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Rheidae
Rheidae
Rheidae
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Rheids
Temporal range: Paleocene–Holocene 56–0 Ma
Greater rhea, Rhea americana
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Infraclass: Palaeognathae
Clade: Notopalaeognathae
Order: Rheiformes
Family: Rheidae
Bonaparte, 1853[1]
Type species
Rhea americana
Genera
Synonyms
  • Rheinae Bonaparte 1849

Rheidae /ˈriːɪdiː/ is a family of flightless ratite birds which first appeared in the Paleocene.[2] It is today represented by the sole living genus Rhea, but also contains several extinct genera.[3]

Taxonomy

[edit]

Order Rheiformes (Forbes, 1884) Furbringer, 1888 [Rheimorphae Bonaparte, 1849; Rheae Forbes 1884][4][5][6][7][8]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Rheidae

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from Grokipedia
Rheidae is a family of large, flightless birds endemic to , consisting of two extant species: the (Rhea americana) and the lesser rhea (Rhea pennata). These birds are characterized by their tall stature, long necks and legs, greyish-brown plumage, and reduced wings ending in a , adaptations that enable them to run at speeds up to 60 km/h across open terrain. The Rheidae family belongs to the order , which encompasses other flightless birds such as ostriches, emus, and cassowaries, distinguished by their flat lacking a for flight muscles. Both species exhibit , with males larger and more brightly colored during breeding; the reaches heights of 1.5–1.7 m and weights of 20–40 kg, while the lesser rhea is smaller at 0.9–1 m tall and 15–28.6 kg. Their three-toed feet and powerful legs facilitate swift evasion of predators like pumas and foxes, and they possess fused adrenal glands, a unique trait among birds. Rheas inhabit diverse open landscapes across the , with the favoring lowland grasslands, savannas, and shrublands from to 1,500 m in countries including , , , , and . In contrast, the lesser rhea occupies open steppes, shrublands, and deserts from to 4,500 m, with northern favoring higher elevations (2,000–4,500 m) and the southern at lower altitudes, ranging from southern through and to , with three (R. p. pennata, R. p. tarapacensis, and R. p. garleppi). Both species are diurnal and gregarious, forming flocks of 5–30 individuals outside breeding season, often associating with grazing mammals for protection. Their diet is omnivorous, comprising seeds, fruits, roots, leaves, , small vertebrates, and even carrion, with water obtained primarily from . is polygynous and seasonal, typically from spring to early summer; males attract multiple females to a nest site, incubate clutches of 13–50 eggs (laid by up to 10 females), and provide sole for the precocial chicks, which can number up to 80 in exceptional cases. Conservation challenges include habitat loss from agriculture and overgrazing, as well as hunting for meat, skins, and eggs; the is classified as Near Threatened by the IUCN, with declining populations across its 8.74 million km² range, while the lesser rhea is Least Concern overall but decreasing, with some subspecies like R. p. garleppi critically low (estimated 100–500 individuals). Both are listed on Appendix II to regulate international trade. Historically, rheas have served as an economic resource for indigenous communities in the and , providing food, feathers, and oil.

Taxonomy and evolution

Classification

The Rheidae family belongs to the order within the infraclass Paleognathae, a group that comprises the ratites—flightless birds such as ostriches, emus, cassowaries, kiwis, and the extinct moas. The family is represented by a single extant , Rhea (Brisson, 1760), which encompasses all living species of rheas. The Rhea includes two recognized extant species: the (Rhea americana) with five (R. a. americana, R. a. araneipes, R. a. intermedia, R. a. nobilis, and R. a. albescens), and the lesser rhea (also known as , Rhea pennata) with three (R. p. pennata, R. p. tarapacensis, and R. p. garleppi). Historically, the lesser rhea (R. pennata, including its highland subspecies formerly classified as R. tarapacensis) was placed in a separate genus, Pterocnemia Gray, 1871, based on morphological differences such as leg structure and plumage patterns, but molecular analyses have demonstrated close genetic similarity to Rhea americana, leading to the synonymization of Pterocnemia under Rhea. In 2022, the IUCN and BirdLife International further lumped the former species R. tarapacensis (puna rhea) as the subspecies R. p. tarapacensis. Phylogenetically, rheas are positioned within the ratites, with tinamous (order Tinamiformes) as their closest living relatives, forming a of South American paleognaths; molecular clock estimates place the divergence between this rhea-tinamou lineage and other ratites in the , approximately 60–70 million years ago.

