Mountain Gorilla

The Mountain Gorilla (Gorilla beringei beringei) is one of two subspecies of Eastern Gorillas. They are only 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). Some claim that the Bwindi population in Uganda's Bwindi Impenetrable Forest is a third subspecies, although no formal taxonomic description has been published.

A census taken in 2003 has shown a 17% increase in population size since 1989. There are now a total of 380 gorillas in 30 social groups ("Mountain Gorilla Census" 2004). However, Mountain Gorillas continue to be considered critically endangered on the IUCN Red List of Endangered Species. They face an extremely high risk of extinction in the wild due to habitat loss, poaching, human disease, and war (Butynski et al. 2003).

Description

Mountain Gorillas have longer and darker hair than other gorillas, enabling them to live at high altitudes and travel into areas where temperatures drop below freezing. They have adapted to a life on the ground more than any other non-human primate, and their feet most resemble those of humans. Gorillas can be identified by nose prints unique to each individual. Researchers often use photographs and illustrations of noses for identification and monitoring.

Mountain Gorillas are highly sexually dimorphic, with males usually weighing twice as much as females. Adult males also have more pronounced bony crests on the top (sagittal) and back (nuchal) of their skulls, giving their heads a more conical shape. These crests anchor the massive muscles of their large jaws. Adult females also have these crests, but they are much less pronounced (Smithsonian National Zoological Park ^*).

Adult males are called silverbacks. When they reach sexual maturity, a saddle of gray or silver-colored hair develops on their backs. The hair on their backs is shorter than on most other body parts, and their arm hair is especially long. Upright, males reach 1.5–1.8 m (5–6 ft) in height, with an arm span of 2.25 m (7 ft 6 in) and weigh 204–227 kg (450–500 lb) (Knight 2003).

Mountain Gorillas are primarily terrestrial and quadrupedal. However, they will climb into fruiting trees if the branches can carry their weight, and they are capable of running bipedally up to 6 m (20 ft). Like all great apes other than humans, their arms are longer than their legs. They move by knuckle-walking (like Common Chimpanzees, but unlike Orangutans and Bonobos), supporting their weight on the backs of their curved fingers rather than their palms.

Mountain Gorillas are 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 their massive bulk. They forage in early morning, rest during the late morning and around midday, and in the afternoon they forage 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 (Schaller, 1963).

Habitat and diet

The Mountain Gorillas inhabit the Albertine Rift montane cloud forests of the Virunga Volcanoes, ranging in altitude from 2225 to 4267 meters (7300-14000 ft). Most are found on the slopes of three of the dormant volcanoes: Karisimbi, Mikeno, and Visoke ("Mountain Gorillas," 2003). The vegetation is very dense at the bottom of the mountains, becoming more sparse at higher elevations, and the forests where the Mountain Gorillas live are often cloudy, misty and cold ("The Life of Mountain Gorillas" 2002).

Mountain Gorillas are primarily herbivores. The majority of their diet is composed of the leaves, shoots and stems (85.8%) of 142 plant species. They also feed on bark (6.9%), roots (3.3%), flowers (2.3%), and fruit (1.7%), as well as larvae, snails and ants (0.1%) (Fossey and Harcourt 1977). Adult males can eat up to 75 pounds of vegetation a day, while a female can eat as much as 40 pounds.

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 (1963) identified ten distinct zones, including: the bamboo forests at 2225–2804 m (7300–9200 ft); the Hagenia forests at 2804–3353 m (9200–11000 ft); and the giant senecio zone at 3444–4267 m (11300–14000 ft). Mountain Gorillas spend most of their time in the Hagenia forests, where gallium vines are found year-round. All parts of this vine are consumed: leaves, stems, flowers, and berries. They travel to the bamboo forests during the few months of the year fresh shoots are available, and they climb into subalpine regions to eat the soft centers of giant senecio trees ("Mountain Gorillas" 2003).

Reproduction

A newborn gorilla weighs about 1.8 kg (4 lb), and spends its first few months of life in constant physical contact with its mother. They begin to walk at around four or five months, and start to put plant parts in their mouths between four and six months. At eight months they regularly ingest solid food (Watts 1985). Weaning occurs around three years of age, although juveniles may remain with their mothers for years after that (Lindsley & Sorin 2001).

Young male and female gorillas are considered infants from birth until three years of age, juvenile between the ages of about three and six, and subadult from six to about eight years old. Blackbacks are sexually immature males from around eight years until they have developed the silver saddle and large canines of maturity (Groves & Meder 2001). Females begin to ovulate at 7 or 8 years of age, and have their first infant between the ages of 10 and 12. Males, generally do not start breeding before the age of 15 (The International Gorilla Conservation Programme ^*).

