Mountain Gorilla Facts | Uganda & Rwanda Wildlife Guide
Mountain gorillas have an extremely restricted habitat, living only in a small range of volcanoes at the intersection of Uganda, Rwanda and the Democratic Republic of Congo, and in Bwindi Impenetrable National Park about 20 miles north. Although European explorers heard rumors of these forest-dwelling giants in the 1880s, they were not officially identified by scientists until 1902, and they were not studied until George Schaller spent a year-and-a-half living with them in 1959–1960. More well-known is Dian Fossey, who studied them for 18 years and is the author of Gorillas in the Mist—the subject of a movie with the same name.
In the 10 years after Schaller did his pioneering study of Rwanda’s mountain gorillas, their total population across their range declined by 40 percent to a low of just a few hundred. With intense pressure put on these fertile hillsides by a rapidly growing human population and increasing levels of poaching, it was unclear whether the mountain gorillas would survive the century in which they had been identified.
In the late 1970s, two of Fossey’s graduate students from the Karisoke Research Center—Amy Vedder and Bill Weber—helped the government of Rwanda see the financial potential of bringing tourists to visit the mountain gorillas. It was a very new concept and a difficult battle, as the government had a plan in place to reduce the size of Volcanos National Park for a cattle ranching project. Vedder and Weber persevered and eventually convinced the government to halt the cattle ranching scheme and give tourism a try.
Once adopted, this tourism initiative slowly expanded to Uganda and the Democratic Republic of Congo and reversed the steady decline in mountain gorilla numbers. Today mountain gorillas are the only great ape whose population is increasing instead of decreasing, with a 2012 population of 880, and a population that is likely to be around 1,000 once the numbers from the 2015–2018 census are released.
The permit price Nat Hab travelers pay, part of which is shared with the communities surrounding the gorilla parks, has very likely prevented this species from going extinct.
PHYSICAL DESCRIPTIONGorillas are the largest of the great apes, and mountain gorillas are slightly smaller than their lowland relatives. A large male can weigh 500 pounds or more, with adult females weighing half that on average. Males and females grow at the same rate until they reach about 6 years old, at which point the females’ growth slows considerably. Males continue growing until they reach sexual maturity at around 12 to 13 years, at which point they will begin getting a distinctive silver saddle—the reason for the title “silverback.” The silverback is 10 times stronger than an adult human and maintains absolute control over his family group.
Young mountain gorillas are considered infants until age 3, juvenile from 3 to 6, young adults from 6 to 10 for females and 6 to 12 in males, and mature adult after they reach sexual maturity.
BEHAVIOROn a typical day for mountain gorillas, the animals get up from their sleeping nests between 6 am and 8 am and eat enthusiastically for about two hours. At about 10 am the group begins a resting period when many of the social bonding behaviors happen. While the adults sit quietly, grab a quick nap or groom each other, the young gorillas play follow the leader, king of the mountain and tag. They somersault down slopes and chase each other, chuckling as they run. They mock fight, wrestling, grappling and tumbling about, and pretend to bite without hurting each other. It would be difficult not to see the resemblance to a human family at home on a weekend.
Mountain gorillas are huge and powerful, yet gentle and patient. The most mischievous little one may scramble all over the most dignified silverback, poking him with knees and elbows, pulling his hair and keeping him from a quiet nap, all with perfect impunity. Babies are the center of attention for the whole group from the moment they are born until they are about 3 years old and able to take care of themselves. Mountain gorillas care for their babies with great love and affection, dangling them, tickling them into giggling fits and cuddling them often. They defend them with their lives.
Unlike the chimpanzees, gorillas do not hoot and holler for hours on end. Except when threatened, they speak softly or not at all. Fossey classified their sounds into roars, screams, cries, grunts, belches, hoots, chuckles, question barks and wraagh sounds. When a group is threatened, the leader, usually a huge male silverback, rears up on his hind legs to stretch to his most imposing height, beats his chest, waves his arms, tears branches from trees and waves them and roars. If the intruder does not immediately break eye contact, drop into a submissive posture or vanish, the silverback charges, stopping just short of contact.
When the roaring, charging, arm-waving, 6-and-a-half foot, 500-pound silverback causes the enemy to run for dear life, the mountain gorilla may give chase, grab the hapless creature, give it one or two good bites and dash back to follow its group that has likely retreated to a safe distance in the forest. The dominant silverback may be backed up by other silverbacks in the group, or by younger males, but ordinarily these males stay with the females and little ones as they run to safety.
