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When you see the large, dark imposing shape of a gorilla mere metres from you, it is hard to conceive that we humans share about 98% of our DNA with them. These are the largest and most powerful primates on the planet, yet so peaceful in their mannerisms and habits. There are two gorilla species in the world: the eastern gorilla and the western gorilla. The endangered mountain gorilla is a subspecies of the eastern gorilla and is found in east-central Africa in just two isolated groups – the Virunga Volcanoes and the Bwindi Impenetrable Forest. Their favoured high-altitude forest habitat is surrounded by agriculture and settlements and the crimping of habitat, exposure to human illness and the bushmeat trade has seen to the reduction of the surviving population to around 1,000 individuals. Seeing gorillas in the wild is indeed a pleasure and privilege.
A band of gorillas
Adapted from Game Ranger in your Backpack © Megan Emmett
Eyes face forward and are relatively close together, giving gorillas excellent depth perception. The position of their eyes and pronounced brow ridges aid in non-verbal communication.
Like humans, gorillas have hairless lips that enhance their facial expressions and can be manipulated for effective feeding. Short muzzles are typical of all primates.
Gorillas have 32 teeth, same as humans, and also have two sets in a lifetime (baby teeth are replaced by a permanent set in adulthood). Teeth are strong to accommodate the coarse plant material they consume. Males develop large, sharp canines as they mature, used in defence and competition.
Gorillas have a distinctive body shape, with their stomachs being larger than their chests. This is to accommodate the large intestines needed to digest the bulky fibre they eat.
Gorillas show extreme sexual size dimorphism: females weigh 70–90 kg (155-200 lb) and males up to 200 kg (440 lb). This trait is common in animals where one sex is the main protector of the group or where competition for mating rights is only between one sex, in this case, the males.
A very strong upper body armed with long, powerful arms enables climbing activities and other movement, although fully-grown adult gorillas rarely climb trees. Gorillas are quadrupedal.
This means they generally move around on all fours, but they occasionally use bipedal (two-legged) motion too. They have a larger arm to height ratio than humans, which also suits their quadrupedal lifestyle because they actually use their hands while walking, supporting their weight on their knuckles. They have a large pelvis, and slightly outward curving legs, which also facilitates their quadrupedal gait, with their legs moving out slightly and then forward as they go.
Like other primates, gorillas have opposable thumbs, an adaptation which evolved to suit a life in the trees. Gorillas spend a fair amount of time climbing, either to find food or get to safety.
This important adaptation serves other useful purposes too. Having opposable thumbs is very handy when grasping and breaking leaves, branches or fruit and for detailed manipulation of these food items. They also have semi-opposable feet. Their big toe is longer, very flexible and situated lower on their foot compared to that of humans. This helps with the grasping and climbing behaviours that are so important in their day-to-day life.
Gorillas generally prefer to live in thickly forested areas (including rainforests and bamboo forests) but they do occasionally wonder into the more open areas on the edges of forests.
Family groups can have home ranges varying in size from about 200 – 4500 hectares (495 – 11 120 acres). There are two main populations of mountain gorillas, one in the mountainous forests of Uganda in Bwindi Impenetrable Forest, and one in Rwanda in Volcanoes National Park.
Diet consists almost exclusively of plant material and gorillas are very selective foragers eating the roots, stem, leaves or fruit of different plant species to obtain the best nutrition.
Because they live in lush, productive environments, they don’t have to move far for food, averaging between 500–1,000 m (1,640-3,280 ft) per day. Gorillas have been known to occasionally eat larvae, snails, ants, termites, bark and rotting wood (a good source of sodium).
A typical day follows a familiar routine where animals begin foraging in the morning, rest during the heat of the day and then forage again before bedding down for the evening.
They spend about a quarter of their day foraging in total. Younger gorillas occasionally climb trees to look for food, but adults spend most of their time on the ground. Groups sleep together in nests on the ground, or in trees, that they make from foliage. Infants share their mother’s nests for warmth and comfort.
Gorillas have 16 different types of calls, although both verbal and non-verbal communication play a crucial role in gorilla society. The most common sound uttered is a content “ohh-ohh-ohh”.
This is used to let members of the family know that everything is fine and to let each other know where they are when feeding in very dense forests. Short barks are used to indicate alarm or curiosity. A variety of other sounds indicate aggression, distress, contentment or are used to coordinate the family’s activities. One of the most common expressions used – especially in younger gorillas – is a playful one, which usually consists of an open mouth with a low-hanging bottom lip but no teeth exposed, combined with the gorilla grunting equivalent of laughter. On the non-verbal side, dominance signalling is common and can be as subtle as moving out of the way of a dominant individual. More intense displays include chest beating, stand-off stances, vegetation breaking and throwing, or even more aggressive behaviours like hitting. Chest beating can be heard over one kilometre away and is a direct indicator of the size of a male gorilla.
Mountain gorillas live in stable family groups of around 10 individuals known as troops. Troops are led by a single, older, dominant male called the silverback due to the patch of silver hair covering his otherwise dark lower back.
The rest of the troop is made up of several young males, some females and their offspring. There is often a subordinate adult male in the troop, almost fully grown but without the silver fur, known as a blackback (usually males between eight and 12 years old). There are a few records of silverbacks coexisting and of both producing offspring. Typically, if a silverback hands over dominance to the next in line, he is then allowed to remain in the family as long as he doesn’t try to exert mating rights.
Females become sexually mature around seven or eight years old but don’t begin to breed until they are eight or ten years old. Males mature at an even later stage. Females give birth to only one baby every four to six years.
Generally, a female will only raise three or four youngsters in her entire lifetime. The gestation is similar in length to humans and youngsters only wean between three and five years of age. Only after that will the female mate again.
Newborns are weak and weigh just 2 kg (4 lb), but they develop quickly, with their movements awkward to start. Infants nurse and are gradually weaned at an age of three to five years old once they are more independent.
Care of the infants is shared by the whole family, with both males and females in the group embracing, carrying and playing with them. Babies are classed as infants until they reach around three-and-a-half years old, and adults from around 8 years.
The majority of males leave their biological groups at around 11 yrs old. Others may opt to stay within their natal family and queue for dominance. Males that leave may move alone or in bachelor groups for a few years.
They do this until they can attract females to join them. Around 60% of females also leave their birth group to join other troops, as this prevents inbreeding.
The more you learn about the dignity of the gorilla, the more you want to avoid people
Dian Fossey