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The mountain gorilla (Gorilla beringei beringei) is one of the two subspecies of the eastern gorilla. There are two populations. One is found in the Virunga volcanic mountains of Central Africa, within three National Parks: Mgahinga, in south-west Uganda; Volcanoes, in north-west Rwanda; and Virunga in the eastern Democratic Republic of Congo (DRC). It is listed as critically endangered by the IUCN. The other is found in Uganda’s Bwindi Impenetrable National Park. Some primatologists consider the Bwindi population in Uganda may be a separate subspecies, though no description has been finalized. As of September 2015, the estimated number of gorillas remaining is less than 900.

Gorilla taxonomy

Mountain gorillas are descendants of ancestral monkeys and apes found in Africa and Arabia during the start of the Oligocene epoch (34-24 million years ago). The fossil record provides evidence of the hominoid primates (apes) found in east Africa about 22–32 million years ago. The fossil record of the area where mountain gorillas live is particularly poor and so its evolutionary history is not clear. It was about 9 million years ago that the group of primates that were to evolve into gorillas split from their common ancestor with humans and chimps; this is when the genus Gorilla emerged. It is not certain what this early relative of the gorilla was, but it is traced back to the early ape Proconsul africanus. Mountain gorillas have been isolated from eastern lowland gorillas for about 400,000 years and these two taxa separated from their western counterparts approximately 2 million years ago. There has been considerable and as yet unresolved debate over the classification of mountain gorillas. The genus was first referenced as Troglodytes in 1847, but renamed to Gorilla in 1852. It was not until 1967 that the taxonomist Colin Groves proposed that all gorillas be regarded as one species (Gorilla gorilla) with three sub-species Gorilla gorilla gorilla (western lowland gorilla), Gorilla gorilla graueri (lowland gorillas found west of the Virungas) and Gorilla gorilla beringei (mountain gorillas including, Gorilla beringei found in the Virungas and Bwindi). In 2003 after a review they were divided into two species (Gorilla gorilla and Gorilla beringei) by The World Conservation Union (IUCN).

Physical description

The fur of the mountain gorilla, often thicker and longer than that of other gorilla species, enables them to live in colder temperatures. Gorillas can be identified by nose prints unique to each individual. Males, at a mean weight of 195 kg (430 lb) upright standing height of 150 cm (59 in) usually weigh twice as much as the females, at a mean of 100 kg (220 lb) and a height of 130 cm (51 in). This subspecies is on average the second largest species of primate; only the Eastern Lowland Gorilla, the other subspecies of Eastern Gorilla, is larger. Adult males have more pronounced bony crests on the top and back of their skulls, giving their heads a more conical shape. These crests anchor the powerful temporalis muscles, which attach to the lower jaw (mandible). Adult females also have these crests, but they are less pronounced. Like all gorillas they feature dark brown eyes framed by a black ring around the iris. Adult males are called silver backs because a saddle of gray or silver-colored hair develops on their backs with age. The hair on their backs is shorter than on most other body parts, and their arm hair is especially long. Fully erect, males reach 1.9 m (6 ft 3 in) in height, with an arm span of 2.6 m (8 ft 6 in) and weigh 220 kg (490 lb). The tallest silver back recorded was a 1.94 m (6 ft 4 in) with arm span of 2.7 m (8 ft 10 in), a chest of 1.98 m (6 ft 6 in) and weigh 219 kg (483 lb), shot in Alimbongo, northern Kivu in May 1938. There’s an unconfirmed record of another individual, shot in 1932, that was 2.06 m (6 ft 9 in) and weighed 218.6 kg (482 lb). The heaviest silver back recorded was a 1.83 m (6 ft) shot in Ambam, Cameroon which weighed about 266 kg (586 lb).

The mountain gorilla is primarily terrestrial and quadrupedal. However, it will climb into fruiting trees if the branches can carry its weight, and it is capable of running bipedally up to 6 m (20 ft). Like all great apes other than humans, its arms are longer than its legs. It moves by knuckle-walking (like the common chimpanzee, but unlike the bonobo and both orangutan species), supporting its weight on the backs of its curved fingers rather than its palms.

