The Mountain Gorilla- Gorilla Tracking in Uganda
General Information about Gorilla Tracking in Uganda
Mountain Gorilla: Engagi (in Rukiga, the local language)
Gorilla gorilla beringei
Weight: Males 140-200 kg (300-440 lbs)
:Females 70-110 kg (150-250 lbs)
Height: Males 1.4-1.85m (55-73 inches)
Females: up to 1.5m (up to 60 inches)
Diet: Leaves, fruits, shoots, stems, flowers, bark, grubs, insects (especially
ants), ferns
Gestation Period: 8.5 months
Estimated Life Span: Up to 50 years in the wild, longer in captivity
Predators: Man, leopard (in Bwindi Impenetrable National Park and Mgahinga National Park we now have no leopard)
History and Taxonomy
Gorillas occur exclusively in the dense rain forests of West and Central Africa and have been known to people here for thousands of years. To the rest of the world, however, gorillas were mysterious and largely unknown. The stories brought home by early European explorers were greatly exaggerated descriptions of ferocious, man-like beasts that bore little resemblance to the actual animals. The name “gorilla” may originate from one such story. Over 2,000 years ago, an explorer named Hanno from the North African city of Carthage encountered apes on the coast of West Africa. He described a wild battle where several of his men were wounded and scratched trying to capture live specimens. In his language gorilla meant
“the scratched’ and while the apes he met were probably chimpanzees, the name has remained with gorillas throughout the centuries. The gorilla was first formally described as a species by scientists in 1847; and while the taxonomic names have changed slightly over the years, they are grouped with humans and the other apes into the super family Hominoidea. More specifically, gorillas
belong with chimpanzees and orangutans in the great ape family known as Pongidae. Human beings are the sole species in the Hominidae family, while the lesser apes (gibbons and siamangs) of Southeast Asia have their own family. Of all these primates, chimps and gorillas are the most closely related to humans, sharing 98 percent of the same genetic material. Three subspecies of gorilla are recognized; the western lowland gorilla (Gorilla, gorilla, gorilla), the eastern lowland gorilla (Gorilla, gorilla graueri), and the mountain gorilla (Gorilla, gorilla beringei). The mountain gorilla is the rarest of the apes and was only “discovered” by the scientific community in the 20th century. Most gorillas collected and described previously were of the more common western lowland variety, and rumors of gorillas living in the mountains of Central Africa weren’t confirmed until two animals were shot on the slopes of Mt. Sabinyo in 1902. The hunter was a German Army officer named Oscar Von Beringe,
whose name was later attached to the subspecies (beringei). After Beringe’s expedition, numerous mountain gorillas were shot and preserved for museum collections, but they have never been reared successfully in captivity like their lowland cousins. In the 1950s, serious research on mountain gorillas began with the ground-breaking work of George Schaller. He was the first to habituate gorillas for research in the Virunga volcanoes and also spent a brief time in Bwindi. Dian Fossey greatly expanded on this work and her research station at Karisoke in Rwanda is still in operation today
Distribution
Gorillas once inhabited the entire rain forest that stretched from the coast of West Africa to the western arm of the Great Rift Valley, Bwindi. During glacial periods, however, much of the moisture in the Earths atmosphere became trapped in the polar ice caps, and the climate dried out accordingly. Forests receded into pockets, and the gorillas, being exclusively forest dwellers, became divided into western and eastern populations. Over time, genetic drift and habitat specialization led to the development of three subspecies. The western lowland gorilla (G.g. gorilla) lives in the lowland rainforests of West Africa (Congo, Zaire, Cameroon, Gabon, etc.) while 1,000 kilometers (621 miles) to the east, the eastern lowland gorilla (G.g. graueri) inhabits the forests of eastern Zaire. The mountain gorilla (G.g. beringei) is found only in the Bwindi Forest and the Virunga volcanoes, on the eastern side of the Great Rift Valley. Apparently, the drier climate of the valley has been a barrier to gorilla migrations long enough for the mountain gorilla to become its own subspecies. Isolated in their pockets of mid to high altitude forest, the mountain gorillas developed longer hair, wider chests, broader jaws, wider nostrils and other adaptations that distinguish them from their lowland counterparts. It should be noted that most of these traits developed as a result of high altitude living. Bwindi gorillas, being found in medium altitude forest, show physical traits somewhere between Virunga and lowland populations, and were often considered lowland gorillas in the past. Genetic comparisons, however, have shown that the Virunga and Bwindi gorillas are part of the same subspecies. The two forests have been separated by cultivation for only 500 years. Before that, Bwindi and Virunga gorillas were likely part of a single population that inhabited a range of altitudes. Further research is currently being conducted on this interesting subject.
