Systematics

Other rheas were previously separated in genus Pterocnemia, on basis mainly of tarsal scutellation and feathered upper tarsus, but genetic studies reveal them to be closely related to the present species, and probably more aptly considered congeneric (10 Delsuc, F., M. Superina, G. Ferraris, M. K. Tilak, and E. J. P. Douzery (2007). Molecular evidence for hybridisation between the two living species of South American ratites: potential conservation implications. Conservation Genetics 8(2):503–507. Close , 11 SACC (2008). Proposal (#348) to South American Classification Committee: Incluir Pterocnemia dentro de Rhea. URL: http://www.museum.lsu.edu/~Remsen/SACCprop348.htm (download July 2013). Close ). The Greater Rhea has hybridized with the Lesser Rhea (R. pennata) in captivity.

Several subspecies are poorly known, and their listed ranges remain somewhat provisional; birds in eastern Bolivia and Mato Grosso do Sul (west-central Brazil), which are currently listed for subspecies araneipes, may instead be referable to albescens (12 Folch, A. (1992). Rheidae (rheas). In Handbook of the Birds of the World. Volume 1 (J. del Hoyo, A. Elliott, and J. Sargatal, Editors), Lynx Edicions, Barcelona, Spain. pp. 84–89. Close ).

Five subspecies currently recognized (6 Blake, E. R. (1977). Manual of Neotropical Birds. Volume 1. Spheniscidae (Penguins) to Laridae (Gulls and Allies). University of Chicago Press, Chicago, IL, USA and London, UK. Close , 12 Folch, A. (1992). Rheidae (rheas). In Handbook of the Birds of the World. Volume 1 (J. del Hoyo, A. Elliott, and J. Sargatal, Editors), Lynx Edicions, Barcelona, Spain. pp. 84–89. Close , 4 Davies, S. J. J. F. (2002). Ratites and Tinamous: Tinamidae, Rheidae, Dromaiidae, Casuariidae, Apterygidae, Struthionidae. Oxford University Press, Oxford and New York. Close , 13 Dickinson, E. C., Editor (2003). The Howard and Moore Complete Checklist of the Birds of the World. Third edition. Christopher Helm, London, UK. Close , 14 Dickinson, E. C., and L. Christidis, Editors (2014). The Howard and Moore Complete Checklist of the Birds of the World. Fourth Edition. Volume 2: Passerines. Aves Press, Eastbourne, UK. Close , 15 del Hoyo, J., and N. J. Collar (2014). HBW and BirdLife International Illustrated Checklist of the Birds of the World. Volume 1. Non-passerines. Lynx Edicions, Barcelona, Spain. Close , 16 Clements, J. F., T. S. Schulenberg, M. J. Iliff, S. M. Billerman, T. A. Fredericks, B. L. Sullivan, and C. L. Wood (2019). The eBird/Clements Checklist of Birds of the World: v2019. Cornell Laboratory of Ornithology, Ithaca, New York, USA. Close , 17 Gill, F., D. Donsker, and P. Rasmussen, Editors (2020). IOC World Bird List v10.1. Close ), although Blake (6 Blake, E. R. (1977). Manual of Neotropical Birds. Volume 1. Spheniscidae (Penguins) to Laridae (Gulls and Allies). University of Chicago Press, Chicago, IL, USA and London, UK. Close ) cautioned that “the characters and ranges of the several races of americana are tentative.”

DNA Properties

The complete mitochondrial DNA molecule contains 16,710 nucleotides, and its organization most resembles that of ostriches (Struthio spp.) and the chicken (19 Griffin, D. K., and M. Schmid (2003). Comparative chromosome painting of chicken autosomal paints 1–9 in nine different bird species. Cytogenetic and Genome Research 103:173–184. Close , 20 Sales, J. (2006). Digestive physiology and nutrition of ratites. Avian and Poultry Biology Reviews 17:41–55. Close ).

