Female gorillas are shorter with smaller arm spans. Adult male gorillas are known as silverbacks due to the characteristic silver hair on their backs reaching to the hips. Occasionally, a silverback of over 1.8 m (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.