Life History Patterns of Western Gorillas - Insights from Mbeli Bai
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).
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Side profiles of gorillas of different life-history classes. The two photos in each row present a typical example of the life-history classes used in this study (from top: infang, juvenile, subadult, blackback, young silverback and adult female/adult silverback Photos: Thomas Breuer |
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.
Developmental stages (life-history classes) in the life cycle of mountain gorillas (MG) and western gorillas (WG)
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.
Thomas Breuer
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
Dr. Thomas Breuer has been working in Central Africa for around a decade. Since 2002 he has been working for the Wildlife Conservation Society as the principal investigator of the Mbeli Bai Study in the Noubalé-Ndoki National Park, studying the social organization and behavior of western gorillas.