2. Everything you never thought you needed to know about the Gorilla

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The Gorilla
Infraorder: Catarrhini
Superfamily: Hominoidea
Family: Pongidae

Often referred to as the Great Apes, the Pongidae include the orang-utans (Pongo pygmaeus), chimpanzee (Pan troglodytes), the bonobo (Pan paniscus), the gorilla (Gorilla gorilla) and humans (Homo sapiens).

This family is controversial as humans are normally put in their own family, but the genetic distance between Homo and the other great apes is extremely small. It would also not be cladistic to group gorilla and chimpanzees in their own family and the humans in another as humans, and chimpanzees appear to share the most recent common ancestor with the gorillas being a sister group to them both.

The great apes differ from the Hylobatidae in many small details. For example, there is a Y-shaped fissure in the lower molars caused by having five cusps and their brains are larger and more complex. The pongids have the dental formula 2/2, 1/1, 3/3, 3/3 = 36 and have a diet of fruit, leaves, insects and meat (though gorillas and orangutans do not eat meat). Their behaviour and social structure is variable intra-specifically and inter-specifically they range from monogamous to polygynous to male bonded multi-male, multi-female groups.

Genus: Gorilla

The Gorillas great strength, size and impressive male displays have given the Gorilla an unfair reputation as a brute (see King Kong). Gorillas are extremely intelligent and less belligerent than chimpanzees. It is thought that they use plants as medicines and in captivity have been able to learn several hundred words in sign language (a fact once doubted).

Gorilla groups are held together by a dominant male, the silver back. If this male dies suddenly, rather than being ousted by another male, the group of unrelated females and their offspring usually disband. Female bonding is therefore not a strong force in gorilla culture.

Gorillas are not entirely folivorous, lowland gorillas have a large amount of fruit in their diet, often eating the same foods as chimpanzees. This is contradictory to the belief that animals inhabiting the same ecosystem will diversify their diets to avoid competition.

Status

Gorillas are the largest of the great apes, and among the world’s most endangered species. All gorilla subspecies are listed as endangered on the World Conservation Union’s Red List of Threatened Animals.

Their precise taxonomic hierarchy has been hotly debated but they can be viewed as two species with four sub-species. The two species would be the western, including the Cross River gorilla (Gorilla gorilla diehli) and the western lowland gorilla (Gorilla gorilla gorilla); and the eastern, including the eastern lowland gorilla (Gorilla berengei graueri) and the mountain gorilla (Gorilla berengei berengei).

Mountain gorillas have been widely studied in the wild since the 1950s. Only about 700 are left. The western lowland gorilla is much more common, with a population size of about 94,000. However, recent surveys show they are declining rapidly due to poaching and disease.

Habitat

Gorillas live in moist tropical forests, often in secondary, or re-growing, forests or along forest edges, where clearings provide an abundance of low, edible vegetation. Mountain gorillas range up into cloud forest.

Diet in the Wild

Gorillas are primarily herbivorous, eating the leaves and stems of herbs, shrubs, and vines. In some areas, they raid farms, eating and trampling crops. They also will eat rotten wood and small animals.

The diet of western lowland gorillas also includes the fleshy fruits of close to a hundred seasonally fruiting tree species; the diets of other gorilla subspecies include proportionally less fruit. Gorillas get some protein from invertebrates found on leaves and fruits. Adult male gorillas eat about 45 pounds (32 kg) of food per day. Females eat about two-thirds of that amount.

Reproduction

Female gorillas reach maturity at seven or eight years old, but they usually don’t breed until ten years or older.

Due to competition between males for access to females, few wild males breed before they reach 15 years old. Eight and a half months after mating, a female gives birth to one young, which can usually walk within three to six months. Young are usually weaned by three years old, and females can give birth every four years.

Upon reaching sexual maturity, between ages seven and ten, young gorillas strike out on their own, seeking new groups or mates. Zoo gorillas may reach sexual maturity before seven years old, and may have young every two to three years.

Life Span

Gorillas may live about 35 years in the wild, and up to 54 in zoos.

Behaviour

Gorillas live in groups. Each group usually contains one or more silverbacks and two to ten females and young. Newly established silverbacks may kill young not sired by them, but otherwise, gorilla family life is mostly peaceful.

Bloody battles sometimes occur between silverbacks when they square off to compete over female groups or home ranges. Gorillas spend their mornings and evenings feeding, usually covering only a small area of forest at a time. Groups spend the middle of the day sleeping, playing, or grooming (females groom their young or a silverback). At night, gorillas fashion nests of leaves and branches on which to sleep; un-weaned infants sleep in their mothers’ nests.

Social Structure

Gorillas are behaviourally flexible. This means that their behaviour and social structure is not set in stone; there is great variety. The information below should only be used as a general guide.

Gorillas live in groups, or troops, from two to over 30 members. Western lowland data seem to indicate smaller group sizes, averaging about five individuals. Groups are generally composed of a silverback male, one or more black back males, several adult females, and their infant and juvenile offspring. This group composition varies greatly due to births and deaths and to the immigration and emigration of individuals.

