Food
Gorillas are largely herbivorous (plant-eating). Plant material contains cellulose, which is indigestible to many non-herbivorous animals. With regard to digestion, herbivorous animals that do not ruminate (re-chew their food as part of the digestive process) rely solely on the microbes (microscopic bacteria) living in their colon. The bacteria function to breakdown the indigestible plant cellulose and turn it into digestible carbohydrates through the fermentation process.
Food availability affects both diet and foraging behaviour of gorillas. High quality herbs that are easily digestible and rich in proteins and minerals are scarce and patchily distributed in outside swamp forest areas. Fruit is relatively widely available in their habitats and forms an important part of the diet of Western Lowland Gorillas. The availability of seasonal fruit appears to shape foraging and ranging patterns of gorillas (Remis, 1997). When fruit is abundant, it may constitute most of the diet.
High-quality herbs (rich in minerals and proteins contents) are eaten all year round, while low-quality herbs are eaten only when fruit is scarce. More leaves and woody vegetation are consumed during the dry season (January-March) when few fleshy fruits are available. In habitat where the leguminous tree Gilbertiodendron dewevrei is present, gorillas feed heavily on its seeds and can travel some distance during mass fruiting events (occurring at five year intervals) to congregate in stands of G. dewevrei (Blake & Fay, 1997). Insects are also part of their diet (termites and ants), although their relative importance is still undetermined (Tutin & Fernandez 1992; Remis, 1997; Deblauwe, 2003; 2006). Western gorillas travel farther when more fruit (and termites) are available in the forest and have shorter day ranges when they must rely on leaves and woody vegetation (Goldsmith, 1999).
Social Behaviour
Gorillas form harems. Reproductive groups of Western Gorillas almost always contain only one dominant silverback male plus three or four females and four or five offspring (Fay, 1989). Adult females in any group are mostly unrelated, and the social ties that exist between them are weak. In contrast to many other primates, it is the bond between each individual female and the silverback, rather than bonds between the females, that hold the group together. Upon reaching maturity, both males and females leave the natal group. The females usually join another group or a lone young adult male, whereas the males remain solitary until they can attract females and establish their own groups (Parnell, 2002).
Western Lowland Gorillas generally form stable cohesive groups. The takeover of a group during which another male from outside ousts the group’s silverback has never been reported, and a group splitting has been reported on only one occasion (Remis, 1997). However, western gorillas do not appear to be as cohesive on a daily basis as their eastern counterparts. In some groups, members spread out with distances of over 500m between them, other groups split up during the day and then reunite at the nest site. Recent studies at bais also suggest that around those particularly attractive locations, population dynamics might be more active than previously thought (Gatti et al, 2004), with frequent exchange of individuals between groups.
The very large groups sometimes observed among eastern gorillas have not been reported to occur in western gorillas. Group size appears to be influenced by the size of the foraging patches and fruit abundance. Western gorillas eat considerably more fruit than eastern gorillas, and this preference for clumped food resources may constrain their group size. Total group size ranges from two to 32 individuals with an average of four to six adults.
Distribution (current and historical)
The historical distribution was probably very close to the current distribution: the Western Lowland Gorilla inhabits the tropical forests of Cameroon, Central African Republic, Gabon, Congo, and Equatorial Guinea. It is also reported to be present in the Mayombe forests in extreme southwest DRC, with at least some groups said to migrate seasonally across the Angola (Cabinda) border, north of the Congo River (Redmond, 2006). The Congo/Oubangi River seems to be the eastern limit of the distribution; the Sanaga River represents the northern limits of the closed forest and concomitantly of the Western Lowland Gorillas. However, a small population exists North of the river in Ebo Forest (Morgan et al., 2003). The Atlantic Ocean represents the western boundary of the distribution and the southern edge of the Western Lowland Gorilla’s distribution is defined by forest-savannah.
