Gorillas comprise the eponymous genus Gorilla, the largest extant genus of primates by size. They are ground-dwelling, predominantly herbivorous apes that inhabit the forests of central Africa. The genus is divided into two species and either four or five subspecies. The DNA of gorillas is highly similar to that of a human, from 9599% depending on what is counted, and they are the next closest living relatives to humans after the bonobo and common chimpanzee. Gorillas' natural habitats cover tropical or subtropical forests in Africa. Although their range covers a small percentage of Africa, gorillas cover a wide range of elevations. The mountain gorilla inhabits the Albertine Rift montane cloud forests of the Virunga Volcanoes, ranging in altitude from 2,2004,300 metres (7,20014,100 ft). Lowland gorillas live in dense forests and lowland swamps and marshes as low as sea level, with western lowland gorillas living in Central West African countries and eastern lowland gorillas living in the Democratic Republic of the Congo near its border with Rwanda. The American physician and missionary Thomas Staughton Savage and naturalist Jeffries Wyman first described the western gorilla (they called it Troglodytes gorilla) in 1847 from specimens obtained in Liberia. The name was derived from Greek (Gorillai), meaning "tribe of hairy women", described by Hanno the Navigator, a Carthaginian navigator and possible visitor (circa 480 BC) to the area that later became Sierra Leone. Evolution and classification The closest relatives of gorillas are chimpanzees and humans, all of the Hominidae having diverged from a common ancestor about 7 million years ago. Human genes differ only 1.6% on average from their corresponding gorilla genes in their sequence, but there is further difference in how many copies each gene has. Until recently, gorillas were considered to be a single species, with three subspecies: the western lowland gorilla, the eastern lowland gorilla and the mountain gorilla. There is now agreement that there are two species with two subspecies each. More recently, a third subspecies has been claimed t exist in one of the species. The separate species and subspecies developed from a single type of gorilla during the Ice Age, when their forest habitats shrank and became isolated from each other. Primatologists continue to explore the relationships between various gorilla populations. The species and subspecies listed here are the ones upon which most scientists agree. Gorillas move around by knuckle-walking, although they sometimes walk bipedally for short distances while carrying food or in defensive situations. Wild males gorillas weigh 135 to 180 kg (300 to 400 lb) while adult females usually weigh half as much as adult males at 68113 kg (150250 lb). Adult males are 1.7 to 1.8 m (5.6 to 5.9 ft) tall, with an arm span that streaches from 2.3 to 2.6 m (7.5 to 8.5 ft). Female gorillas are shorter with smaller arm spans. Occasionally, a silverback of over 1.8 metres (5 ft 11 in) and 230 kg (510 lb) has been recorded in the wild. Obese gorillas in captivity can weigh as much as 270 kg (600 lb). Gorilla facial structure is described as mandibular prognathism, that is, the mandible protrudes farther out than the maxilla. Adult males also have a prominent sagittal crest. The eastern gorilla is more darkly colored than the western gorilla, with the mountain gorilla being the darkest of all. The mountain gorilla also has the thickest hair. The western lowland gorilla can be brown or grayish with a reddish forehead. In addition, gorillas that live in lowland forests are more slender and agile than the more bulky mountain gorillas. The eastern gorilla also has a longer face and broader chest than the western gorilla. Studies have shown gorilla blood is not reactive to anti-A and anti-B monoclonal antibodies, which would, in humans, indicate type O blood. Due to novel sequences, though, it is different enough to not conform with the human ABO blood group system, into which the other great apes fit. Like humans, gorillas have individual finger prints. Their eye color is dark brown, framed by a black ring around the iris. Similar to humans, the leading cause of death in gorillas is cardiovascular disease