For the hours between 3 pm and 5 pm, the gorillas move to the area where they will sleep, snacking along the way. At 6 pm darkness begins to fall, and the group builds its nests for the night. Mountain gorillas spend about 40 percent of their time at leisure, 30 percent eating and the remaining 30 percent traveling. Schaller reported that gorillas move anywhere from 300 to 6,000 feet per day within a home range of 10 to 15 square miles; Fossey found that the gorillas moved about 400 feet per day, perhaps a result of their more restricted territory compared to Schaller’s day.
Sleeping nests are made of branches and vines, broken or bent to form a crude bowl. The nests are cupped in the middle with a rim all the way around and are built either in trees or, more often, on the ground. Leafy branches are used to form a thick cushion in the bottom. Only plants not used for eating are used in nesting. In the rainy season, choice nesting places are in sheltered tree hollows, since gorillas are not fond of cold rain.
Each nest is used only once, since gorillas move on to a new nesting site each night. Each individual sleeps in its own nest except for young up to 3 years old who sleep with their mothers. Gorillas defecate into their nests at night, a habit that, while slightly disgusting to a human observer, helps researchers determine family size and age structure.
Babies begin making practice nests before they are 1 year old. The youngest gorilla Fossey observed building a solid, serviceable nest in which he slept by himself was 34 months old. Mountain gorillas prefer to sleep on knolls and open slopes where anyone who approaches must hike up to the nesting site. The silverbacks do sentry duty, sleeping well below the rest of the group, making it impossible to approach the group undetected.
MOUNTAIN GORILLA SOCIETYWhen trying to generalize behaviors or characteristics of mountain gorilla families, it is important to remember that they are very closely related to humans (with 98 percent identical DNA), and just like humans, each gorilla has a distinct personality. One advantage to doing multiple gorilla treks on this trip is that you will be able to see how these different personalities play out, particularly the personalities of the dominant males.
A silverback that grew up in a habituated family is likely to be much calmer and more tolerant of human visitors than a silverback who was more recently habituated. Some silverbacks will tolerate the presence of other adult male gorillas, sometimes even the previous silverback that he unseated. Some will allow subdominant males to mate with females in the group, others will violently prevent any attempt to do so. Our park guides during the treks know each of the gorillas by name and will be able to share their stories and point out how their individual personalities play out in the group setting.
Mountain gorilla groups range in size from 4 to more than 30 gorillas of all ages, from infants to the elderly. Each group is led by a silverback, usually the largest or strongest of the group. In groups with several silverbacks, the largest of them will lead the group. The silverback decides when the group will move, when it will stop, where it will rest and where it will sleep for the night. He sets the pace of travel and often slows everyone down so that a young, old, sick or injured member can keep up with the group. Females establish their own hierarchy, but mothers with babies seem to be at the top of the pecking order with both older and younger females somewhere below.
Some adult males strike out on their own, wandering alone. They may drop in on different groups and visit, staying a few hours or a few days, depending on how well they are accepted by each group’s silverbacks. The groups’ males usually breed with the mature females, but not always. Breeding-age females that are consistently ignored by the silverback may take themselves off to another group where their advances receive a warmer reception.
Since female status in the group hierarchy is determined by having a baby, the transfer may allow the female to achieve a higher status within a new group. The fact that the silverbacks form the first line of defense when the group is in danger means that when they are facing guns or spears, they are often killed. As groups break up due to the death of the silverbacks, the females may transfer to other groups, or to a lone silverback to form a new group.
FEEDING HABITSMountain gorillas are vegetarians. Leaves, shoots and stems make up 86 percent of the gorilla’s diet, and fruit only 2 percent. They spend most of their time on the ground, although they will occasionally climb trees to reach fruit when it is in season. At higher elevations, trees generally are not strong enough to support the weight of a massive mountain gorilla, so youngsters may do the fruit picking for the group.
The bulk of the mountain gorilla’s diet is gallium—a thin, scraggly vine related to what we call bedstraw. The gorilla also eats a great deal of the bitter wild celery, prickly thistles and stinging nettles. Gorillas also eat dirt to obtain necessary minerals such as sodium and potassium, and they eat their own dung, perhaps to obtain vitamin B12, which may pass through the upper intestine without being absorbed the first time around.
It is possible that gorilla brain development is a reflection of their diet. Compared to chimpanzees, gorillas have a smaller brain-to-body-size ratio. Chimpanzees, being primarily fruit eaters, have perhaps developed larger brains because they need to travel more widely and remember where fruiting trees are at different times of year. Gorillas, on the other hand, always have vegetation close at hand and don’t need to travel very far to get their next meal. They also have no need to out-compete other gorillas for a limited food source, so they do not need to be more clever than their compatriots.
