The rheas (/ˈriːə/) are large ratites (flightless birds without a keel on their sternum bone) in the order Rheiformes, native to South America, distantly related to the ostrich and emu. Most taxonomic authorities recognize two extant species: the greater or American rhea (Rhea americana) and the lesser or Darwin's rhea (Rhea pennata). The IUCN lists the puna rhea (Rhea tarapacensis) as a separate species. 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 smaller, they are 100 cm (39 in) tall. Their wings are large for a flightless bird (250 cm (8.2 ft) and are spread while running, to act like sails. Unlike most birds, rheas have only three toes.