The western lowland gorilla (Gorilla gorilla gorilla) is one of two Critically Endangered subspecies of the western gorilla (Gorilla gorilla) that lives in montane, primary and secondary forest and lowland swampland in central Africa in Angola, Cameroon, Central African Republic, Republic of the Congo, Democratic Republic of the Congo, Equatorial Guinea and Gabon. It is the nominate subspecies of the western gorilla, and the smallest of the four gorilla subspecies. The western lowland gorilla is the only subspecies kept in zoos with the exception of Amahoro, a female eastern lowland gorilla at Antwerp Zoo, and a few mountain gorillas kept captive in the Democratic Republic of the Congo. The western lowland gorilla is the smallest subspecies of gorilla but still has exceptional size and strength. This species of gorillas exhibits pronounced sexual dimorphism. They possess no tails and have jet black skin along with coarse black hair that covers their entire body except for the face, ears, hands and feet. The hair on the back and rump of males takes on a grey coloration and is also lost as they get older. This coloration is the reason why older males are known as "silverbacks". Their hands are proportionately large with nails on all digits (similar to those of humans) and very large thumbs. They have short muzzles, prominent brow ridges, large nostrils and small eyes and ears. Other features are large muscles in the jaw region along with broad and strong teeth. Among these teeth are strong sets of frontal canines and large molars in the back of the mouth for grinding fruits[4] and vegetables.[5] A male standing erect can be up to 1.83 m (6 ft 0 in) tall and weigh up to 227 kg (500 lb).[6] Males have an average weight of 140 kg (310 lb), females of 90 kg (200 lb).[7] Males in captivity, however, are noted to be capable of reaching weights up to 275 kg (606 lb).[5] Males stand upright at 1.67 m (5 ft 6 in), females at 1.5 m (4 ft 11 in).[8][6] Zoo owner John Aspinall claimed a silverback gorilla in his prime has the physical strength of seven or eight Olympic weightlifters, but this claim is unverified. Western gorillas frequently stand upright, but walk in a hunched, quadrupedal fashion, with hands curled and knuckles touching the ground; as a result, their arm span is greater than their standing height.