Gorilla

About the gorilla

The gorilla profiled here is the western gorilla (Gorilla gorilla), one of two gorilla species and a member of the great ape family. It is geographically isolated from its relative, the eastern gorilla, and is the second largest living primate after that species. Two subspecies make up the population: the western lowland gorilla, found across most of West Africa and numbering around 276,000 individuals, and the Cross River gorilla, confined to a much smaller range on the border of Cameroon and Nigeria, with only 250 to 300 animals thought to remain. Both subspecies carry the same critically endangered status.

Western gorillas are generally lighter in color than eastern gorillas, with black, dark grey, or dark brown-grey hair and a brownish forehead. Males average 1.67 m (5.5 ft) tall and can reach 1.76 m (5.8 ft), weighing 145 to 191 kg (320 to 421 lb) in the wild; females weigh 58 to 72 kg (128 to 159 lb). Gorillas in captivity tend to run heavier, averaging 157 kg (346 lb) for males and 80 kg (176 lb) for females. The Cross River gorilla can be told apart from the western lowland gorilla by differences in skull and tooth dimensions.

The species was formally described in 1847 by Thomas Savage, who grouped it with the chimpanzee under an early classification before it was recognized as its own genus. Western gorillas live in groups of two to twenty animals, built around a dominant male silverback, several females, and their offspring.

Behavior and diet

A gorilla group centers on one silverback male; younger males typically leave once they mature, while females transfer into new groups before they begin breeding at eight or nine years old. Mothers care for infants for the first three to four years of life, and because of this long dependency period, a female will usually only raise one offspring to maturity every six to eight years. That slow reproductive pace is part of why the species is so vulnerable to poaching. A group's home range can run as large as 30 km² (12 sq mi), though groups do not actively defend it. Western gorillas are known to use tools in the wild, and researchers have documented them throwing sticks and clumps of grass at perceived threats — an unusual reaction, since western gorillas more typically flee from encounters with humans rather than confront them.

Diet is high in fiber and includes leaves, stems, ferns, fruit, piths, flowers, bark, invertebrates, and soil. What a group eats, and how often, varies by location and season, and different groups favor different plants and invertebrates — evidence, researchers suggest, of a kind of food culture. Fruit dominates the diet when it is available and drives how far and how long gorillas travel each day; favored fruits come from the genera Tetrapleura, Chrysophyllum, Dialium, and Landolphia. In the dry season, from January to March, when fruit is scarce, gorillas fall back on tougher vegetation such as the leaves and bark of Palisota and Aframomum. Termites and ants make up most of the animal matter in the diet, with caterpillars, grubs, and larvae eaten more rarely.

Some of what western gorillas eat may serve purposes beyond nutrition. Gorillas in Gabon consume the fruit, stems, and roots of Tabernanthe iboga, a plant containing the compound ibogaine, which acts on the central nervous system and produces effects comparable to both hallucinogens and caffeine. Seed pods of Aframomum melegueta, a regular part of the diet for many wild populations, appear to carry some cardiovascular benefit as well.

Range and habitat

Western gorillas live in a region of midwest Africa, separated geographically from the eastern gorilla. The western lowland gorilla subspecies occupies most of West Africa, while the Cross River gorilla is limited to a small area straddling the Cameroon-Nigeria border, with the nearest population of western lowland gorillas roughly 250 km (160 mi) away. Surveys by the Wildlife Conservation Society in 2006 and 2007 turned up around 125,000 previously unreported gorillas living in the swamp forests of the Lake Tele Community Reserve and in the neighboring Marantaceae dryland forests of the Republic of the Congo — a discovery that could more than double earlier population estimates for the species. With that finding factored in, the current population of western lowland gorillas may run between 150,000 and 200,000, even as the animals remain vulnerable to disease, deforestation, and poaching.

Conservation status

The western gorilla is listed as Critically Endangered, the most severe classification short of extinction, on the IUCN Red List. The Ebola virus has been especially damaging: it reduced populations in protected areas by 33% between 1992 and 2007, a decline that researchers estimate could equal 45% over the longer period from 1992 to 2011. Poaching, commercial logging, and civil wars across the gorillas' range compound the threat, and the species' very low reproductive rate — a maximum intrinsic growth rate of about 3% — means that hunting and disease losses have driven population declines of more than 60% over the past 20 to 25 years. Even under optimistic scenarios, full recovery would take close to 75 years, and continuing habitat loss from agriculture, timber extraction, mining, and climate change is expected to push the overall decline past 80% over three generations, or 66 years, running from 1980 to 2046.

For the Cross River gorilla, the situation is more acute still: an estimated 250 to 300 individuals remain, spread across nine to eleven locations, though genetic research suggests some migration between them. Habitat loss and hunting for bushmeat have driven this subspecies' decline. In response, Cameroon created Takamanda National Park on its border with Nigeria, forming a transboundary protected area with Nigeria's Cross River National Park that together safeguard an estimated 115 Cross River gorillas — about a third of the subspecies' total population — with the hope that animals can move freely between the two reserves.