Long-term Studies for the Conservation of Western Gorillas - Mbeli Bai

Large mammals, including western gorillas, chimpanzees and forest elephants, are important flagship species because it is often the largest and most charismatic threatened species that raise public support for conservation. Additionally they can play substantial roles in rain forest ecology through processes such as seed dispersal. Of course, large mammals exhibit slow life histories making data acquisition time-consuming, and in addition they are extremely difficult to study in the rain forest because of the dense vegetation, so their populations have to be estimated by indirect methods, such as dung and nest counts.
When conservationists in the 1990s were looking at maps of western Equatorial Africa, they quickly realized that in the decades to follow, increased logging activities would cause major changes in forest cover in the area. Furthermore, increases in logging activities are often accompanied by increases in illegal activities, the most notable of these being bushmeat hunting. More recently, western gorilla populations in Gabon and the Republic of Congo have experienced tremendous declines due to Ebola outbreaks. This year, western gorillas were classified as critically endangered by the IUCN and it is therefore necessary, now more than ever, to assess their vulnerability to these threats.
Currently there are less than a handful of habituated western gorilla groups, and it has taken years to get to the stage where gorillas can be followed and observed daily. While data from these habituated groups can provide us detailed information on the daily activities, ranging patterns and social behaviours of western gorillas, there is still a lack of knowledge on species-typical group dynamics, life-history patterns (e.g. mortality) and other important demographic parameters.
Luckily, associated with the increased conservation activities in western Equatorial Africa, was the discovery of forest clearings, called "bais" in the local languages. Bais are focal points in the rain forest, attracting large numbers of many different mammals because of their mineral rich soils, plants and clay that appear to be of particular nutritional importance to them. It has already been shown that forest buffaloes show clumped distribution around bais, and that bais are located in the middle of the home ranges of bongo (a large forest antelope). Large numbers of elephants and gorillas come to these clearings, and their consequent visibility means that individuals can be identified and tracked over time. Bai studies are advantageous as they allow for the collection of demographic data on many different groups that make regular visits to the clearing.
The swampy Mbeli Bai in the south-west of the Nouabalé-Ndoki National Park is the largest of several forest clearings in the region. This park suffers little from human disturbance, and therefore represents an important stronghold for western gorillas and other large endangered forest mammals. Together with the adjoining Dzanga-Ndoki National Park in the Central African Republic, and the Lobéké National Park in Cameroon, it forms the core protected zone of the Sangha Trinational Conservation Landscape, which harbours one of the largest remaining populations of great apes in Africa. In addition to western gorillas and forest elephants, Mbeli Bai is visited by sitatungas, forest buffaloes, black-and-white colobus monkeys, and two species of otter.
In 1993 and 1994 my colleagues from the Wildlife Conservation Society (WCS) conducted pilot studies at Mbeli Bai with the aim of finding out if western gorillas make regular visits, and if they can be individually identified. In February 1995 monitoring of the gorilla population started with the aim of providing detailed data on western gorillas' social organization, life-history patterns and population dynamics.
Working at a forest clearing is very different to following a habituated group in the forest. Our research is very passive and consists principally of waiting for the animals to come to the clearing where we can observe them with spotting telescopes. Our research demands a huge amount of patience; after hours (or even sometimes days) without gorillas, two different groups may enter the bai simultaneously, demanding our full attention and vigilance for data collection. Generally at least two observers sit on the 9 m-high platform at the edge of the forest clearing during all daylight hours and it generally takes new observers at least 3 months to learn the identity of the gorillas - over the course of a year, the bai is visited by about 130 gorillas belonging to 14 social units, and 13 solitary silverbacks.
We do not aim to follow the groups into the forest, as we want to guarantee that they have an absolutely undisturbed life. Only on rare occasions do we enter into the swampy clearing to get gorilla dung samples; these are analyzed for parasite load by the Field Veterinary Program of WCS. Additionally the Mbeli Bai Study is contributing towards a great ape monitoring and surveillance program in the Nouabalé-Ndoki National Park through visual health observations and parasitological analysis. Genetic analysis of the gorillas is done at the Max Planck Institute for Evolutionary Anthropology.
We are present at the bai for approximately 10 hours every day, which provides us with detailed data to further our understanding of the ecological and social factors that determine the gorillas' bai visiting patterns. This research is complemented by monthly monitoring of more than 400 fruiting trees in the study site (including the Nauclea trees around the bai that attract many gorillas in September/October of each year). Analyses of the bai visitation and grouping patterns of other large mammals, especially elephants, provide important information on the role of bais in determining large-scale associations and population structure in large forest mammals.
