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Gorilla Facts

Gorillas are primates (a group of mammals that includes monkeys, apes and humans), and more specifically great apes.  Adult males can weigh between 180 and 250kg depending on the species; which are distinguishable, not only by their range and habitat, but their skull shape, fur length and colouring.

How are gorillas classified?

Living things can be organised into different groups.  Species that are alike are grouped together.  This is called classification. Gorillas are the largest species of great ape and are classified as follows:

Class:                 MammalsOrder:                 PrimatesFamily:                Hominidae (great apes) Species:             Gorilla gorilla (Western gorillas),  Gorilla berengei (Eastern gorillas)

Each of these two gorilla species have two subspecies:

Western gorillas:

Western lowland gorilla G.g.gorilla © Ian Redmond
Western lowland gorilla G.g.gorilla © Ian Redmond
Cross River gorilla G.g.diehli © Julie Langford
Cross River gorilla G.g.diehli © Julie Langford

Eastern gorillas:

Eastern lowland gorilla, G.b. graueri  © Ian Redmond
Eastern lowland gorilla, G.b. graueri © Ian Redmond
Mountain gorilla, G. b. berengei  © Ian Redmond
Mountain gorilla, G. b. berengei © Ian Redmond

Geographic distribution and population size

Gorillas are found in two regions of Africa. The eastern gorilla occurs on the tri-national boundary between DRC*, Rwanda and Uganda. They are found in the Virunga Massif which lies between Rwanda, Uganda and DRC and the Bwindi Impenetrable Forest in Uganda and the forests of eastern DRC. The western lowland gorilla is found in southern Cameroon, Central African Republic, the mainland of Equatorial Guinea, Cabinda (Angola), Congo and DRC. The Cross River gorilla is only found on the Nigeria Cameroon border area.

About 480 mountain gorillas remain in the Virunga Mountains (including within Virunga National Park), with an additional 300 occurring in the Bwindi Impenetrable Forest. Numbers of the eastern lowland gorilla (also known as Grauer’s gorilla) have fallen dramatically over the last two decades and now it is thought only a few thousand remain in eastern DRC. The western lowland gorillas was recently estimated at 100,000 – 150,000 individuals, but is now believed to have declined significantly. The Cross River gorilla number only 200-300 individuals making it the rarest of the gorilla subspecies.

Habitat

Gorillas are restricted to closed forest including lowland and swamp forest, sub-montane and montane forest. Mountain gorillas range at the highest elevation between 2,000-4,000 metres. Eastern lowland gorillas range between 600-4,000 metres.  Cross River gorillas are largely restricted to sub-montane forest and range to over 2,000 metres, while the western lowland gorilla occurs in more lowland areas, up to about 600 metres.

Diet

Gorillas, like chimpanzees, feed on a variety of foods including fruit, leaves, herbs, bark, roots, seeds and invertebrates. Diversity in the gorilla diet and fruit availability decreases with increasing altitude. Subsequently, western gorillas eat more fruit than eastern gorillas, which are more herbivorous. Mountain gorillas in particular eat a lot of herbaceous vegetation.

Ecology and Behaviour

A typical gorilla group is composed of one mature male and a number of females and their young. Gorilla group size varies widely however, and groups composed of as many as 40-50 individuals have been known. Multi-male groups occur but they are rare among western lowland gorillas. When they mature, males and females disperse from the group they were born in. Males may go through a non-breeding bachelor period either solitarily or in all-male groups. 

One adult male dominates, while among themselves females are egalitarian, neither forming hierarchies nor coalitions. Friendly behaviour between females occurs among close relatives who keep in close proximity and groom each other. Relationships between males are competitive and agonistic and therefore the strongest social bonds are between males and females. Adult males protect females and their offspring from predators and stranger males who might attack and kill young.

Gorillas are non-territorial and group ranges can overlap extensively. Although groups avoid each other within overlapping areas encounters between groups, when they occur, are largely aggressive. Western lowland gorilla groups are more social, tolerating other groups peacefully at natural clearings in the forest called bais.

Gorillas have a range of calls: the belch vocalization is a soft contact call used when individuals are out of visual contact; when excited, aroused individuals will bark, hoot, roar scream and beat their chests; male displays which use a combination of these calls also involve charging, strutting and throwing vegetation.  Individuals also laugh when they play.

Gestation is about 8.5 months in length and infancy lasts for about four years. Females reach maturity at six to seven and a half years of age and typically start reproducing at about 10-11 years. As with chimps male gorillas reach maturity later, at about fifteen years. Eastern gorilla adult males over the age of twelve years have a grey-silver saddle and are known as silver-backs.

Threats

The western gorilla is largely threatened by the bushmeat trade. In addition, since the early 1990s, Ebola, with its 95% mortality rate has negatively impacted remote populations. Since 2000, it has been estimated that at least a third of all western lowland gorillas occurring in protected areas have been killed by Ebola. Because the global population of Cross River gorilla is so small and fragmented, it is particularly vulnerable to threats.

*Democratic Republic of Congo

 

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