Rhea is a bird that is also known as the South American ostrich. The rheas are large ratites (flightless birds without a keel on their sternum bone) in the order Rheiformes, native to South America, related to the ostrich and emu. Rheas are large, flightless birds with grey-brown plumage, long legs and long necks, similar to an ostrich. Large males of R. Americana can reach 170 cm (67 in) tall at the head, 100 cm (39 in) at the back and can weigh up to 40 kg (88 lb.). The lesser rhea is somewhat smaller as they are only 90 cm (35 in) tall at the back.