Description
The Mountain Gorilla has longer and darker hair than other gorilla species, allowing it to live in hot or cold weather and travel into areas where temperatures drop below 0 °C (32 °F). It has chosen a life on the ground more than any other non-human primate, and its feet most resemble those of humans. Gorillas can be identified by nose prints unique to each individual.[citation needed] Males usually weigh twice as much as the females, and this subspecies is on average the largest of all gorillas. 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 masseter muscles, which attach to the lower jaw, or mandible. Adult females also have these crests, but they are less pronounced.
Adult males are called silverbacks 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. Upright, males reach 1.73 m (5 ft 8 in) in height, with an arm span of 2.3 m (7 ft 7 in) and weigh 155 kg (340 lb).[4] The tallest silverback recorded was a 1.94 m (6 ft 4 in) individual shot in Alimbongo, northern Kivu in May 1938 and the heaviest was a 1.83 m (6 ft) silverback shot in Ambam, Cameroon which weighed about 266 kg (590 lb).[3]
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).[citation needed] 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.[citation needed]
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 diet
The Mountain Gorilla inhabits the Albertine Rift montane cloud forests of the Virunga Volcanoes, ranging in altitude from 2,200–4,300 metres (7,200–14,000 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 an 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 (9,200–11,000 ft); and the giant senecio zone at 3,400–4,300 metres (11,000–14,000 ft). The Mountain Gorilla spends most of its time in the Hagenia forests, where gallium 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.
Reproduction
A newborn gorilla weighs about 1.8 kilograms (4.0 lb), and spends its first few months of life in constant physical contact with its mother. In their first few months of life, infant Mountain Gorillas ride on their mother's backs. At an earlier stage, the mother will almost constantly be holding the infant. It begins to walk at around four or five months, and starts to put plant parts in its mouth between four and six months. At eight months it regularly ingests solid food. Weaning occurs around three years of age, although juveniles may remain with their mothers for years after that.
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. 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 Mountain Gorilla has no mating season and females usually initiate mating behavior. The length of their menstrual 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 bear 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.
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 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.
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 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.
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.
The dominant silverback 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. 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. Experienced silverbacks are capable of removing poachers' snares from the hands or feet of their group members.
When the dominant silverback dies or is killed by disease, accident, or poachers, the family group may be severely disrupted. 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. 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 two silverbacks can sometimes engage in a fight to the death, using their canines to cause deep, gaping injuries. 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, two-legged to four-legged, (8) slapping and tearing vegetation, and (9) thumping the ground with palms to end display.
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.
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 silverbacks. Deep, rumbling belches suggest contentment and are heard frequently during feeding and resting periods. They are the most common form of intragroup 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. Koko, the gorilla trained in sign language, is afraid of crocodiles and alligators, even though she was born in captivity and has never seen them. They are also afraid of water and will cross streams only if they can do so without getting wet (ie. 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 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. 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 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. 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. A census in 2004 showed a population of 380, most of it in the Karisoke sector and a significant increase since the time of Fossey's last count.
In April 2007 it was announced that a census of the Mountain Gorillas in Uganda's Bwindi Impenetrable National Park had recorded a 6% increase in population since a census in 2002.
