The Mbeli Bai Gorillas - Social Structure and Behaviour
The first pilot study at Mbeli Bai, a 10 hectare swampy clearing in the
southwest of the Nouabalé-Ndoki National Park, was conducted in
1993. A further pilot study in 1994 by Stephen Blake confirmed that the
site attracted unusually large numbers of gorillas. Full-time monitoring
and study at Mbeli Bai began in February 1995, under the direction of
Claudia Olejniczak. At the beginning of 1997, Richard Parnell became the
principal investigator for the study.
Until the commencement of the study, virtually nothing was known regarding
the intricacies of western lowland gorilla social organization. Studies
of mountain gorilla social structure have been detailed and wide ranging.
By comparison, studies on western lowland gorillas have mostly described
aspects of ecology such as feeding and ranging that can be studied using
indirect evidence such as feeding trails, nutritional analysis and nest
counts. However, few advances have been made in addressing questions regarding
social organization and interaction. Descriptions of social structure
have relied heavily on secondary trail evidence and low sample sizes of
known groups. Hypotheses regarding group dynamics, and in particular,
the possibility that gorillas exhibit a fission-fusion organization not
dissimilar to that observed in chimpanzees have been proposed but these
also rely too heavily on night nest counts, a methodology that has been
shown to be lacking in precision.
These "indirect" methodologies have been utilized primarily
due to difficulties in habituating western lowland gorillas in their normal
forest habitat. Even where the habituation of gorillas in the forest has
been partially successful, only one or two groups have been studied. Mbeli
Bai allows researchers visibility of complete social groups of gorillas.
We are currently able to identify all individuals from 13 groups plus
8 solitary silverbacks; a study population of over 100 animals. This allows
us to follow the dynamic shifts in group composition for a large number
of groups, all of which are habituated to our presence at an observation
tower at the bai edge.
The preliminary results show that groups with 2 silverbacks are rare and
to date, no evidence has been collected to support a fission-fusion organization.
Our findings indicate a much wider range of possible group compositions
than have so far been described for the subspecies. In the context of
our demographic monitoring, we have so far recorded 18 female transfers,
5 male emigrations, the birth of 16 infants and the death of 2 silverbacks,
with the consequences for their groups. Several adult females in the study
population have given birth to their second offspring since the start
of the study, while some of those whose offspring died in early infancy
have now given birth to their third infants, allowing us to monitor infant
development and reproductive parameters. The number of individuals and
groups known to the study has already enabled us to compile the most complete
demographic data set for the sub-species yet extant; a resource that becomes
increasingly important with each year of unbroken monitoring.
While attempts have been made to describe western gorilla social structure,
data on individual gorilla social interactions are absent, with the exception
of isolated anecdotal evidence. In previous studies examples of intra-group
interaction have occasionally been witnessed, but such opportunities are
rare and likely to be limited in their context, by poor observation conditions
and an incomplete knowledge of the individuals involved. At Mbeli Bai,
we often witness two or more groups using the clearing simultaneously
and intergroup interactions are not uncommon. A significant characteristic
of these encounters is the high degree of tolerance shown by group silverbacks.
When agonistic encounters do occur, they are frequently initiated by,
and confined to other group members such as black-backs, subadults and
juveniles. By recording all aspects of these encounters, whether agonistic
or affinitive, we are beginning to uncover layers of complexity and social
awareness that are all too often hidden using more traditional study methods.
Our data so far hint at the ways in which young animals gain the social
skills required of them later in life, or even create "friendships"
or animosities that may later affect the likelihood of female transfer
or of tolerance between group silverbacks.
Data are also taken on the form of these interactions, which will enrich
wild gorilla ethograms and provide information in areas such as gestural
and vocal communication as well as positional aspects of interaction.
Most encounters recorded are dyadic in nature, but analysis will also
be performed on encounters that affect more than one member of each group.
Observations are also analyzed temporally to understand to what extent
gorillas of various ages observe a form of "etiquette" in their
interactions with each other. Are there set rules governing the behaviour
of different age and sex classes, or does the form and outcome of interaction
depend more on past experiences or the proximity of a silverback from
either group?
So far, observations are taken only at the bai and no attempt has been
made to habituate groups to being followed into the forest, though it
is expected that this may become the logical next step with one or more
of the groups that visit the bai. The philosophy of the study is that,
wherever possible, long-term research of vulnerable species such as gorillas
should not be divorced from concerns about their conservation status.
Though a full-time research project, this philosophy has led us to undertake
conservation education efforts in the Upper Sangha region, through the
creation and distribution of a conservation newspaper for high-school
age children and the setting up of a children's conservation club. A pilot
launch of this project at the local school has generated considerable
interest throughout the region, and we hope to expand these activities
considerably in the future.
Richard Parnell
Richard Parnell began his research at Mbeli Bai, Nouabalé-Ndoki National Park, in 1996. Between 1989 and 1995, he studied gorillas in the Lopé Reserve, Gabon.
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 A gorilla group in the bai |