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’.
Kunga, the black-back in the Makumba group, showing his bravado (Copyright by Michelle Klailova)

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. TThe juvenile, Essekerende, eating Haumania, a staple food for western gorillas (Copyright by Angelique Todd)heir 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).  Silo and Etefi, 2 sub-adults in the Makumba group, playing (Copyright by Michelle Klailova)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.