The mountain gorilla is diurnal, most active between 6:00 a.m. and 6:00 p.m. Many of these hours are spent eating, as large quantities of food are needed to sustain its massive bulk. It forages in early morning, rests during the late morning and around midday, and in the afternoon it forages again before resting at night. Each gorilla builds a nest from surrounding vegetation to sleep in, constructing a new one every evening. Only infants sleep in the same nest as their mothers. They leave their sleeping sites when the sun rises at around 6 am, except when it is cold and overcast; then they often stay longer in their nests.

Habitat and ecology

The mountain gorilla inhabits the Albertine Rift montane cloud forests and of the Virunga Volcanoes, ranging in altitude from 2,200–4,300 metres (7,200–14,100 ft). Most are found on the slopes of three of the dormant volcanoes: Karisimbi, Mikeno, and Visoke. The vegetation is very dense at the bottom of the mountains, becoming more sparse at higher elevations, and the forests where the mountain gorilla lives are often cloudy, misty and cold.

The mountain gorilla is primarily a herbivore; the majority of its diet is composed of the leaves, shoots and stems (85.8%) of 142 plant species. It also feeds on bark (6.9%), roots (3.3%), flowers (2.3%), and fruit (1.7%), as well as small invertebrates. (0.1%). Adult males can eat up to 34 kilograms (75 lb) of vegetation a day, while a female can eat as much as 18 kilograms (40 lb).

The home range size (the area used by one group of gorillas during one year) is influenced by availability of food sources and usually includes several vegetation zones. George Schaller identified ten distinct zones, including: the bamboo forests at 2,200–2,800 metres (7,200–9,200 ft); the Hagenia forests at 2,800–3,400 metres (9,200–11,200 ft); and the giant senecio zone at 3,400–4,300 metres (11,200–14,100 ft). The mountain gorilla spends most of its time in the Hagenia forests, where galium vines are found year-round. All parts of this vine are consumed: leaves, stems, flowers, and berries. It travels to the bamboo forests during the few months of the year fresh shoots are available, and it climbs into subalpine regions to eat the soft centers of giant senecio trees.

Social structure

The mountain gorilla is highly social, and lives in relatively stable, cohesive groups held together by long-term bonds between adult males and females. Relationships among females are relatively weak.These groups are non territorial; the silver back generally defends his group rather than his territory. In the Virunga mountain gorillas, the average length of tenure for a dominant silver back is 4.7 years.

61% of groups are composed of one adult male and a number of females and 36% contain more than one adult male. The remaining gorillas are either lone males or exclusively male groups, usually made up of one mature male and a few younger males. Group sizes vary from five to thirty, with an average of ten individuals. A typical group contains: one dominant silver back, who is the group’s undisputed leader; another subordinate silver back (usually a younger brother, half-brother, or even an adult son of the dominant silver back); one or two black backs, who act as sentries; three to four sexually mature females, who are ordinarily bonded to the dominant silver back for life; and from three to six juveniles and infants.

Most males, and about 60% of females, leave their natal group. Males leave when they are about 11 years old, and often the separation process is slow: they spend more and more time on the edge of the group until they leave altogether. They may travel alone or with an all-male group for 2–5 years before they can attract females to join them and form a new group. Females typically emigrate when they are about 8 years old, either transferring directly to an established group or beginning a new one with a lone male. Females often transfer to a new group several times before they settle down with a certain silver back male.

The dominant silver back generally determines the movements of the group, leading it to appropriate feeding sites throughout the year. He also mediates conflicts within the group and protects it from external threats. When the group is attacked by humans, leopards, or other gorillas, the silver back will protect them even at the cost of his own life. He is the center of attention during rest sessions, and young animals frequently stay close to him and include him in their games. If a mother dies or leaves the group, the silver back is usually the one who looks after her abandoned offspring, even allowing them to sleep in his nest. Experienced silver backs are capable of removing poachers’ snares from the hands or feet of their group members.