Ecology
Gorillas are the most terrestrial of the great apes, spending over 90% of their time on the ground. They move quadrupedically over all fours, but can stand upright for short periods, particularly to reach food plants or as a part of a chest-beating sequence. Gorillas are primarily vegetarians, although some insects and mineral soil are eaten, and there have been several instances of dead infants being consumed by other group members. In the Virungas, the mountain gorilla diet includes at least 58 different food plants, consisting mostly of leaves, bark and herbaceous material. As a result of the high quantity of water in their diet, gorillas 17 can go for much of the year without drinking water. Occasionally, during the dry season, animals have been observed drinking water and can sometimes be seen licking water from their fur during heavy rainstorms. Bwindi Forest is much more diverse in terms of plant species than the higher- altitude habitat of the Virungas, so the total number of food species here is undoubtedly higher and probably includes a larger amount of fruit. Gorillas have a single-chambered mono-gastric digestive system, which is less efficient for digesting vegetation than the ruminant system or the multi-chambered stomachs of colobus monkeys. Gorillas must, therefore, consume large quantities of vegetation daily (over 20 kg – 44 lbs. for an adult male), and always appear to have bloated stomachs. Many gorilla foods are concentrated in the lush secondary growth of forest clearings and disturbed sites. The presence of these areas may help determine the abundance of gorillas in some habitats. For example, in West Africa there is evidence that clearings maintained by foraging elephants have a strong impact on the distribution of gorillas. The high[1]altitude habitats of the Virunga Volcanoes have a more open canopy, with lush undergrowth spread evenly throughout the forest. Gorillas there are less dependent on disturbed sites and tend to have smaller home ranges than those in Bwindi and other dense forests. Clearings and disturbed areas are certainly important to Bwindi gorillas, and most home ranges will include a core of heavily utilized sites. Gorillas are the largest primate species in the world, with adult males known to top 200 kg (440 lbs.). Females are approximately half that size, a difference that probably evolved due to the males role as defender of the social group and to the competition among males for breeding privileges. Mountain gorillas are generally larger than their lowland cousins, although the largest wild specimen on record was an eastern lowland gorilla from the Kahuzi-Biega region in Zaire
Social Organisation
Gorillas live in family groups led by a dominant adult (silverback) male. The lead silverback is the groups primary defender, and probably fathers the majority of infants born to females in the group. The number of females can vary greatly. The group may contain other, secondary silver- backs who follow the leader in a ranking order based on age and size. Standing among females is also determined by age, familial links and relatedness to the dominant silverback as well as by the presence of young and the current favor of the silverback. Blackback males (those nearing maturity, but yet to begin growing silver hair) come next in the hierarchy, but may outrank females in some instances. An average group of mountain gorillas contains approximately 13 animals, with one or two silverbacks, a blackback male, several females and their offspring. Gorillas, however, follow no set pattern. Groups of over 30 animals (with five silverbacks!) have been recorded, and smaller families like Katendegyere here in Bwindi are common. Gorilla groups are fairly stable units, but a flow of individuals is main tained through the population by a constant process of emigration where nearly all females and most males leave the group of their birth shortly after reaching maturity. Females will transfer to another group or to lone males during interactions, while males leave their group to travel alone until they are strong enough to attract females of their own. These processes help to avoid inbreeding in a population where mating between related adults is rare. Gorilla social behavior is notably calm and stable among the primates. Where fluctuating group structure and frequent dominance struggles are common for chimpanzees and many monkeys, conflict within gorilla groups is relatively rare. A dominant silverback may fight with subordinate males if challenged, but can usually maintain control through vocalizations and displays rather than physical confrontation. Transfer of power in a gorilla group can be violent, with younger males or intruding silverbacks driving out the leader. But it is also common for a dominant male to grow old and even die of natural causes while still leading his group. The most common conflicts within a group are minor confrontations over feeding areas or right-of-way. These can be between males, females or sub-adults, and are most noticeable in the field by lunges and “pig grunt” vocalizations. 19 Conflicts between groups are somewhat more typical, although infre quent, and play an important role in regulating population dynamics. Gorillas aren’t territorial, and home ranges often overlap considerably as with the Katendegyere and Mubare groups here in Bwindi. When two groups come together, or when a group meets a solitary male, dramatic displays between the silverbacks are common and, occasionally, the whole group will become involved. It is during these encounters, while the males chest beat and charge, that females may decide to transfer from one group to another. They aren’t stolen and physically herded by the males, but appear to make the choice themselves, and a female may realign herself several times during her lifetime. How long she remains with a particular male will depend upon her status within the group and her success in rearing offspring. As with intra-group encounters, actual physical contact is often avoided through mock charging and chest beating displays. Sometimes, the confrontations can be avoided altogether by barks and chest beats which communicate a group’s location over long distances. In the case of actual fighting, gorillas use their long canine teeth to bite and slash their opponent, causing deep and sometimes serious wounds. If a male is actually driven from his group, one of the more brutal gorilla behaviors asserts itself. After a violent takeover, the new lead male will often kill all nursing infants in the group to bring the females back into estrus. This ensures that all offspring will be his and is a common way for animals to improve breeding success.
