be used and tested at other western gorilla research sites.
The slower life history and longer period of dependency of immature western gorillas could have major consequences for other aspects of western gorilla biology. For example, it might have effects on the likelihood of multi-male (kin) groups in western gorillas. If male tenure length is shorter than a male's age to maturity it is unlikely that father-son multimale groups can form. It might also impact infant mortality patterns and population growth rates that will affect recovery from population crashes of this critically endangered species.
Infant mortality to weaning age at Mbeli Bai is higher than 50%. Our study emphasizes the importance of long-term studies in providing accurate baseline demographic and life-history data of undisturbed primate populations in assessing the vulnerability of populations to their threats.
I would like to thank the Ministère de l'Économie Forestière for permission to work in the Nouabalé-Ndoki National Park and WCS's Congo Program for crucial logistical and administrative support. Special thanks are due to numerous research assistants who contributed to the demographic data at Mbeli Bai. Financial support for the Mbeli Bai Study is provided by Brevard Zoo, The Columbus Zoo and Aquarium, Cincinnati Zoo and Botanical Garden, Cleveland Metropark Zoo, Disney Worldwide Conservation Fund, Margot Marsh Biodiversity Fund, Houston Zoo, Jacksonville Zoo, Knoxville Zoo, Little Rock Zoo, National Geographic Society, Santa Barbara Zoo, Sea World & Busch Gardens Conservation Fund, Toronto Zoo, USFWS, WCS and Woodland Park Zoo.
The Results of this study and the pictures were originally published in the following article: Breuer, T., Breuer-Ndoundou Hockemba, M., Olejniczak, C., Parnell, R. J. & Stokes, E. J. (2009): Physical maturation, life-history classes and age estimates of free-ranging western gorillas - insights from Mbeli Bai, Republic of Congo. American Journal of Primatology 71, 106-119