Mature offspring typically leave their natal group to find a mate. At about eight years old, females generally emigrate into a new group of her choosing. She seems to choose which silverback to join based on such attributes as size and quality of his home range, etc. This seems to be related to the silverback’s size, but not always.

A female may change family groups a number of times throughout her life. When leaving their natal group, some sexually mature males may attempt to replace the silverback in an already established group. However, they usually spend a few years as solitary males. Nevertheless, a new troop can be easily formed when one or more non-related females join a lone male.

The group is lead by the adult, dominant, silverback male. He has exclusive breeding rights to the females. At times he may allow other sub-adult males in the group to mate with females. The silverback mediates disputes and also determines the group’s home range. He regulates what time they wake up, eat and go to sleep.

Gorillas are most active in the morning and late afternoon. They wake up just after sunrise to search for food, and then eat for several hours. Midday, adults take a siesta and usually nap in a day nest while the young wrestle and play games. After their midday nap they forage again. Before dusk each gorilla makes its own nest, infants nest with their mothers.

All gorillas over three years make nests, day nests for resting and night nests for sleeping. Infants share their mothers’ nests. Gorillas form nests by sitting in one place and pulling down and tucking branches, leaves, or other vegetation around themselves. Adult males usually nest on the ground.

Females may nest on the ground or in trees. Juveniles are more apt to nest in trees. Studies of western lowland gorillas have shown that the number of nests found at a site does not necessarily coincide with the number of weaned animals observed in a group.

The western lowland gorilla is characterized as a quiet, peaceful, and non-aggressive animal. They never attack unless provoked. However, males do fight over acquisition and defence of females, and the new leader of a group may kill unrelated infants. This causes the females to begin cycling sooner.

An adult male protecting his group may attempt to intimidate his aggressor by standing on his legs and slapping its chest with cupped or flat hands while roaring and screaming. If this elaborate display is unsuccessful and the intruder persists, the male may rear his head back violently several times. He may also drop on all fours and charge toward the intruder. In general, when they charge they do not hit the intruder. Instead, they merely pass them by.

This demonstration of aggression maintains order among separate troops and reduces the possibility of injury. It is thought that size plays an important role in determining the winner of an encounter between males (the larger male wins). Because of gorilla variability, some or all of these behaviours may not be seen.

Gorillas exhibit complex and dynamic relationships. They interact using grooming behaviours, although less than most other primates. Also affiliation may be shown by physical proximity.

Young gorillas play often and are more arboreal than the large adults. Adults, even the silverback, tolerate infant play behaviour. He also tolerates, to a lesser extent, and often participates in the play of older juveniles and black back males.

The duration and frequency of sexual activity in gorillas are low in comparison to other great apes. The silverback has exclusive mating rights with the adult females in his group. The reproductive success of males depends upon the maintenance of exclusive rights to adult females. The female chooses to mate with the silverback by emigrating into his family group. Normally quiet animals, some gorillas are unusually loud during copulation.

Communication

Gorillas communicate using auditory signals (vocalisations), visual signals (gestures, body postures, facial expressions), and olfactory signals (odours). They are generally quiet animals, grunting and belching, but they may also scream, bark, and roar. Dian Fossey heard 17 different kinds of sounds from mountain gorillas. Other scientists have heard 22 different vocalisations, each seeming to have its own meaning.

Gorillas crouch low and approach from the side when they are being submissive. They walk directly when confident and stand, chest beat (actually they slap with open hands), and advance when being aggressive.

Past/Present/Future

Until several decades ago, gorilla populations enjoyed the seclusion of vast tracts of forest. Today, Africa’s growing population puts many pressures on these declining primates. Logging roads snake into forests, opening frontiers to settlers and loggers, while hunters kill or capture gorillas for their meat, parts (sometimes sold as souvenirs), or because the animals raid farm fields. Gorilla meat is eaten by hunters and loggers, and is also sold in city markets and restaurants.

While protection laws exist in most countries still inhabited by gorillas, enforcement is often lacking. Civil wars in Rwanda and the Democratic Republic of Congo have harmed conservation efforts in these countries and opened parks to poachers. Gorillas also stumble into snares set for other animals, and may be killed or injured. Increased political stability, better public awareness, and carefully protected parks would go a long way toward reversing the gorillas’ decline.

A Few Gorilla Neighbours

Leopard (Panthera pardus): The only animal in its range, aside from humans, that can harm an adult gorilla, although these animals rarely tangle with each other.

African elephant (Loxodonta africana): By downing trees, forest-dwelling elephants help create gorilla feeding areas.

African grey parrot (Psittacus erithacus): Despite a large range, this forest parrot is disappearing from many areas due to capture for the cage bird trade and forest cutting. By saving gorilla habitat, one can protect these and many other animals.

Fun Facts

Despite their size and current popularity, gorillas remained a mystery to people living outside of Africa until a missionary described them in 1847.

After chimpanzees, gorillas are our closest relatives, sharing about 98 percent of our genes.

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Freely adapted for internal use from sources at:

© The Smithsonian National Zoological Park

© The BBC