The Western Lowland Gorilla is still a relatively widespread species, but if the recent decline continues, a notable reduction in their distribution is to be expected. Surveys conducted in the 1980s indicated that healthy populations existed in many areas remote from human settlements. However, despite the fact that western equatorial Africa has one of the lowest human population densities of any tropical forest area in the world, gorilla (and chimpanzee) populations in this region are, today, in dramatic decline. This is due to increased commercial hunting, the spread of logging, which alters forest structure and facilitates poaching, and outbreaks of Ebola. Figures in areas where studies have take place are frightening: the average annual rate of decline in Gabon between 1983–2000 (Walsh et al., 2003) was 4.7% and the high mortality (>80%) recorded in two known populations affected by Ebola (Bermejo et al., 2006; Caillaud et al., 2006) underline a critical state for conservation and the urgent need for protection.
Evaluation and evolution of populations
Accurate population estimates for gorillas are often difficult to establish, because their vast range has not yet been thoroughly surveyed. However the recent paper of Walsh et al (2003) used two countrywide surveys twenty years apart to show that roughly half of Gabon’s gorillas vanished between 1983-2000. A recent set of surveys encompassing much of Northern Congo showed that there are very high densities in Raphia swamps and that well protected logging concessions can also have medium densities (Stokes et al., 2008).
Previous estimates of Western Lowland Gorillas were based on habitat suitability (Harcourt 1996) but unfortunately in this region, even good habitat type does not necessarily mean that there are gorillas present. Hunting and Ebola have taken their toll and continue to do so. Numbers have been boosted by additional populations discovered in previously unsurveyed swamps in northern Congo, but it should be remembered that of the oft-quoted 125,000 gorillas in that area, at least 46,000 were already known about, in the Nouabale-Likouala landscape (Stokes et al 2008). Many of them live outside of existing protected areas, and their survival is essentially due to remoteness (from villages) of the areas recently surveyed.
The vast majority of Western Lowland Gorillas occur in Congo and Gabon, followed by southwest Cameroon. Equatorial Guinea still harbours some, but there is no country estimate. There is a population in the Dzanga-Sangha area of southwest Central African Republic, shared with the contiguous Nouabale-Ndoki area of Congo. Finally, gorilla numbers in the contiguous Cabinda province (Angola) and Mayombe (Bas-Fleuve region) are unknown but probably very low – perhaps a few dozen- as a result of the combined effects of habitat loss, fragmentation and poaching (Redmond 2006).
The Western Lowland Gorilla (Gorilla gorilla gorilla) is listed as Critically Endangered on the IUCN Red List (IUCN 2008, CR A4cde) even though the taxon occurs over a wide area with relatively low human population densities. The subpopulations generally occur at low densities and their distribution is patchy. It has been estimated that 80% of the population live outside protected area (Harcourt 1996).
Ranging behaviour
Western Lowland Gorilla groups travel within a home range averaging 5 to 30 km². Gorillas do not display territorial behaviour, and neighbouring groups often overlap ranges (Bermejo, 2004, Doran et al., 2004). The group usually favours a certain area within the home range but seems to follow a seasonal pattern depending upon the availability of ripening fruits and, at some sites, localised large open clearings (swamps and “bais”). Gorillas normally travel 0.5-3.0 km per day (Remis, 1997b, Doran et al., 2004). Populations feeding on high-energy foods that vary spatially and seasonally tend to have greater day ranges than those feeding on lower-quality but more consistently available foods. Larger groups travel greater distances in order to obtain sufficient food (Remis, 1997b). Human hunters and leopards (Panthera pardus) can also influence the movement patterns.
The annual home ranges (the areas used by a group over a year) of western gorillas are larger than those of mountain and eastern lowland gorillas and the home ranges of different groups overlap quite extensively. Wherever these home ranges are divided by international boundaries, the movement of gorillas back and forth across the border in search of seasonally available food plants is considered migratory under the terms of the Convention on Migratory Species. Transboundary populations of Western Lowland Gorillas occur between Gabon and Equatorial Guinea, Gabon and Cameroon, Gabon and Congo, Cameroon and Equatorial Guinea, Congo and the Central African Republic, Congo and Angola and DRC.
Republic of Congo:
Home to some of the largest known populations, including protected sites like Nouabalé-Ndoki National Park and Odzala-Kokoua National Park. These forests contain mixed-species stands of Gilbertiodendron, Marantaceae understory, and high fruit diversity, supporting year-round foraging.
Gabon:
Gorillas are distributed across Loango, Moukalaba-Doudou, and Ivindo National Parks. The country has positioned itself as a regional leader in eco-conservation, with forest management zones incorporating wildlife corridors—seasonal flooding in swampy basins forces periodic migration across elevated terrain.
