The specific name was bestowed in 1837 by Darwin's contemporary and rival Alcide d'Orbigny, who first described the bird to Europeans from a specimen from the lower Río Negro south of Buenos Aires, Argentina.

[12] Three subspecies have traditionally been recognized: The IUCN formerly considered the two northern taxa R. p. tarapacensis and R. p. garleppi as a separate species, the puna rhea (R.

The lesser rhea is mainly a herbivore, with the odd small animal (lizards, beetles, grasshoppers) eaten on occasion.

[7] Darwin's rhea lives in areas of open scrub in the grasslands of Patagonia and on the Andean plateau (the Altiplano), through the countries of Argentina, Bolivia, Chile, and Peru.

[15] During the second voyage of HMS Beagle, the young naturalist Charles Darwin made many trips on land, and around August 1833 heard from gauchos in the Río Negro area of Northern Patagonia about the existence of a smaller rhea, "a very rare bird which they called the Avestruz Petise".

He continued searching fruitlessly for this bird, and the Beagle sailed south, putting in at Port Desire in southern Patagonia on 23 December.

On the following day, Darwin shot a guanaco (similar to a llama) which provided them with a Christmas meal, and in the first days of January, the artist Conrad Martens shot a rhea which they enjoyed eating before Darwin realised that this was the elusive smaller rhea rather than a juvenile, and preserved the head, neck, legs, one wing, and many of the larger feathers.

On 26 January the Beagle entered the Straits of Magellan, and at St Gregory's Bay Darwin met Patagonians he described as "excellent practical naturalists".

As every living being had been created in a fixed form, as accepted by the science of his time, they could only change their appearance by a perfect adaptation to their way of life, but would still be the same species.

[15] However, they are classified as Rhea tarapacensis by the IUCN, which regards it as being near threatened, with the primary threats being hunting, egg-collecting, and fragmentation of its habitat due to conversion to farmland or pastures for cattle-grazing.

[15][7] Patagonia National Park in Chile's Aysén Region hosts the Centro de Reproducción para la Conservación del Ñandú ('Reproduction Centre for Darwin's rhea Conservation').