ARE THEY REALLY “MOUNTAIN” GORILLAS?Dian Fossey discovered that mountain gorillas dislike rain and huddle up in groups, looking miserable during the downpours that frequently drench the mountains. This, plus the statements of both Schaller and Fossey about being constantly soaked to the skin after tramping through the forest with the gorillas, piqued the curiosity of American researcher Charles James. What, he wondered, were animals that loved to sunbathe and that did not seem well adapted to a cold, damp climate doing in the Virungas at elevations of more than 10,000 feet, where it is often damp, misty, rainy and chilly? Why did it often seem that Schaller’s observations were at odds with Fossey’s when both were excellent observers?
The clues were everywhere. Schaller had observed more mountain gorillas between 1,500 and 5,000 feet than above 10,000 feet. He reported that they preferred secondary growth to the sparse undergrowth of mature forest. He counted 100 plant species in their diet while Fossey counted 58. Fossey observed the gorillas were susceptible to respiratory diseases, particularly pneumonia. She pointed out that the boundaries of the Parc des Volcans had been reduced in 1959 and again in 1966, and that the Rwandan government was considering reducing the park by another 40 percent. Each time the park was reduced, the boundaries were moved higher up the mountain in order to open the lower slopes to cultivation.
The pieces of the puzzle began to fall into place. Perhaps the mountain gorilla is not native to the mountains; perhaps it is retreating up the mountains as an ever-increasing human population occupies the arable lower elevations. At least half the plants mountain gorillas consider edible do not grow in the cooler climate of the higher elevations. Although Schaller speaks of the gorillas moving up the mountain to eat bamboo shoots each season, Fossey mentions the gorillas going down to the bamboo zone. The gorillas are miserable-looking in the drenching rains and being cold and wet causes them to burn enormous amounts of energy just to maintain body temperature. That does not leave adequate reserves to fight off respiratory infections, which may eventually develop into pneumonia.
The higher the gorillas go, the less secondary growth there is and, hence, the less food is available. Numerous animal behavior studies have demonstrated that crowded conditions cause social disruption. This may explain why Schaller saw no fighting between groups of gorillas, while Fossey saw a great deal. (Of course, it may also be that Fossey saw more fighting because she observed the animals for 18 years compared to Schaller’s year and a half.) Schaller’s map of home ranges shows considerably less overlap between groups than Fossey’s. Schaller describes the gorilla’s dung as “quite solid, not at all messy, and the hair of the apes is not soiled by contact.” Fossey reports numerous instances of diarrhea, a recognized ape response to stress and agitation.
Although Fossey itemizes a number of adaptations to the cold, wet climate of the mountains, where night temperatures can drop below freezing, there is no mention of adaptations to serious cold such as a thick layer of fat to maintain body heat, a coat of underfur to retain surface warmth, or a coarse or oily top coat of fur that sheds water. All of these points suggest the mountain gorilla might not be endemic to the mountains, as has been assumed, but instead migrates vertically following seasonal forage. With the establishment of the gorilla conservation area and the constant pushing back of the boundaries to higher and higher elevations, it may be the gorillas have become trapped in an unsuitable environment that would limit their numbers, even if all poaching and the capturing of wild gorillas were stopped entirely.
There is danger in this sort of speculation, but the fact remains that a close study of Schaller’s and Fossey’s work makes one wonder if they were studying the same animals. The differences between theory and practice are all too apparent in the case of the mountain gorilla. A substantial cause of the ever-shrinking park boundaries is the Common Market’s decision to purchase pyrethrum (a daisy that produces a natural insecticide) from Rwanda in order to provide hard currency for that desperately poor country. The land to grow the pyrethrum has been taken from the Parc des Volcans, the sanctuary set aside for the gorillas, where Schaller did the first census of the mountain gorilla in 1960.
The Rwandan sector of the Parc des Volcans sanctuary has been halved, reducing the area by 50 percent. (The Parc des Volcans is made up of land in Congo, Uganda and Rwanda and has a different name in each country.) Fossey declared that this loss of land and forage alone (without taking poaching into account) was enough to account for the 60 percent reduction in the mountain gorilla population in the Rwandan sector of the Parc des Volcans, which she noted after Schaller’s census.
CONSERVATIONAlthough their numbers have increased in recent decades, mountain gorillas face many threats to their survival. Their habitat is always under pressure, as the lands around these national parks have some of the fastest-growing human populations in the world. Poaching is a constant danger, particularly in the Democratic Republic of Congo, and disease transmission from humans is a concern when locals enter the forest to harvest food, wood or medicines. Also, with growing mountain gorilla populations constrained by a very limited range, inbreeding is a concern that researchers are carefully watching.
Perhaps the biggest threat to the mountain gorilla is political instability in the region. Although Rwanda and Uganda are both very stable, the DRC is not, and if unrest spilled across the borders, a decline in tourism could reduce the support from the governments and communities to protect the gorillas.
Header Credit: Steven Schonfeld
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