The Lesser or Puna Rhea is the only member of the Family Rheidae known to occur in Peru. Rheas are large, flightless birds with long legs and necks, similar to an ostrich. Their wings are large for a flightless bird. Unlike most birds, rheas do not have a keel on their sternum bone and have only three toes. They also store urine separately in an expansion of the cloaca. Rheas are polygamous, with males courting between two and twelve females.
After mating, the male builds a nest, in which each female lays her eggs. The male incubates from ten to sixty eggs. The male may use another subordinate male to incubate his eggs, while he finds another harem to start a second nest. The females, meanwhile, may move on and mate with other males. During the non-breeding season they may form flocks, but flocks break up during the breeding season.