Daily habits
Gorillas spend approximately 30% of their time feeding, 30% of their time moving and foraging and the 
remaining 40% of the day at rest. They rise at daybreak for a period of movement and foraging in the morning before settling down for a long rest time at mid-day. When foraging, the gorillas may move and find food along the way, but usually look for a clearing or other area where they can spread out and concentrate on feeding. The lead silverback usually chooses these routes and locations. During the afternoon, the group will typically move again before finding a place to spend the night. Each gorilla makes its own nest every night, except for infants who sleep with their mother until completely weaned at age three to four years or when their mother has a new infant. Nests are usually grouped around the dominant male, but may be spread over 100 meters or more, particularly in groups where several silverbacks are present. Night nests are only used once, although 20 gorillas may “sleep late” on some rainy days or on mornings after a long movement. Gorilla groups move on average less than one kilometer per day, and rarely more than two. Longer movements typically occur following an aggressive interaction or other incident that has stressed the group. When tracking, you can often predict a day of long movement by the absence of feeding sites and the presence of diarrhetic dung, a typical indicator of stressed animals.
Communication
Gorillas communicate using a wide variety of vocalizations, gestures and facial expression. Sixteen distinct vocal signals have been identified, from the scream of a charging silverback to the variable “belch” vocalizations of the group at rest. Adult males, particularly the lead silverback, dominate a groups vocalizations. Calls like the various barks and roars are exclusively male, often used in aggressive situations or to help coordinate group movements. Non-vocal communication in gorillas can also be complex. Males use bared teeth, a “stiff-legged walk,” lunges and charges to signal various levels of aggression. These are often combined with vocal cues, as in the classic nine-step chest-beating sequence. The complete sequence begins with a distinctive hooting vocalization and symbolic feeding before the gorilla stands bipedally. The animal then beats his chest, kicks out and runs sideways before tearing at surrounding vegetation and slapping the ground. This impressive display is used to intimidate an opponent and may help males avoid unnecessary physical confrontations. Other group members may mimic parts of the sequence, (i.e., juveniles beating the chest), but only adult males have been observed completing all nine steps. Non-aggressive communication is less dramatic, but includes the various play behaviors and interactions between infants and mothers. Grooming is less common than in many primates, restricted mostly to females tending their young or the lead silverback, but grooming between other males or females, particularly related individuals, is not unheard of
Reproduction
Female gorillas reach sexual maturity at around age eight, but usually don’t have their first offspring until at least the age ten. They have an estrus (sexually receptive) period of one to three days each month, during which time they may invite copulation from, or be pursued by, the dominant male. Mating behavior and “sexual play” by sub-dominant males may be tolerated by the silverback when a female is not in estrus, but he will vigorously defend access to a female during her estrus times. These periods may be signified by a certain smell or other cue as all males seem to be aware of them and aggressive encounters are correspondingly higher. Males reach maturity between eight and fifteen years. Gorilla pregnancies last for eight and one half months, but are difficult to observe in the field, as their stomachs are always distended with food. Newborns have light brown skin and are highly dependent, rarely leaving arm’s reach of their mothers in the first six months. Twins have been born to gorillas, but it is extremely rare. Babies ride ventrally at first, clinging to the mother’s chest, but slowly begin spending more time on her back as they mature. During the second year, they become more independent and begin playing with others, interacting with the silverback and feeding themselves. It is notable that this period of experimentation with food is the primary way that new items are added to a gorilla group’s diet. At between three and four years, young gorillas become relatively independent and begin building their own nests next to their mothers. When no longer lactating, the mother returns to her regular estrus cycle and can produce another child. Infant mortality in gorillas is high, however, with 30% or more of the offspring dying before they reach the age of six. In a life of 50 years, a female should be able to produce 10 offspring, but with this high infant mortality, most mothers probably raise only about four to six offspring. Infant gorillas interact noticeably more with adult males than in most other primate species. Silverbacks can often be seen playing with several juveniles and have even been known to “adopt” an infant whose mother has died. It is thought that since the dominant male defends access to estrus females in the group, he can treat all the offspring as his own and take a greater interest in their care. Most primates live in multi-male societies where parentage is less clear and child-rearing left entirely up to the females
Age and Sex Classes
Even with the following guidelines, judging the age of animals can be very difficult, especially for females. Usually, female maturity is judged by the presence and age of their young. Females can often be confused with subadult males and remain difficult to tell apart until either a female gives birth or the male begins to develop a sagittal crest and chest muscles. Once a young male begins to develop a sagittal crest and chest muscles, he becomes a blackback and is easily distinguishable from females. The age of older males or silverbacks is usually judged by the amount of silver hair on the back. The more silver hair, the older the gorilla.