Fossil record

The fossil record of Rheidae documents the family's origins in the early , with the earliest definitive remains attributed to the early Eocene Itaborai Formation in , represented by the genus Diogenornis (e.g., D. fragilis), consisting of fragmentary hindlimb elements indicative of a terrestrial, flightless . Some analyses suggest potential precursors based on associated palaeognathous material, though these are debated and not conclusively assigned to crown Rheidae. The temporal range extends through the and , with the crown genus Rhea appearing by the late (Messinian stage, ~7 Ma), and continuing into the , including Pleistocene species that overlap with extant forms. s are predominantly fragmentary, including isolated tibiotarsi, tarsometatarsi, and phalanges from South American localities, reflecting the family's adaptation to open habitats. Key extinct members include Opisthodactylus patagonicus from the early Santa Cruz Formation in Patagonia, , known from hindlimb bones that suggest a body size similar to modern rheas but with more robust proportions for locomotion. In the late of northwestern , Rhea mesopotamica (originally described as Pterocnemia mesopotamica) is represented by associated leg elements from the Ituzaingó Formation, indicating a slightly smaller form than the greater rhea (R. americana). Pliocene records from include species like Rhea fossilis, based on distal tibiotarsi and tarsometatarsi that display osteological features aligning closely with living rheas, though some taxa such as Rhea pampeana have been synonymized with extant due to poor preservation and overlapping morphology. A new late , Opisthodactylus kirchneri, from northwestern extends the genus's range northward and highlights regional paleobiogeographic variation within Rheidae. The evolutionary history of Rheidae reflects the development of flightlessness and ratite traits—such as reduced wings, a flat sternum, and elongated hindlimbs—in isolation on the South American continent following the Cretaceous-Paleogene extinction and the progressive fragmentation of Gondwana. This isolation, beginning around 80 Ma but intensifying after 66 Ma, allowed Rheidae to diverge as specialized terrestrial herbivores without competition from flying palaeognaths. Molecular clock estimates place the divergence of Rheidae from other ratite lineages (e.g., Casuariiformes, Struthioniformes) at approximately 50-60 million years ago, aligning with early Paleogene fossil evidence and supporting multiple independent losses of flight within Palaeognathae. These adaptations underscore Rheidae's role as one of the oldest surviving ratite clades, with a fossil record emphasizing hindlimb dominance and minimal skeletal variation across time.

Description

Physical features

Rheidae, the family encompassing the rheas, consists of large, flightless birds native to , characterized by their robust terrestrial adaptations. The greater rhea (Rhea americana) is the largest species, standing up to 170 cm tall at the shoulder and weighing 20–40 kg, with males typically larger than females. In contrast, the lesser rhea (Rhea pennata, including its subspecies such as the Puna rhea R. p. garleppi) is smaller, reaching heights of 90–100 cm and weights of 15–28.6 kg, with the Puna subspecies exhibiting slightly more compact dimensions overall. As ratites, rheas possess an unkeeled , a flat breastbone lacking the ridge that anchors flight muscles in volant birds, reflecting their evolutionary loss of flight capability. Their body structure emphasizes locomotion, featuring a long neck and elongated legs that support rapid terrestrial movement, a small head with a flat, broad bill suited for grazing, and three-toed feet with reduced phalanges that enable efficient running at speeds up to 60 km/h. The tarsi are scutellated, providing additional protection and stability on open terrains. The wings of rheas are vestigial and incapable of sustaining flight, but relatively long and serving primarily as balancing aids during high-speed chases, functioning like sails to enhance maneuverability. Each wing terminates in a sharp , which can be used defensively, particularly in males during territorial interactions. Rheas exhibit sensory adaptations suited to diurnal lifestyles, including large eyes that provide acute vision for detecting predators across vast grasslands, a trait common among ratites. Like all birds, they lack teeth, relying instead on a muscular reinforced by ingested grit to grind tough plant material and small prey.