When the silver back dies or is killed by disease, accident, or poachers, the family group may be disrupted. Unless there is an accepted male descendant capable of taking over his position, the group will either split up or adopt an unrelated male. When a new silver back joins the family group, he may kill all of the infants of the dead silver back. Infanticide has not been observed in stable groups.

Analysis of mountain gorilla genomes by whole genome sequencing indicates that a recent decline in their population size has led to extensive inbreeding. As an apparent result, individuals are typically homozygous for 34% of their genome sequence. Furthermore, homozygosity and the expression of deleterious recessive mutations as consequences of inbreeding have likely resulted in the purging of severely deleterious mutations from the population.

Aggression

Although strong and powerful, the mountain gorillas are generally gentle and very shy. Severe aggression is rare in stable groups, but when two mountain gorilla groups meet, the two silver backs can sometimes engage in a fight to the death, using their canines to cause deep, gaping injuries. For this reason, conflicts are most often resolved by displays and other threat behaviors that are intended to intimidate without becoming physical. The ritualized charge display is unique to gorillas. The entire sequence has nine steps: (1) progressively quickening hooting, (2) symbolic feeding, (3) rising bipedally, (4) throwing vegetation, (5) chest-beating with cupped hands, (6) one leg kick, (7) sideways running four-legged, (8) slapping and tearing vegetation, and (9) thumping the ground with palms. Jill Donisthorpe stated that a male charged at her twice. In both cases the gorilla turned away, when she stood her ground.

Affiliation

The midday rest period is an important time for establishing and reinforcing relationships within the group. Mutual grooming reinforces social bonds, and helps keep hair free from dirt and parasites. It is not as common among gorillas as in other primates, although females groom their offspring regularly. Young gorillas play often and are more arboreal than the large adults. Playing helps them learn how to communicate and behave within the group. Activities include wrestling, chasing, and somersaults. The silver back and his females tolerate and even participate if encouraged.

Vocalization

Twenty-five distinct vocalizations are recognized, many of which are used primarily for group communication within dense vegetation. Sounds classified as grunts and barks are heard most frequently while traveling, and indicate the whereabouts of individual group members. They may also be used during social interactions when discipline is required. Screams and roars signal alarm or warning, and are produced most often by silver backs. Deep, rumbling belches suggest contentment and are heard frequently during feeding and resting periods. They are the most common form of intra group communication.

Fears

For reasons unknown, mountain gorillas that have been studied appear to be naturally afraid of certain reptiles. Infants, whose natural behavior is to chase anything that moves, will go out of their way to avoid chameleons and caterpillars. They are also afraid of water and will cross streams only if they can do so without getting wet (i.e. crossing over fallen logs). Dian Fossey observed and noted the mountain gorilla’s obvious dislike of rain, as well.

Research

In October 1902, Captain Robert von Beringe (1865–1940) shot two large apes during an expedition to establish the boundaries of German East Africa. One of the apes was recovered and sent to the Berlin Zoological Museum, where Professor Paul Matschie (1861–1926) classified the animal as a new form of gorilla and named it Gorilla beringei after the man who discovered it. In 1925 Carl Akeley, a hunter from the American Museum of Natural History who wished to study the gorillas, convinced Albert I of Belgium to establish the Albert National Park to protect the animals of the Virunga mountains.

George Schaller began his 20-month observation of the mountain gorillas in 1959, subsequently publishing two books: The Mountain Gorilla and The Year of the Gorilla. Little was known about the life of the mountain gorilla before his research, which described its social organization, life history, and ecology. Following Schaller, Dian Fossey began what would become an 18-year study in 1967. Fossey made new observations, completed the first accurate census, and established active conservation practices, such as anti-poaching patrols. The Digit Fund, which Dian Fossey started, continued her work and was later renamed the Dian Fossey Gorilla Fund International. The Fund’s Karisoke Research Center monitors and protects the mountain gorillas of the Virungas. Close monitoring and research of the Bwindi mountain gorillas began in the 1990s.

