Rheas are the South American version of the Ostrich and on the Pampas they fulfil a similar ecological niche, feeding mainly on vegetation, but taking insects and small vertebrates when possible.
There are two species of Rhea, the Greater or Common Rhea, Rhea americana, which is the larger of the two and stands about 1.6 m (5 ft) high and weighs about 25 kg (55 lbs). Pterocnemia pennata, or Darwin's Rhea, is slightly smaller and lives not only on part of the Pampas, but also high up in the Andes (up t0 5000 m or 16000 ft). Both species tend to live in medium sized flocks of 5-50 individuals. Rheas, like Ostriches, are adapted for running as a means of escaping predation and they run very quickly. Their wings are proportionally larger than those of other ratites and they use them to help them gain speed and stability for quick sharp turns.
Rheas travel freely for most of the season, but males become territorial in the breeding season. This begins with them evicting the other males from the flock. Younger males leave first, then the older males fight using their heads and necks until on dominant male emerges. He chooses a territory near some water and scrub and makes a nest there. After mating the females start laying and as soon as some eggs are laid the male starts incubating them and becomes very broody and protective - to such an extent that females wishing to lay more eggs cannot get to the nest and must lay them nearby. Many eggs are lost this way. Once he is firmly in place the flock goes off and the females mate with and lay eggs for another male. The eggs take 30-40 days to hatch and again, like other ratites, the young are precocial and ready to leave the nest within hours.
Rheas have never suffered the commercial interests the Ostriches have and though farming has intruded into their habitat, both populations are still healthy.