Plumage and variation

The of rheas is characterized by loose, fluffy feathers that lack hooks on the barbules and preen glands for oiling, resulting in an unusually soft texture adapted to their terrestrial . These feathers are generally gray-brown overall, providing a mottled appearance through subtle variations in shading and spotting. Among the species, the (Rhea americana) exhibits predominantly gray , with darker brown tones on the crown, , upper back, and a blackish collar around the head, neck, and shoulders; the underparts are whitish, and the legs are covered in paler gray feathers. In contrast, the lesser rhea (Rhea pennata), including , displays grayer to yellowish-brown upperparts with extensive white tipping on the back and wing feathers, creating a distinct spotted pattern, while the head and neck are similarly grayish-brown. The Puna subspecies (R. p. garleppi) is notably paler overall, with a grayer head, reduced white spotting, and rufous-brown upperparts where white tips are limited to longer feathers. Sexual dimorphism in plumage is subtle, with males typically darker than females, particularly during the breeding season when males develop a prominent neck ring and intensified shading on the head and upperparts. Females maintain duller, less contrasted tones year-round, though both sexes share the overall gray-brown palette without stark differences in pattern. Rheas undergo an annual complete prebasic molt following the breeding season, replacing all flight and body feathers in a process that temporarily affects their appearance but aligns with their flightless nature; juveniles transition from a uniform brownish lacking spots to the adult pattern over the first three to four years. This molt occurs progressively, starting with body feathers and extending to wings, and is part of the continuous feather replacement typical of ratites.

Distribution and habitat

Native range

The Rheidae family, comprising flightless birds native exclusively to , occupies a broad indigenous range spanning eastern and southern portions of the continent, from northeast southward to . This distribution encompasses diverse open landscapes across multiple countries, reflecting the family's adaptation to expansive, non-forested environments. The (Rhea americana) is the most widespread species, inhabiting the , Chaco woodlands, and savannas primarily in , , , , and . The lesser rhea (Rhea pennata, also known as Darwin's rhea) occupies southern Patagonian grasslands and Andean plateaus, with a range extending from southern Peru through western Bolivia, Chile, and Argentina to Patagonia. It includes three subspecies: the nominate R. p. pennata in Patagonia (Argentina and Chile); R. p. garleppi in the altiplano of southern Peru, western Bolivia, and northwestern Argentina; and R. p. tarapacensis (puna rhea), a highland specialist restricted to the Andean puna regions from southern Peru and northern Chile to northwestern Argentina. Rheas preferentially select open habitats such as grasslands, shrublands, and savannas, where visibility aids predator detection and ; they consistently avoid dense forests and closed-canopy areas. These birds exhibit seasonal movements, often migrating short distances in search of water sources and optimal during dry periods. Altitudinally, the ranges from in lowland like the to elevations exceeding 4,500 m in highland forms such as the puna rhea.

Introduced populations

The (Rhea americana) has been introduced outside its native South American range primarily for agricultural purposes, such as meat, leather, and feather production, with escapes from farms leading to small populations in . In , the only established population of greater rheas in originated from escapes in August 2000, when a small group fled a farm near Groß Grönau in , eventually settling in adjacent areas of Mecklenburg-Western . The population grew steadily due to successful breeding in the region's open agricultural landscapes, which resemble the ' native habitats, reaching approximately 247 individuals as of spring 2020; earlier counts reached around 550–600 in 2018–2020, but has since limited expansion, with the population remaining stable as of 2025. These birds have adapted well to temperate grasslands and mild winters, enabling with clutches of 10–50 eggs per season, though colder snaps and legal to mitigate crop damage have limited further growth. In the , smaller groups of greater rheas have appeared as escaped pets or from private collections, but no self-sustaining populations have formed. A notable incident occurred in March 2021 near Maple Cross in , where up to 20 birds were reported running loose in residential and roadside areas, prompting police warnings about aggressive behavior toward dogs. Similar escapes, such as 12 individuals from a in Stalham, , in 2023, have been contained or recaptured, preventing establishment. Ecological impacts from these introduced populations remain minor due to their limited size and containment efforts. In , rheas occasionally damage crops like corn and rape by foraging in fields, leading to conflicts with farmers and authorized hunting since 2017, but they pose no significant hybridization risk with and have not spread widely beyond a 500 km² area. In the UK, transient groups have shown potential to exploit open habitats but have not bred successfully in the wild, maintaining low invasive potential.