Conservation

Conservation efforts have led to an increase in overall population of the mountain gorilla (Gorilla beringei beringei) in the Virungas and at Bwindi. The overall population is now believed to be at least 880 individuals. Three more – infants who suffered a traumatic poaching experience, injuries from snares, and/or losing their mothers in brutal killings – are currently in care of the Senkwekwe Centre orphanage in the DR of Congo.

In December 2010 the official website of Virunga National Park announced that “the number of mountain gorillas living in the tri-national forested area of which Virunga forms a part, has increased by 26.3% over the last seven years – an average growth rate of 3.7% per annum.” The 2010 census estimated that 480 mountain gorillas inhabited the region. The 2003 census had estimated the Virunga gorilla population to be 380 individuals; which represented a 17% increase in the total population since 1989 when there were 320 individuals The population has almost doubled since its nadir in 1981, when a census estimated that only 254 gorillas remained.

The 2006 census at Bwindi indicated a population of 340 gorillas, representing a 6% increase in total population size since 2002 and a 12% increase from 320 individuals in 1997. All of those estimates were based on traditional census methods using dung samples collected at night nests. Conversely, genetic analyses of the entire population during the 2006 census indicated there were only approximately 300 individuals in Bwindi. The discrepancy highlights the difficulty in using imprecise census data to estimate population growth.

In both Bwindi and the Virungas, groups of gorillas that were habituated for research and ecotourism have higher growth rates than unhabituated gorillas, according to computer modeling of their population dynamics. Habituation means that through repeated, neutral contact with humans, gorillas exhibit normal behavior when people are in proximity. Habituated gorillas are more closely guarded by field staff and they receive veterinary treatment for snares, respiratory disease, and other life-threatening conditions. Nonetheless, researchers recommended that some gorillas remain unhabituated as a bet-hedging strategy against the risk of human pathogens being transmitted throughout the population.

Despite their recent population growth, the mountain gorilla remains threatened. As of 2008, mountain gorillas were listed as Critically Endangered on the IUCN Red List and are dependent on conservation efforts to survive.

Threat Effect on Gorilla Population Conservation Efforts
Poaching
  • Gorillas are maimed or killed by traps set for other animals.
  • Abduction of infants for illegal selling to zoos and as pets. This often results in other adult gorillas being killed in the process.
  • Increased patrolling using armed guards in protected forest areas.
  • Karisoke’s guards find and remove some 1,000 snares each year.
  • Gorilla censuses to monitor gorilla population.
  • Karisoke Research Centre runs a facility for young gorillas rescued from poachers.
Habitat loss
  • Rapidly expanding human settlements are removing the gorilla habitat.
  • Fragmentation of forested areas has resulted in the isolation of gorilla groups from each other, reducing genetic diversity.
  • Expansion of areas of national park to protect habitat.
Disease
  • Regular contact of tourists with the gorillas allows transmission of diseases from humans to the gorillas.
  • Domestic animals and livestock also contribute to disease transmission.
  • Requirement to stay at least 7 meters from gorillas at all times.
  • Better management of ecotourism.
  • More education for local communities to minimize impact of livestock.
War and unrest
  • Refugees removing trees to create settlements and farms.
  • Increased use of gorillas for meat by displaced peoples.
  • Gorillas can be killed by land mines placed along forest paths.
Local communities
  • Habitat removal.
  • Lack of support for conservation efforts due to insufficient education and awareness amongst locals.
  • Poverty encourages the use of slash and burn agriculture to grow food.
Karisoke Research centre has:

  • Created Virunga biodiversity inventory and field guides, to support public education and ecotourism.
  • Supplied primary school students with books and other materials.
  • Run conservation programs for secondary schools.
  • Provided biodiversity training for local park staff and conservation groups.
  • Provide field courses and internships for Rwandan college students.