- Order: Rheiformes
- Family: Rheidae
- Polytypic 5 Subspecies
The Greater Rhea is among the largest living birds. Its long, powerful legs carry a football-shaped body with a long protruding neck that can dip to the ground to feed and rise above tall grass to scan for danger. The plumage is gray, with black around the head, neck, and shoulders, lighter gray feathering the legs, and white on the underparts. The rhea, like all ratites, cannot fly, but retains large wings with which it supports bursts of running and performs impressive displays for mates and competitors. Its distinctive profile can be identified from a distance, and in its native habitat could only be mistaken for its sister species, the Lesser Rhea (Rhea pennata).
The Lesser Rhea (Rhea pennata) is the only other species in this family, and the only other ratite in the New World. As its name implies, the Lesser Rhea is slightly smaller, and its plumage is brown with a pattern of white flecks. In contrast the plumage of the Greater Rhea is unspotted and primarily is gray, although the base of the neck often is blackish. The two species replace each other geographically, with little or no overlap.
Young rhea chicks employ several context-specific calls, but lose most vocal capabilities by the age of seven weeks. As the trachea grows, the internal tympanic membranes intrude less into the bronchial passages, contributing to the deterioration and ultimate silencing of the adult rhea's voice (Beaver 1978).
Adult rheas are mostly silent. During breeding the male gives a low-pitched, two noted boom or roar, which can be heard for up to 1 km (Sick 1993). An alarm call is a "hoarse grunt" (Sick 1993).
Greater Rhea chicks frequently vocalize (see Vocalizations); the adult male responds with "light bill snapping" (Sick 1993).
Detailed Description (appearance)
The following description is based on Blake (1977):
Very large, with long neck and tarsi. Tarsus entirely bare, with transverse scutes. Males larger and darker than female. Plumage of upperparts generally gray or grayish brown. Crown, nape, base of neck, and upper back usually dark brown or black. Underparts whitish.
From Blake (1977):
Bill: Yellowish brown.
Tarsi: Yellowish brown.
Height: 1.5 to 1.7 m
Mass: 20-25 kg (Blake 1977)
Linear measurements (from Blake 1977):
Rhea americana americana
Culmen length, males: 95 mm (n=1)
Culmen length, females: mean 88 mm (range 80-94 mm, n=3)
Tarsus length, males: 370 mm (n=1)
Tarsus length, females: mean 358 mm (range 335-370 mm, n=3)
Rhea americana araneipes:
Culmen length, males: 100.3 mm (range 96-104 mm, n=3)
Culmen length, females: mean 92.5 mm (92, 93 mm, n=2)
Tarsus length, males: mean 352.7 mm (range 351-354 mm, n=3)
Tarsus length, females: mean 363.5 mm (range 344-370 mm, n=4)
Five subspecies currently are recognized (Folch 1992, Dickinson 2003), although Blake (1977) cautions that "the characters and ranges of the several races of americana are tentative":
Rhea americana americana widespread in Brazil, except for extreme south and southwest
Rhea americana intermedia southern Brazil (Rio Grande do Sul) and Uruguay
Rhea americana nobilis eastern Paraguay
Rhea americana araneipes western Paraguay, Bolivia, southwestern Brazil (southern Mato Grosso)
Rhea americana albescens northern Argentina
The complete mitochondrial DNA molecule contains 16,710 nucleotides, and its organization most resembles that of the ostrich and chicken (Sales 2006).
Rheas, ostriches, emus, cassowaries, and kiwis comprise the group of large flightless birds called ratites. The relationships between the different ratite groups long has been controversial. The distribution of ratites across southern land masses has suggested that the group originated from the large ancient paleocontinent Gondwana, which is believed to have broken into several of the land masses ocupying the Southern Hemisphere. Ratites often are considered to be monophyletic, paired with the sister group of flighted tinamous, which suggested a single loss of flight in the ratite ancestry. A recent phylogenetic analysis (Harshman et al. 2008) suggests tinamous should be counted among the ratites, making the group polyphyletic. This study proposes that the loss of flight evolved separately at least three times in ratites, and explains many of their morphological and behavioral similarities. It also calls 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 (Haddrath and Baker 2001).
rheas/emus: ca 45 million years (Harlid et al. 1998).
rheas/ostrich: ca51 million years (Harlid et al. 1998).
75-80 million years (Sibley and Ahlquist 1990).
The relationships of rheas within the ratites remains unresolved. Mitochondrial DNA sequencing of representative ratites suggests that rheas are grouped with emus while ostriches are basal (Van Tuinen et al. 1998). A separate sequence analysis suggests that rheas, not ostriches, are basal (Lee et al. 1997). Most molecular-based trees place the rhea as the most basal living ratite (Zelenitsky and Modesto 2003). Harshman et al. (2008) favor a topology in which rheas are sister to tinamous, although some results from their research suggest that rheas are basal to a clade of tinamous + Australasian ratites (kiwis, emus, and cassowaries).
Linnaeus described the Greater Rhea in 1758.
The Greater Rhea also is known as the Gray Rhea or the Common Rhea. In Spanish, it is called "Ñandú" or "Ñandú Común".
Hodes, C.. 2010. Greater Rhea (Rhea americana), Neotropical Birds Online (T. S. Schulenberg, Editor). Ithaca: Cornell Lab of Ornithology; retrieved from Neotropical Birds Online: http://neotropical.birds.cornell.edu/portal/species/overview?p_p_spp=55956