Ecology and behavior

Diet and foraging

Rheas are primarily herbivorous, consuming a diet dominated by grasses, leaves, seeds, and fruits, with opportunistic intake of animal matter such as , small vertebrates, and occasionally carrion comprising about 1% of their overall diet. In agricultural landscapes, greater rheas (Rhea americana) preferentially forage on broad-leaved plants like (Medicago sativa) and wild dicots, showing little preference for grasses, while also incorporating crop weeds such as thistles (Carduus spp.) and nightshade (Solanum spp.). Lesser rheas (Rhea pennata) exhibit an even more strictly , with leaf matter from shrubs and forbs making up over 94% of intake, supplemented by seeds (about 6%) and minimal (less than 0.1%). Darwin's rheas (Rhea pennata) focus on highland vegetation, including (Atriplex spp.), fruits, grasses, roots, and leaves, with animal components remaining minor. Foraging occurs diurnally, with rheas walking slowly while holding their heads low to the ground (typically under 50 cm) to peck at , occasionally pausing for vigilance. To process tough plant material, they ingest small stones (gastroliths) that aid grinding in the muscular , with stones comprising around 2% of dry stomach contents in lesser rheas. These birds often near water sources, though most water is obtained from ; they drink when available despite their arid-adapted habitats. Dietary composition shows seasonal shifts, with increased consumption of protein-rich during the breeding season to support reproductive demands, particularly among adults. In non-breeding periods, reliance on herbaceous intensifies, reflecting resource availability in grasslands and steppes. Species-specific adaptations highlight dietary breadth: greater rheas exploit a wider range including cultivated crops in lowland , whereas Darwin's rheas, inhabiting Andean highlands, emphasize alpine grasses and succulents suited to their elevated, drier environments.

Social structure

Rheas are gregarious birds that typically form flocks outside the breeding season, with group sizes ranging from 5 to 30 individuals, though larger loose herds of up to 100 have been observed in areas with abundant resources. In non-breeding periods, these flocks often include both sexes and various age classes, facilitating and predator detection through collective vigilance. During the breeding season, shifts, with males becoming solitary to establish territories and females often traveling nomadically in smaller groups or pairs. Males exhibit territorial primarily during breeding, aggressively defending areas against intruders using displays and vocalizations. Females, in contrast, remain largely nomadic year-round, with less emphasis on fixed territories. Communication among rheas relies on a combination of vocal, visual, and behavioral signals. Adults are generally silent outside specific contexts, but males produce deep booming calls, while females emit hisses during agonistic encounters. Visual displays include wing spreading to signal threats, and alarm responses involve zig-zag running patterns to evade predators. Rheas employ acoustic, visual, tactile, and chemical cues overall for social interactions. Rheas are diurnal, with activity peaking at dawn and dusk, during which they engage in and movement across open habitats. Predator evasion relies on their speed, reaching up to 60 km/h, combined with group vigilance where individuals alternate scanning for threats while others feed. This social strategy enhances survival in predator-rich environments like grasslands and steppes.

