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. There are two extant species: the greater or American rhea (Rhea americana) and the lesser or Darwin's rhea (Rhea pennata). Both are currently rated as near-threatened in their native ranges; a feral population of the greater rhea in Germany appears to be growing.