History of Western Lowland Gorilla Research

Up until the mid-60’s, most information on western gorillas came from expeditions led by zoos and museums to collect live or dead specimens from the wild. In 1964 the first studies on wild western gorillas were carried out by Jones and Sabater Pi in Equatorial Guinea, albeit in disturbed and degraded habitat. In the mid-1980’s Tutin and Fernandez, later joined by Rogers and Williamson, began their study of western gorillas at Lopé in Gabon. Although never quite successful in their attempts to habituate gorillas, this long-term study provided a multitude of baseline ecological data and clarified gorilla distribution across Gabon. At the same time Richard Carroll and Michael Fay undertook the first rigorous surveys of western gorillas in south-western CAR. The latter two confirmed an abundance of gorillas in the Dzanga-Sangha region, as well as exceptionally high densities of other wildlife. With the aid of WWF, the results of these surveys greatly contributed to the establishment of the Dzanga-Sangha Special Dense Forest Reserve including the Dzanga-Ndoki National Park in 1990.

Soon after their initial survey work, both Carroll and Fay began long-term ecological research on the gorillas in the region (at Bai Hokou and Ndakan respectively). These studies were followed by a number of others in the 90’s (in CAR: Remis, Goldsmith, Blom, and Cipolletta at Bai Hokou; on the CAR/ Congo border: Doran at Mondika; in Congo: Kuroda, Mitani, and Nishihara at Ndoki, Olejniczak and Parnell at Mbeli bai, Bermejo at Lossi, and Magliocca at Maya Nord bai). These studies provided more detailed information on western lowland gorilla group size and composition, diet, tree use, nesting, foraging, and ranging behaviour. The onset of bai studies (Mbeli and Maya Nord) in particular, finally allowed direct observations and the collection of demographic data. However, in reality gorillas spend little overall time in bais and the lack of habituated groups still restricted data collection in the forest to indirect signs and chance encounters. In this respect, Bermejo’s study proved to be a breakthrough, the Lossi gorillas allowing prolonged direct observations in the forest, apparently due to their infrequent contact with humans in the past.

Then in 2001, under the direction of Chloe Cipolletta and funded by WWF, the Primate Habituation Programme at Bai Hokou made Dzanga-Sangha home to the first ever truly habituated group of western gorillas, the Munye group. Since the habituation of the Munye group, one other site, Mondika (CAR/Congo) has also successfully habituated gorillas, and several other gorilla research sites in Gabon and Cameroon are attempting habituation. Thus after over 4 decades of study, researchers finally have access to habituated western gorillas to detail their social lives and ecological influences on their behaviour. However now time may be the issue: the number of habituated groups across sites remains low, while the threat of Ebola remains imminent.

Western Lowland Gorilla Morphology

Gorillas, split into western and eastern species, are the largest extant primate and one of our closest living relatives. Western gorillas are physically distinguishable from those in the east by their shorter hair and typically reddish-brown crest. Individuals can vary from brown to blackish in colour, with young individuals exhibiting a white tuft. Adult males (up to 200 kilos), are twice the size of females and exhibit the characteristic ‘silver back’ on maturity and a pronounced head resulting from a bony structure known as the sagittal crest. Individual gorillas can be distinguished through their nose-prints: the shape and size of their nostrils, alongside associated lines and markings.

Western Lowland Gorilla Biology

Western gorillas live in family groups consisting of a silverback male and several females and offspring. Average group size is approximately 9 individuals although much larger groups of around 30 individuals have been known to exist. Gestation is close to that of humans with females giving birth every 4-6 years. Newborns are completely dependant on their mother and are initially carried on the female’s underside, and later on her back. Young have been observed suckling up to 4 years of age. However, by this time, they are frequently seen travelling alongside the silverback male, resulting in the gorilla-equivalent of a ‘kids-club’.

Juveniles increasingly gain confidence until sub-adulthood, when both male and female offspring disperse. Sub-adult black-back males leave the group and roam as solitary males, for up to and over 5 years, before establishing their own family group. Females never travel as lone individuals. Maturing females are attracted away from their natal group during interactions, either by solitary males or by group silverbacks. During their lifetime, females may make additional transfers to other silverbacks.

The stability of western lowland gorilla groups and the length of a silverback’s tenure are yet to be determined. Based on long term monitoring of groups and approximate age estimates, silverback males can hold tenure for at least 10 years. Groups disintegrate on the death of the silverback, leaving the females and young vulnerable to attack by leopards, or other silverbacks. As groups with more than one silverback are rarely observed, the number of solitary males in the environment, and hence competition for females, must be high. Gorillas in captivity have been known to live for over 50 years, but considering the degree of male-male competition, it is doubtful whether this is true in the wild.

Gorillas are active during daylight hours (from approximately 6:00 to 18:00 in equatorial Africa). They exploit a number of habitat types within the tropical forest mosaic, including primary, secondary, monodominant (Gilbertiodendron spp.), and herb (Maranataceae) forest, and forest clearings (or “bais”). For the most part western gorillas are terrestrial but both males and females also spend a considerable amount of time in trees. They dedicate up to 70% of their time to feeding. Due to the abundance of food remains left on gorilla trail, diet is the most studied aspect of their biology. Western gorillas have been found to be eclectic yet highly selective feeders. Unlike their mountain cousins, which mostly feed on abundant herbs, western gorillas consume up to 180 species of plants, as well as a several species of insects. Their staple diet is made up of the shoots and pith of a few common herbs. Depending on the season, these staples are supplemented by a variety of other items including leaves, roots, bark, and insects, as well as seasonally available fruits. As fruit is highly preferred but rare in the environment, western gorillas are thought to experience higher levels of competition compared to mountain gorillas, which in turn influences their socio-ecology. Direct conflicts over food appear more evident, and in their search for favoured fruits, western gorillas spend more time travelling (approximately 10% of their time), range further (up to 4 kilometres a day), and show considerably larger home ranges than mountain gorillas (totalling up to and over 30 km², PHP long-term data). 

When not found eating, western gorillas spend the remainder of their day resting, allowing time for the young to engage in extended play sessions. At night individuals either sleep directly on the ground, or in constructed nests (on the ground or in trees). In order to protect his family, the silverback always sleeps on the ground. During the wet season, individuals are more likely to make constructed nests, sometimes making an additional nest during the night, probably to protect themselves from the wet soil. These seasonal variations in nest-building create a problem when estimating group size from nest sites and well as confounding census data. Our current knowledge on western gorilla distribution and abundance today are based on nests counts, however, if gorillas decide not to make a nest or indeed make 2 nests, these density estimates are likely to be over-estimates. Researchers are now investigating new and more precise methodologies for counting gorillas. These are vital in order to detect future population trends and threats (such as Ebola), providing the conservation community essential information towards the continued protection of this critically endangered species.