Reproduction

Mating system

Rheidae, the family encompassing the greater rhea (Rhea americana) and lesser rhea (Rhea pennata), exhibit a polygynandrous mating system in which males are simultaneously polygynous, mating with multiple females, while females are sequentially polyandrous, mating with multiple males over the course of a breeding season. This reproductive strategy allows for communal egg-laying in male-defended nests, with males assuming sole responsibility for incubation and chick-rearing after females depart to seek additional mates. In both species, only a subset of males—typically those in prime condition—secure territories and harems of 2 to 12 females, leading to high variance in male reproductive success. Greater rhea females typically lay more eggs per season (median 40) compared to lesser rhea females (median 18). The breeding season occurs during the Southern Hemisphere's spring and summer, generally spanning July to January, though regional variations exist based on latitude and local conditions. This timing aligns with increasing day length, as rheas are long-day breeders responsive to photoperiod cues that synchronize gonadal development and hormonal changes. Rainfall and temperature also modulate the season's onset and intensity; optimal reproduction favors mild temperatures (around 17–21°C) and low daily rainfall (<11 mm), with excessive precipitation reducing egg production and nest success. In arid regions, the season may shorten or shift in response to sporadic rains that enhance forage availability. Courtship is initiated by territorial males who perform elaborate displays to attract and herd females into their defended areas, often forming temporary aggregations that facilitate without true lek structures. These displays include deep bowing with wings spread wide to reveal black flight feathers, rapid wing-shaking, zigzag running toward females, and deep booming vocalizations produced by inflating the . Females respond by approaching receptive males or, in groups, forming solicitation circles while emitting hisses and wing-flaps to encourage copulation. In some populations, males compete aggressively with charges and wing-slaps to monopolize females. Clutch formation involves multiple females sequentially laying eggs in a single ground nest scraped by the male, typically a shallow depression lined with grass and feathers. Each female contributes 5 to 13 eggs, laid at 36- to 48-hour intervals over 10 to 12 days, resulting in communal of 20 to 50 eggs from 2 to 8 females, though exceptional nests may hold up to 60. Eggs are large (averaging 130 × 90 mm for greater rheas, slightly smaller for lesser rheas), pale yellow to buff, and unmarked. Once laying concludes, the male begins incubation, which lasts 35 to 40 days, while females abandon the site to join other males' territories.

Incubation and parental care

In the family Rheidae, males assume sole responsibility for incubation, a behavior observed in both the (Rhea americana) and the lesser rhea (Rhea pennata). After females deposit in a communal nest, the male begins incubating once the clutch is sufficiently complete, typically 5–7 days after the first is laid, to synchronize . The lasts 35–40 days, during which the male remains on the nest for approximately two-thirds of the time, rotating the periodically to ensure even heating and periodically leaving to during the warmest parts of the day. Rhea eggs are large, measuring about 13 cm in length and 9 cm in width, with a pale yellow-green or greenish-yellow shell that provides in grassy habitats. A single nest may receive up to 60 eggs from multiple females, but due to the male's limited ability to cover and heat them all effectively, only 20–30 eggs are typically viable per . If the nest is significantly disturbed by predators or human activity, the male often abandons it entirely, leading to high rates of nest desertion (up to 65% in some populations). Upon , rhea chicks are precocial, covered in downy gray-brown with darker stripes for , and capable of following the immediately after emerging from the , which occurs over 4–5 days in a synchronized manner. The leads broods of 20–40 chicks, protecting and guiding them in while providing no direct feeding, as the young begin pecking at food shortly after . continues for 4–6 months until the chicks achieve independence, though juveniles may remain in loose family groups for up to two years. Chick survival is challenged by high predation rates, particularly from mammals such as pampas foxes (Lycalopex gymnocercus) and feral dogs, with high mortality in the first few months. Males defend their broods aggressively, using powerful kicks from their strong legs and charging at threats to deter attackers, which can sometimes result in injuries to predators. This paternal investment contributes significantly to the species' reproductive success despite environmental pressures.

