Mountain Gorillas in Rwanda

Mountain gorillas are only found in Central Africa, on the border between the eastern Democratic Republic of Congo (DRC), north-west Rwanda and south-west Uganda. They are confined to four national parks, separated into two forest blocks no more than 45 kilometres apart and comprising approximately 590 sq km of afromontane and medium altitude forest. One population of mountain gorillas inhabits the Bwindi Impenetrable National Park in Uganda. The 2002 census recorded between 310-315 individuals. The second population of mountain gorillas is found in the habitat shared by Mgahinga Gorilla National Park (Uganda), Volcanoes National Park (Rwanda) and Virunga National Park -Southern Sector (DRC). The Virunga population numbers at least 358 individuals and has grown by 11% in the past 12 years.

Taxonomy

Current taxonomy of gorillas (IUCN Red Data Book 2000) places the mountain gorilla (Gorilla beringei beringei) as a distinct subspecies of the eastern gorilla species, along with the eastern lowland gorilla (Gorilla beringei graueri). More data are needed to determine whether the Bwi, di gorilla should be considered a separate subspecies. At present, much of the scientific and conservation community still views the Bwindi gorillas – together with their Virunga counterparts – as mountain gorillas (Gorilla beringei beringei). This nomenclature was recently changed (2000) from the previous name (Gorilla gorilla beringei).

Physical Characteristics

Mountain gorillas are generally somewhat larger than other subspecies and have longer hair. An average adult male weighs 160 kg (350 lbs), and an average adult female weighs 98 kg (215 lbs).

Habitat & Diet

The mountain gorillas inhabit high altitude and montane forests, as well as bamboo forests, ranging in altitude from 2,500 to 4,000 metres (8,200 – 13,100 feet). They are primarily vegetarian and almost 86% of their diet is composed of leaves, shoots and stems of herbaceous vegetation. They will eat small amounts of wood, roots, flowers and fruit, and occasionally feed on larvae, snails and ants. They eat a total of 142 different plant species, but only three different types of fruit. All ages and genders will occasionally eat their faeces, possibly to prevent the loss of minerals through digestion, although the exact reason has not yet been determined.

Life History & Social Structure

Mountain gorillas live in stable groups consisting of one dominant male and a number of females. Most groups (61%) are composed of one adult male and a number of females. 36% of groups have more than one adult male. The remaining gorillas are either loan males, or exclusively male groups. Group territories overlap and the dominant male generally defends his group rather than his territory.

Adult males develop a silver spray of hair across their back and hips, earning them the name ‘silverback’. This generally takes place around 15-17 years of age. Lifespan is 40-50 years and females will generally have their first infant between the ages of 10 and 12. Their gestation period is 9 months and their inter-birth interval 3-5 years. The age categories generally used for classification are:

• Infant: Birth to 3-4 years

• Juvenile: 4 to 7 years

• Subadult: between 9 and 16 years (depending on gender and the onset of reproductive maturity, which varies between individuals)

• Adult: for females, from 9 years onwards; for males, around 16.

The reproductive cycle for females is about 28 days, of which they are fertile only 1-3 days.

In general, most males and about 60% of females leave their natal group and emigrate to other groups. Males leave when they are about 11 years old and may travel alone or with another male for 2-5 years before they can attract females to join them and form a new group. Females typically leave their natal group around the age of 8 and usually transfer to an established group.

Behaviour

Gorillas are diurnal (active during the day) and mainly terrestrial. The dominant silverback male generally determines the movements of the group. There appears to be no discernible female hierarchy, although this is still debated.

The cohesion of the group tends to be attributed to the desire of all the members to stay close to the silverback male, who protects the juveniles and infants in the group. The silverback’s threat display includes ‘hooting’, chest beating and slapping of the ground, as well as the throwing and dragging of vegetation. Generally the groups are very peaceful, unless the animals are threatened or a female is trying to transfer to a nearby group.

Gorillas spend approximately 25% of their day eating, and rest during the middle of the day. The young animals spend much of their time playing with each other, and even with adults. When they sleep at night, they prepare a nest on the ground, using non-food plants. Mothers will share a nest with their young offspring.

Gorillas are generally quiet animals. They have an estimated 12 different vocalizations. The ‘belch’ is considered a contact call (to keep the group informed of an individual’s whereabouts in the dense vegetation) as well as a sign of contentment while foraging.

This information has been supplied by: International Gorilla Conservation Programme (www.igcp.org)

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