Cameroon:
Populations are found in Lobéké National Park and the surrounding TRIDOM transboundary zones. Illegal logging and expanding agriculture have restricted natural ranges. Nevertheless, dense evergreen forest patches still host viable family groups.
Central African Republic:
Dzanga-Sangha Special Reserve supports a long-term habituation program for research and tourism. The gorillas here frequently occupy terra firme forest near mineral-rich clearings known as “bais,” which attract multiple species simultaneously.
Equatorial Guinea:
Fewer studies have been conducted on gorilla populations here. Monte Alén National Park holds potential habitat, although deforestation and poor monitoring infrastructure hinder systematic surveys.
Angola (Cabinda enclave):
This is the westernmost limit of their range. Verified presence remains limited, and political instability has delayed comprehensive ecological assessments.
Western Lowland Gorillas prefer primary tropical rainforest but also utilize secondary forest, swamp forest, and seasonally inundated floodplains.
They avoid open savannah and montane zones.
Elevational range typically lies between sea level and 1,300 meters, though most sightings occur below 800 meters.
Habitat use shifts seasonally in response to fruiting patterns.
During dry seasons, gorillas tend to concentrate near swamp margins and bais due to higher fruit availability and mineral deposits.
These behavioral patterns make seasonal tracking essential for accurate ecological assessments.
Several major rivers, including the Congo, Sangha, and Ogooué, act as natural barriers to gene flow, reinforcing the need for conservation strategies that incorporate regional connectivity.
Threats to Survival
Habitat Loss and Fragmentation
Logging, road construction, and agricultural expansion have significantly degraded the Western Lowland Gorilla’s forest environment. In the Congo Basin, large-scale industrial timber concessions continue to fragment habitat, reducing home-range continuity and limiting gene flow between gorilla groups.
Even certified sustainable logging operations have introduced permanent human presence, noise, and edge effects that disrupt feeding and nesting. Gorilla avoidance of disturbed areas can lead to population isolation and heightened vulnerability to disease.
Swamp forests, though once viewed as natural refugia, are now targeted for oil palm, rubber, and cocoa plantations. The resulting conversion displaces gorilla groups and eliminates essential fruit trees.
Poaching and Illegal Trade
Hunting for bushmeat remains the most immediate anthropogenic threat. Although protected under national and international law (CITES Appendix I), the Western Lowland Gorilla is routinely killed for meat, with infants sometimes trafficked into the exotic pet market.
Poaching is not always for subsistence. In logging zones, commercial hunters use newly built roads to access deep forest areas previously inaccessible. The carcasses are then transported to urban markets such as Brazzaville, Bangui, and Yaoundé.
Enforcement remains weak. Rangers often lack equipment, salary, or jurisdiction to police vast concession zones. Arrests rarely result in prosecution, and fines seldom match the severity of the offense.
Infectious Disease
Ebola virus disease has caused some of the most catastrophic die-offs ever recorded among great apes. Outbreaks in northern Congo and Gabon between 2002 and 2004 reduced local gorilla populations by up to 95 percent. Field studies suggest the virus spreads via contact with infected carcasses and contaminated vegetation.
The International Union for Conservation of Nature (IUCN) estimates that Ebola alone could account for a projected 80 percent decline over three generations unless vaccine strategies or mitigation are adopted.
Other infectious diseases, including respiratory viruses and intestinal parasites, have also been documented, especially in regions with high human interface, including ecotourism zones. Zoonotic transmission remains a persistent concern.
Human Encroachment and Conflict
Conflict zones and informal settlements bordering parks and reserves bring unregulated human activity into contact with gorilla populations.
Charcoal production, illegal mining, and bushfire clearance all contribute to further habitat degradation.
Armed conflict in the Central African Republic and parts of northern Congo has disrupted conservation infrastructure. In some areas, researchers and rangers have been forced to evacuate, leaving gorilla groups unmonitored for extended periods.
Conservation areas that lack clear legal boundaries or benefit-sharing frameworks remain hotspots for encroachment.
Displaced communities, lacking alternatives, often turn to forest resources as a survival strategy.