Rheas belong to the order Struthioniformes, or ratites. There are two genera within this family, Rhea and Pterocnemia, and most classification schemes recognize one species within each genus. They are found exclusively in South America, usually in open grasslands (campo).

Rheas are tall, long-legged birds, well adapted to a terrestrial lifestyle. They are excellent runners, achieving speeds of up to 60 km/h. Their wings lend them great maneuverability while running. These paleognath birds are characterized by large, fenestrated maxillo-palatines that do not articulate with the vomer, and short palatines that do articulate with the vomer. They have a tracheobronchial syrinx with one pair of intrinsic muscles and modified bronchial and tracheal rings. Their tarsi are scutellated, and like most other ratites, they have three toes and short middle phalanges. Their plumage is greyish-brown, and the sexes are not very dimorphic, though males develop a black ring on the neck when in breeding plumage. Rheas lack oil glands, and their feathers lack hooks, making them unusually soft. Unlike the similar-looking ostriches, the head, neck, and thighs of rheas are feathered. They do not have retrices (tail feathers), but do have a claw on each wing.

The diet of rheas consists mostly of insects, other small invertebrates, and plant matter. They obtain most of the water they need from the vegetation they eat.

Though they could probably outrun most (if not all) predators, they tend to run in a zig-zag fashion, making sharp turns, then doubling back to flatten their bodies against the dirt.

Rheas are gregarious in habit, and tend to live in flocks ranging in size from 5-30 individuals. While feeding they often mix with groups of various mammals, including pampas deer, alpacas, and cattle. During the breeding season, however, males claim territories and females travel in smaller groups. Rheas have an interesting breeding system, common in ratites, in which males are responsible for all of the parental care. Males incubate the eggs, and a complete clutch might consist of 13-30 eggs from several females, though extremes of 6 and 80 eggs have been recorded in a nest. They will defend the nest and the fledglings aggressively. Males and females are both polygamous.

Several populations of Rheas are in danger of extinction, mainly due to the conversion of their habitat to farmland. Puna rheas (Pterocnemia pennata garleppi) are in immediate danger, and the total population size is thought to only be several hundred birds. Greater rheas (Rhea americana) are not globally at risk, but populations have declined and are considered 'near-threatened'.

Fossil specimens of four different species belonging to Rheidae have been found, dating back approximately 40 million years, making them one of the oldest bird families to live in South America. Their closest living relatives are the Ostriches of Africa. The phylogeny of Rheidae is still under considerable debate, as they are sometimes given their own order, while other times classified as a subfamily. One accepted phylogeny classifies Rheidae as a family, with two species and several races. The two living species are greater rheas (Rhea americana), and lesser rheas (Pterocnemia pennata). The latter is sometimes broken up into 3 different species or subspecies with disjunct ranges, Darwin's rheas (P. pennata pennata), puna rheas (P. pennata garleppi), and P. pennata tarapacensis.

Bock, W.J. 1963. The cranial evidence for ratite affinities. Proc. XIII Intern. Ornith. Congr., p. 39-54.

Campbell, B. & E. Lack, eds. 1985. A dictionary of birds. Buteo Books, Vermillion, South Dakota.

del Hoyo, J., Elliott, A., & J. Sargatal. 1992. Handbook of the birds of the world, volume 1. Lynx

Edicions, Barcelona.

Payne, R.B. pers. obs.

Sick, Helmut. 1993. Birds in Brazil: a natural history. Princeton Univ. Press, Princeton.

Thomson, A. 1964. A new dictionary of birds. British Ornithologists' Union. McGraw-Hill Book Co., NY.


Danielle Cholewiak (author).



uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


having the capacity to move from one place to another.


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate


uses sight to communicate