Phylogenetic Relationships

Rheas, ostriches, emus (Dromaius), cassowaries (Casuarius spp.), and kiwis (Apteryx spp.) comprise the group of large flightless birds called ratites. The relationships between the different ratite groups has long been controversial. The distribution of ratites across southern landmasses has suggested that the group originated on the large ancient paleocontinent Gondwana, which is believed to have broken into several of the landmasses that now occupy the Southern Hemisphere. Ratites often are considered to be monophyletic, sister to the volant tinamous (Tinamidae), suggesting a single loss of flight in the ratite ancestry. A recent phylogenetic analysis (21 Harshman, J., E. L. Braun, M. J. Braun, C. J. Huddleston, R. C. K. Bowie, J. L. Chojnowski, S. J. Hackett, Han Kin-Lan, R. T. Kimball, B. D. Marks, K. J. Miglia, W. S. Moore, S. Reddy, F. H. Sheldon, D. W. Steadman, S. J. Steppan, C. C. Witt, and T. Yuri (2008). Phylogenomic evidence for multiple losses of flight in ratite birds. Proceedings of the National Academy of Sciences USA 105(36):13462–13467. Close ) suggested that tinamous should be counted among the ratites, making the group polyphyletic. This study proposed that loss of flight evolved separately at least three times in ratites, thereby explaining many of their morphological and behavioral similarities. It also called for a reconsideration of the proposed link between ratite evolution and continental drift.

Divergences between the rhea and other members of the order Struthioniformes have been estimated as follows: Greater Rhea/Lesser Rhea: 13.7 million years (22 Haddrath, O., and A. J. Baker (2001). Complete mitochondrial DNA genome sequences of extinct birds: ratite phylogenetics and the vicariance biogeography hypothesis. Proceedings of the Royal Society of London B 268:939–945. Close ); rheas/emus: ca. 45 million years (23 Harlid, A., A. Janke, and U. Arnason (1998). The complete mitochondrial genome of Rhea americana and early avian divergences. Journal of Molecular Evolution 46:669–679. Close ); and rheas/ostriches: ca. 51 million years (23 Harlid, A., A. Janke, and U. Arnason (1998). The complete mitochondrial genome of Rhea americana and early avian divergences. Journal of Molecular Evolution 46:669–679. Close ) or 75‒80 million years (24 Sibley, C. G., and J. E. Ahlquist (1990). Phylogeny and Classification of Birds: A Study in Molecular Evolution. Yale University Press, New Haven, CT, USA. Close ).

The relationships of rheas within the ratites remains unresolved. Mitochondrial DNA sequencing of representative ratites suggests that rheas group with emus, whereas ostriches are basal (25 van Tuinen, M., C. G. Sibley, and S. B. Hedges (1998). Phylogeny and biogeography of ratite birds inferred from DNA sequences of the mitochondrial ribosomal genes. Molecular Biology and Evolution 15(4):370–376. Close ). However, a separate sequence analysis suggested that rheas, not ostriches, are basal (26 Lee, K., J. Feinstein, and J. Cracraft (1997). The phylogeny of ratite birds: resolving conflicts between molecular and morphological data sets. In Avian Molecular Evolution and Systematics (D. P. Mindell, Editor), Academic Press, San Diego, CA, USA. pp. 173–208. Close ). Most molecular-based trees place rheas as the basal extant ratite (27 Zelenitsky, D. K., and S. P. Modesto (2003). New information on the eggshell of ratites and its phylogenetic significance. Canadian Journal of Zoology 81:962–970. Close ). Harshman et al. (21 Harshman, J., E. L. Braun, M. J. Braun, C. J. Huddleston, R. C. K. Bowie, J. L. Chojnowski, S. J. Hackett, Han Kin-Lan, R. T. Kimball, B. D. Marks, K. J. Miglia, W. S. Moore, S. Reddy, F. H. Sheldon, D. W. Steadman, S. J. Steppan, C. C. Witt, and T. Yuri (2008). Phylogenomic evidence for multiple losses of flight in ratite birds. Proceedings of the National Academy of Sciences USA 105(36):13462–13467. Close ) favored a topology in which rheas are sister to tinamous, although some results from their research suggested that rheas are basal to a clade of tinamous + Australasian ratites (kiwis, emus, and cassowaries).

Also known as the Gray Rhea or the Common Rhea. In Spanish, it is called Ñandú or Ñandú Común; in Portuguese (Brazil) ema.

Rhea fossils date back to the Eocene (ca. 40 million years ago), with four species of Rhea identified in Pliocene deposits (12 Folch, A. (1992). Rheidae (rheas). In Handbook of the Birds of the World. Volume 1 (J. del Hoyo, A. Elliott, and J. Sargatal, Editors), Lynx Edicions, Barcelona, Spain. pp. 84–89. Close ). The family first appeared near the start of the Tertiary, originating in South America, making them some of this continent’s oldest birds.