Formerly included with Ostriches in the Struthioniformes, Rheas are now classified in their own order, the Rheiformes. Formerly placed in the genus Pterocnemia based on differences in tarsal feathering and number of tarsal scutes, but now included in Rhea. Some authors (e.g. HBW) split this species into "Darwin's Rhea," this bird found in Patagonia, and Puna Rhea (R. tarapacensis) found in Altiplano. I follow 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 from 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 is 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.
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