The western gorilla (Gorilla gorilla) is a great ape found in Africa, one of two species of the hominid genus Gorilla. Large and robust with males weighing around , the hair is significantly lighter in color than the eastern gorilla, Gorilla beringei, and geographically isolated from them in a region at the midwest of the African continent. Two subspecies are recognised, Gorilla gorilla diehli (Cross River gorilla) is limited to a smaller range in the north at the border of Cameroon and Nigeria. Both subspecies are listed Critically Endangered. Taxonomy A formal description of the species was provided by Thomas Savage in 1847, allying the new species to an earlier description of the chimpanzee as Troglodytes gorilla in a group of eastern simians he referred to as "orangs". The author selected the specific epithet for the name given by Hanno to "wild men" he had noted on the east coast of Africa, presumed by Savage to be a species of orang. The population is recognised as two subspecies: Nearly all of the individuals of this taxon belong to the western lowland gorilla subspecies, whose population is approximately 95,000 individuals. Only 250 to 300 of the only other western gorilla subspecies, the Cross river gorilla, are thought to remain. Description Western lowland gorillas are generally lighter colored than eastern gorillas. Western lowland gorillas have black, dark grey or dark brown-grey hair with a brownish forehead. Males average although reaching a height up to , with males having an average weight of and females weighing . Captive western gorillas average in males and in females. Another source describes the weight of wild male western lowland gorillas as . The Cross River gorilla differs from the western lowland gorilla in both skull and tooth dimensions. Behavior and ecology Western lowland gorillas live in groups that vary in size from two to twenty individuals. Such groups are composed of at least one male, several females and their offspring. A dominant male silverback heads the group, with younger males usually leaving the group when they reach maturity. Females transfer to another group before breeding, which begins at eight to nine years old; they care for their young infants for the first three to four years of their lives. The interval between births, therefore, is long, which partly explains the slow population growth rates that make the western lowland gorilla so vulnerable to poaching. Due to