Because of the differences in their environment, such as higher density and diversity of fruit trees and more patchily distributed herbaceous vegetation, western gorilla ecology and behaviour is expected to differ from that of their counterparts in the Virungas. Indeed, studies at other sites, using indirect methods or following habituated groups, have demonstrated that western gorillas exhibit a more frugivorous diet, longer daily path lengths, increased intergroup encounters and larger group spread.
In the past years our picture of western gorilla social structure and socioecology has changed tremendously as a result of the insights we have gained from our observations in these forest clearings, and results from Mbeli Bai have contributed substantially to this new understanding.
Currently, after 12.5 years of almost continuous monitoring, we have a much clearer picture of western gorillas and can compare our knowledge with the long-term results on mountain gorillas from the Karisoke research centre in the Virunga Volcanoes (Rwanda). We have been able to show that, while up to 50% of groups are multi-male in mountain gorillas, western gorilla groups contain almost exclusively only one fully adult silverback; a large proportion of adult males are solitary, and this has wide-ranging effects on the variability in male reproductive success.
As in mountain gorillas, female western gorillas show both natal and secondary transfer and also involuntary transfer. Involuntary transfer happens after group disintegration after the presumed death of the harem holder. During such occasions we have observed females transfer with their unweaned offspring. Although we have not yet made direct observations of infanticide we witnessed two infanticide attempts by silverbacks of other units, and many females were seen without their unweaned offspring after they had transferred to other groups.
Increased encounter rates among social units (a consequence of the high population density, longer day ranges and potentially greater home range overlap), and the presence of many solitary silverbacks in a population, can also affect the nature of interactions between silverbacks. It could lead to increased levels of male-male aggression, but also to a more peaceful pattern of interaction because mainly familiar males could reside in a population - a scenario known as a male-network structure. Furthermore, because of the absence of multi-male groups, it is not surprising to find higher rates of group disintegrations for western gorillas.
It has been argued that the less stable nature of food availability and the increased predation risk in western gorillas' environment, and their lesser degree of folivory, should have direct effects on mortality and lead to slower physical maturation (for example affecting the age when western gorillas become adult). Currently we are investigating if this prediction holds true for the Mbeli Bai gorilla population - we could already confirm at our site that western gorillas are weaned at a later age than mountain gorillas. This information is crucial for scenarios of population recovery from the threats listed above, because higher mortality or lower birth rates would lead to lower annual growth rates. This emphasizes the importance of long-term studies to provide baseline demographic and life history data of undisturbed primate populations.
The Mbeli Bai Study has also reported many spectacular behavioural observations such as twin births (n = 3), silverback splash displays, and the first observation of tool use in free-ranging gorillas.
In addition to baseline population data, the Mbeli Bai Study has been expanding its research into the surrounding forest and has conducted a study to determine the ecological factors that potentially affect western gorilla distribution and density.
In addition to our conservation-oriented research, we also train Congolese research assistants as well as educators to promote wildlife conservation and the Nouabalé-Ndoki National Park - most effectively through our conservation education program "Club Ebobo" which we have been running since 1998. Our study at the forest clearing offers many local children the opportunity to see the mammals, which would not be possible in the dense forest.
A permanent research presence at the bai provides an effective deterrent to poaching and an important early-warning system for population perturbations in what is a highly strategic location. Mbeli Bai was a major elephant poaching area prior to the creation of the Nouabalé-Ndoki National Park in 1993; since the creation of the park and the initiation of the Mbeli Bai Study that followed, poaching levels in and around the bai have declined to zero.
In conclusion, we encourage other scientists and conservation managers to consider the valuable and time-effective conservation-related research that can be conducted at forest clearings, as well as the positive influence having a presence at these clearings can have on conservation activities.

Thomas Breuer

Our sincere thanks go to the Ministère de l'Économie Forestière et de l'Environnement for permission to work in the Nouabalé-Ndoki National Park, and the staff of WCS's Congo Program for crucial logistical and administrative support. I am particularly grateful for my predecessors Claudia Olejniczak, Richard J. Parnell, and Emma J. Stokes for their contribution to this long-term project. Many different research assistants have helped in collecting data at Mbeli Bai and their help is highly appreciated. The long-term continuation of the Mbeli Bai Study would not have been possible without the continuous support provided by our long-term supporters, notably the Columbus Zoo and Aquarium, Cincinnati Zoo and Botanical Garden, Sea World & Busch Gardens Conservation Fund, Toronto Zoo, WCS and Woodland Park Zoo. I am currently writing up my data to obtain a PhD and I am supported by the Max Planck Society.

Thomas Breuer has been working in Central Africa for around a decade. Since 2002 he has been working for WCS as the principal investigator of the Mbeli Bai Study. He has also been leading the local conservation education program Club Ebobo in the villages surrounding the park.

Western gorilla overview

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