Mountain Gorillas have no mating season. Females usually initiate mating behavior. The length of their estrus cycle is about 28 days with 1-3 fertile days, and ovulation ceases for 3–5 years after reproducing. The length of gestation is eight and a half months. Females generally rear one infant every 6 to 8 years, and may leave only 2–6 offspring over a 40 year life span. Males that have harems of 3–4 females increase their reproductive output by fathering 10-20 offspring over 50 years (Lindsley and Sorin 2001).

Social structure

Mountain Gorillas are highly social, and live in relatively stable, cohesive groups held together by long-term bonds between adult males and females. Relationships among females are relatively weak (Stewart and Harcourt 1987). These groups are nonterritorial; the silverback generally defends his group rather than his territory. In the Virunga Mountain Gorillas, the average length of tenure for a dominant silverback is 4.7 years (Robbins 1995).

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 (Harcourt 1988). Group sizes vary from five to thirty, with an average of ten individuals. A typical group contains: one silverback, who is the group's undisputed leader; one or two blackbacks, who act as sentries; three to four sexually mature females, who are ordinarily bonded to the dominant silverback for life; and, from three to six juveniles and infants. (Fossey 1983)

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 (Lindsley and Sorin 2001). They may travel alone or with 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 silverback male (Watts 1990).

The dominant silverback generally determines the movements of the group, leading them to appropriate feeding sites throughout the year. He also mediates conflicts within the group and protects them from external threats ("Life of Mountain Gorillas" 2002). Experienced silverbacks are capable of removing poachers' snares from the hands or feet of their group members (Berggorilla & Regenwald Direkthilfe ^*). 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 silverback is usually the one who looks after his abandoned offspring, even allowing them to sleep in his nest (Stewart 2001).

When the dominant silverback dies or is killed by disease, accident, or poachers, the family group may be severely disrupted ("Mountain Gorillas," 2003). Unless he leaves behind a male descendant capable of taking over his position, the group will either split up or be taken over in its entirety by an unrelated male. When a new silverback takes control of a family group, he may kill all of the infants of the dead silverback (Fossey 1984). This practice of infanticide is an effective reproductive strategy, in that the newly acquired females are then able to conceive the new male's offspring. Infanticide has not been observed in stable groups.

Behavior

Aggression

Severe aggression is rare in stable groups, but when two mountain gorilla groups meet, the leading silverbacks can sometimes engage in a fight to the death, using their canines to cause deep, gaping injuries (Fossey 1983). 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 (SNZP). The entire sequence has nine steps: (1) hooting slow to fast, (2) symbolic feeding, (3) rising bipedally, (4) throwing vegetation, (5) chest-beating with cupped hands, (6) one leg kick, (7) sideways running, two-legged to four-legged, (8) slapping and tearing vegetation, and (9) thumping the ground with palms to end display (Maple and Hoff 1982).

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 silverback and his females tolerate and even participate if encouraged (SNZP).

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 (Harcourt et al. 1993). 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 silverbacks. Deep, rumbling belches suggest contentment and are heard frequently during feeding and resting periods. They are the most common form of intragroup communication (Fossey 1983).

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 (Schaller, 1964). One of the apes was recovered and sent to the Zoological Museum in Berlin, 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 (BRD). 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 (Dian Fossey Gorilla Fund - UK ^*).

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 Mountain Gorillas before his research, which described their social organization, life history, and ecology (DFGFUK). Following Schaller, Dian Fossey began what would become a 13 year study in 1967. Fossey made new observations, completed the first accurate census, and established active conservation practices, such as anti-poaching patrols.

Conservation

Mountain Gorillas are threatened by poaching, loss of habitat, and human disease.

Poaching: Mountain Gorillas are not usually hunted for bushmeat, but they are frequently maimed or killed by traps and snares intended for other animals. They have been killed for their heads, hands, and feet, which are sold to collectors. Infants are sold to zoos, researchers, and people who want them as pets. The abduction of infants generally involves the loss of at least one adult, as members of a group will fight to the death to protect their young.
Habitat loss: The forests where Mountain Gorillas live are surrounded by rapidly increasing human settlement. The humans' need for land, food, and timber encroaches on the gorillas' habitat through roads, slash-and-burn agriculture, and logging. The resulting deforestation confines the gorillas to isolated forest islands. Some groups may raid crops for food, creating further animosity and retaliation.
Disease: Humans and gorillas are genetically similar enough that gorillas are vulnerable to many of the same diseases as humans. However, gorillas have not developed the immunities to resist human diseases, and infections could severely impact the population. Habituated groups that are visited by tourists have the greatest risk.

