Although several other species are known from the fossil record, the only extant representative of the Opisthocomidae is the Hoatzin (Opisthocomus hoazin). The Hoatzin is a pheasant-sized bird of the Amazon and Orinoco basins which feeds on leaves and fruit and is noted for the wing-claws possessed by the young birds. Uniquely among birds, it uses fermentation in its digestive system to break down its vegetable diet, similar to the method used by cattle and other ruminant mammals.