Greater rhea
The Greater rhea (Rhea americana) is a large, flightless ratite bird and the largest species in the Americas, endemic to open South American habitats where it serves as an ecological engineer through seed dispersal and grazing.[1] Adults stand 1.3–1.7 m (4.3–5.6 ft) tall at the shoulder, weigh 20–40 kg (44–88 lb), and exhibit sexual dimorphism with males larger than females; their plumage is shaggy and gray-brown, with blacker tones on the head, neck, and back, while the underparts are whitish.[1][2] They possess long, powerful legs ending in three-toed feet for rapid sprinting up to 60 km/h (37 mph), a long neck for foraging, and vestigial wings that aid in balance and display but preclude flight, classifying them within the ancient ratite lineage alongside ostriches and emus.[1][2] Native to a vast range spanning northeastern and eastern Brazil, eastern Bolivia, Paraguay, Uruguay, and northern and central Argentina, the greater rhea inhabits lowland pampas, savannas, grasslands, and semi-open scrublands, often near water sources like rivers or marshes during breeding.[3][1] These birds are highly adaptable to human-modified landscapes, including agricultural fields and cattle pastures, though they prefer areas with low vegetation for visibility and escape.[1] Outside their native range, introduced populations exist in southeastern Brazil, an established population of about 600 in Germany, and small groups in the United States, primarily from escaped or released captives.[3][4] Greater rheas are social, diurnal omnivores that form loose flocks of 10–30 individuals outside breeding season, foraging on grasses, seeds, fruits, roots, and occasionally insects, small vertebrates, or even carrion, aided by ingested stones for grinding in their gizzard.[2][1] Breeding occurs from July to January in the Southern Hemisphere summer, with polygynandrous mating where dominant males court multiple females (up to 12), construct shallow ground nests, and solely incubate clutches of 10–60 large, yellowish eggs for 35–40 days before raising precocial chicks for up to six months.[2][1] When threatened, they rely on speed and zigzagging runs rather than confrontation, producing low-frequency booming calls primarily from males during courtship.[1] Currently classified as Near Threatened by the IUCN, greater rhea populations are declining across their range due to habitat loss from agricultural expansion and overgrazing, as well as illegal hunting for meat, eggs, and feathers; the global population size has not been quantified, but the species is described as uncommon to fairly common.[3] Conservation efforts include protected areas in key habitats like the Argentine pampas and regulated hunting quotas in countries such as Argentina and Brazil, where the species holds cultural and economic value in ranching communities.[3][1]Taxonomy
Etymology
The genus name Rhea was introduced in 1752 by the German naturalist Paul Heinrich Gerhard Möhring for the South American ratites, drawing from the name of the Titaness Rhea in Greek mythology, who was the mother of the gods and symbolized fertility and the earth; this choice likely reflected the bird's large size and ground-dwelling nature as a flightless species.[5] The specific epithet americana was coined by Carl Linnaeus in 1758, when he described the species as Struthio americanus in Systema Naturae, with "americana" denoting its origin in the New World to distinguish it from African ostriches.[6] Over time, the binomial nomenclature evolved, and by the early 19th century, the bird was reclassified under the genus Rhea as Rhea americana, solidifying its current scientific name.[1] In indigenous languages of South America, the greater rhea is known as ñandú, derived from the Guaraní term ñandú guazú, which literally translates to "big spider" and may allude to the bird's sprawling legs or the way it spreads its wings in displays, resembling a spider's posture. European explorers and naturalists initially misclassified the greater rhea as a relative of the African ostrich due to superficial similarities in appearance and flightlessness, leading to common names like "American ostrich" or "South American ostrich," as noted by Charles Darwin during his voyages.[7] This nomenclature persisted in early scientific literature, highlighting the bird's role as the largest native ratite in the Americas before more precise taxonomic distinctions were established.)Classification
The greater rhea (Rhea americana) is classified within the order Rheiformes and the family Rheidae, belonging to the paleognathous birds, a group characterized by ratite-like features such as reduced wings and a keelless sternum.[8][1] It was first described by Carl Linnaeus in 1758 under the binomial name Struthio americanus in the 10th edition of Systema Naturae, later reclassified into the genus Rhea.[9] Phylogenetically, the greater rhea is most closely related to the lesser rhea (Rhea pennata), with both forming the monophyletic family Rheidae, as confirmed by genetic analyses of nuclear and mitochondrial DNA sequences.[10] Rheas are part of the broader ratite clade, which includes ostriches (Struthio), emus (Dromaius), cassowaries (Casuarius), and kiwis (Apteryx), though recent phylogenomic studies indicate ratites are polyphyletic, with flightless evolution occurring multiple times independently.[10] The divergence between the Palaeognathae (encompassing ratites and tinamous) and other birds occurred around 110 million years ago, while the split between tinamous (Tinamidae) and ratites is estimated at 60–70 million years ago, following the Cretaceous–Paleogene extinction event.[11] The evolutionary history of rheas is tied to the ancient ratite ancestors that originated in the supercontinent Gondwana during the late Cretaceous, with the rhea lineage diversifying in South America after the continent's separation from Gondwana approximately 100 million years ago.[12] The fossil record of Rheidae dates back to the Eocene epoch around 40 million years ago, with multiple species identified from Pliocene deposits (about 12–2 million years ago) in South America, supporting the family's monophyly through shared morphological traits in hindlimb bones.[13] Genetic studies, including analyses of complete mitochondrial genomes, further affirm the monophyly of Rheidae as a distinct South American clade within Palaeognathae.[10]Subspecies
The greater rhea (Rhea americana) is classified into five recognized subspecies, primarily distinguished by subtle morphological variations and geographic isolation within their South American ranges. These subspecies were delineated based on early 20th-century taxonomic assessments, with ongoing recognition in modern ornithological references despite limited genetic data.[13][14]| Subspecies | Distribution | Key Features |
|---|---|---|
| R. a. americana (nominate) | Northern and eastern Brazil | Typical gray plumage; moderate size (males up to 1.5 m tall); standard bill shape.[15][16] |
| R. a. nobilis | Eastern Paraguay, in semi-arid Chaco woodlands and grasslands | Slightly larger body; darker neck feathering; adapted to dry, open habitats.[17][18] |
| R. a. araneipes | Southwestern Brazil (Mato Grosso region), eastern Bolivia, western Paraguay | Robust build; variations in leg feathering; inhabits savannas and floodplains.[16][13] |
| R. a. intermedia | Southeastern Brazil (Rio Grande do Sul), Uruguay, central Argentina | Intermediate size; balanced plumage tones; prefers pampas and grassy lowlands.[15][19] |
| R. a. albescens | Southern Argentina (pampas to Río Negro region) | Largest subspecies (up to 1.7 m tall); paler, grayish-white plumage; straighter bill.[15][20] |