Formerly included with Ostriches in the Struthioniformes, Rheas are now classified in their own order, the Rheiformes. This species formerly placed in the genus Pterocnemia based on differences in tarsal feathering and number of tarsal scutes is now included in the genus Rhea. Some authors (e.g. HBW) split the species into "Darwin's Rhea" found in Patagonia, and "Puna Rhea" (R. tarapacensis) found in Altiplano. Under that taxonomy, this is Darwin's Rhea. However I prefer Clements, IOC and Howard & Moore who treat these forms as subspecies of Lesser Rhea. The two races are separated by a large area of unsuitable habitat and are believed to have diverged after the rise of the Andes which isolated the two populations. Both are listed as "near threatened" by Birdlife International. The primary threats are hunting, egg collecting and habitat fragmentation. Their mating system includes both Polyandry and Polygyny. Males mate with multiple females and females mate with multiple males. The nest is attended only by the male who incubates and cares for young from multiple females.