Relationship with humans

Historical and cultural uses

and gauchos in have long exploited rheas for sustenance and materials. Gauchos traditionally hunted rheas on horseback using —throwing weapons consisting of balls connected by cords—to entangle the birds' legs, often aided by dogs to exhaust them first. This method allowed for the capture of rheas for their , which indigenous groups considered tender and flavorful when prepared by stuffing the carcass with hot stones for cooking, though leg meat was tougher and reserved for times of . Feathers were utilized for dusters, bed filling, and clothing, while neck skins served as bed mats and tobacco pouches among native communities. Eggs, laid in clutches of 12 to 72, were consumed raw, roasted in ashes, or used in large communal meals, providing a rich source of nutrition due to their substantial content. Rheas hold cultural significance in South American indigenous traditions, particularly among groups like the Terena, where the bird features prominently in mythology and ceremonies as a spiritual symbol. Feathers from rheas are incorporated into traditional dances, such as Terena rituals where dancers wear skirts made of them to evoke the bird's form and energy. In Guarani lore, the rhea, known as ñandú guazú or "big spider," appears in narratives tied to the natural world and cosmic chases, reflecting its role in regional folklore. These elements underscore the rhea's symbolic importance beyond mere utility. Intensive hunting in the contributed to significant population declines, particularly in the and . European trade demands for feathers and skins intensified exploitation, with gauchos and indigenous hunters supplying markets; for instance, over two tons of rhea feathers were exported from in 1884 alone, often requiring the killing of dozens of birds per rug. Chroniclers noted that this unmanaged overhunting, combined with pressures, led to alarming reductions in rhea numbers by the late 1800s, as predicted by observers like Roberto Payró in 1898. Archaeological and historical records confirm that while native use was preferential, European-influenced commercial pressures exacerbated the impact from the 16th to 19th centuries. Commercial farming of rheas emerged in the late , primarily in starting in the 1990s, to produce from their durable hides, low-fat similar to , and feathers valued for their oil-absorbing properties in dusters. These products support a global trade, including decorated rhea eggs collected for artistic and symbolic purposes in cultural displays.

Conservation status

The conservation status of Rheidae species varies across the family, reflecting differences in habitat pressures and geographic distribution. The greater rhea (Rhea americana) is classified as Near Threatened on the , with its population suspected to be declining due to ongoing habitat loss and hunting pressures. Darwin's rhea, the southern subspecies of the lesser rhea (Rhea pennata pennata), is assessed as Least Concern globally, though local declines occur in fragmented areas. In contrast, the Puna rhea (R. p. garleppi), a northern subspecies of the lesser rhea, faces more severe risks and is considered Near Threatened, primarily from highland . Overall, the lesser rhea (R. pennata) is rated Least Concern at the species level, but subspecies-specific vulnerabilities highlight the need for targeted monitoring. Major threats to Rheidae populations include widespread habitat conversion for , particularly soybean cultivation and ranching, which fragment grasslands across their South American range. Hunting for meat and leather remains a significant issue, especially for the , exacerbating declines in accessible areas. Additional risks involve road collisions in expanding zones and the introduction of non-native predators, such as dogs, which prey on eggs and juveniles. The global population size of the is unknown, though trends show moderate declines in core habitats. Lesser rhea populations are more stable but localized, with remaining fairly common in Patagonian steppes and the Puna rhea restricted to smaller, isolated highland groups estimated at under 10,000 mature individuals. Conservation efforts focus on habitat protection and regulated trade to mitigate these threats. The is listed under Appendix II, while the lesser rhea is listed under Appendix I (except the nominate subspecies under Appendix II), controlling in skins and live specimens to prevent . Protected areas, such as reserves in the Argentine and Patagonian steppes, cover key habitats and support population recovery through anti-poaching measures. Reintroduction programs, such as ongoing efforts in Patagonia National Park since 2014, have bolstered lesser rhea numbers by releasing captive-bred individuals into suitable grasslands. These initiatives emphasize sustainable , such as maintaining shrub-free grasslands, to enhance long-term viability.

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  57. https://veganfta.com/blog/2023/08/18/the-truth-about-farming-ratites-ostriches-emus-and-rheas/
  58. https://www.ei-nzigartig.com/en/pages/ei-wissen
  59. https://birdsoftheworld.org/bow/species/lesrhe2/cur/conservation
  60. https://birdsoftheworld.org/bow/species/grerhe1/cur/conservation
  61. https://cites.org/sites/default/files/eng/app/2023/E-Appendices-2023-05-04.pdf
  62. https://cites.org/sites/default/files/eng/cop/11/prop/31.pdf
  63. https://www.rewildingchile.org/en/news/recovering-the-rhea-in-patagonia-national-park-10-years-of-collaborative-public-private-work/
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