Conservation requires work at many levels, from local to international, and involves protection and law enforcement as well as research and education.

"Active conservation includes frequent patrols in wildlife areas to destroy poacher equipment and weapons, firm and prompt law enforcement, census counts in regions of breeding and ranging concentration, and strong safeguards for the limited habitat the animals occupy" (Fossey 1983).
"Theoretical conservation seeks to encourage growth in tourism by improving existing roads that circle the mountains, by renovating the park headquarters and tourists' lodging, and by the habituation of gorillas near the park boundaries for tourists to visit and photograph" (Fossey 1983).
Community-based conservation supports African ownership, provides education on the personal as well as environmental benefits of preserving protected areas, and encourages local people to take pride in and assume some of the responsibility for the protection of their parks (Fossey 1983).

References

African Wildlife Federation Retrieved from http://www.awf.org/index.php.
Berggorilla & Regenwald Direkthilfe (n.d.). Retrieved from http://www.berggorilla.org/english/frame.html.
Butynski, T. & Members of the Primate Specialist Group 2000 (2003). Gorilla beringei ssp. beringei. In: 2003 IUCN Red List of Threatened Species. Retrieved from http://www.redlist.org.
Dian Fossey Gorilla Fund - UK. Retrieved from http://www.gorillas.org.
Fossey, D. (1983). Gorillas in the mist. Boston: Houghton Mifflin Company.
--- (1984). Infanticide in mountain gorillas (Gorilla gorilla beringei) with comparative notes on chimpanzees. In: G. Hausfater, S. B. Hrdy (Eds.). Infanticide: Comparative and evolutionary perspectives. New York: Aldine Publishing Company.
Fossey, D., & Harcourt, A.H. (1977). Feeding ecology of free ranging mountain gorillas (Gorilla gorilla beringei). In Clutton Brock (Ed.), Primate ecology: Studies of feeding and ranging behaviour in lemurs, monkeys and apes. London: Academic Press.
Groves, C. (2001). Primate Taxonomy ISBN 1-56098-872-X
Groves, C., & Meder, A. (2001). A model of gorilla life history. In: Australian Primatology.
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The International Gorilla Conservation Programme. Retrived from http://www.mountaingorillas.org/gorillas/gorillas_mountain.htm.
Knight, T. (2003). Gorilla Natural History. In: Gorillas Online. Retrieved from http://homepage.mac.com/wildlifeweb/gorillas/info/nh.html
The Life of Mountain Gorillas (2002). In: The Dian Fossey Gorilla Fund International. Retrieved from http://www.gorillafund.org/005_gorilla_frmset.html.
Lindsley, T., & Sorin, A. (2001). Gorilla gorilla beringei. In: Animal Diversity Web. Retrieved from http://animaldiversity.ummz.umich.edu/site/accounts/information/Gorilla_gorilla_beringei.html.
Maple, T.L., & Hoff, M.P. (1982). Gorilla Behavior. New York: Van Nostrand Reinhold Company.
Mountain Gorilla Census Shows 17 Percent Increase Since 1989 (16 Jan 2004). In: The Dian Fossey Gorilla Fund International. Retrieved from http://www.gorillafund.org/cont_frm/03_dfgfi_pressrel.html.
Mountain Gorillas (21 May 2003). In: Exploring the Environment - Modules and Activities. Retrieved from http://www.cotf.edu/ete/modules/mgorilla/mgorilla.html.
Robbins, M.M. (1995). A demographic analysis of male life history and social structure of mountain gorillas. In: Behaviour.
Schaller, G.B. (1963). The mountain gorilla: Ecology and behavior. Chicago: Univ. Chicago Press.
Smithsonian National Zoological Park. Retrieved from http://nationalzoo.si.edu/Animals/Primates/Facts/FactSheets/Gorillas/default.cfm.
Stewart, K.J., & Harcourt, A.H. (1987). Gorillas: variation in female relationships. In B. B. Smuts, D.L. Cheney, R.M. Seyfarth, R.W. Wrangham and T.T. Struhsaker (Eds.). Primate Societies. Chicago: University of Chicago Press.
Stewart, K.J. (2001). Social relationships of immature gorillas and silverbacks. In: M.M. Robbins, P. Sicotte, K.J. Stewart (Eds.). Mountain Gorillas: Three decades of research at Karisoke. New York: Cambridge Univ Press.
Watts, D.P. (1985). Observations on the ontogeny of feeding behavior in mountain gorillas (Gorilla gorilla beringei). In: American Journal of Primatology.
--- (1990). Ecology of gorillas and its relation to female transfer in mountain gorillas. In: International Journal of Primatology.

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