Ratites () are a polyphyletic group consisting of all birds within the infraclass Palaeognathae that lack keels and cannot fly. They are mostly large, long-necked, and long-legged, the exception being kiwi, which are the only nocturnal extant ratites. The understanding of relationships within the paleognath clade has been in flux. Previously, all the flightless members had been assigned to the order Struthioniformes, which is more recently regarded as containing only the ostrich. The modern bird infraclass Palaeognathae consists of ratites and the flighted Neotropic tinamous (compare to Neognathae). Unlike other flightless birds, the ratites have no keel on their sternum—hence the name, from the Latin ratis ('raft', a vessel which has no keel—in contradistinction to extant flighted birds with a keel). Without this to anchor their wing muscles, they could not have flown even if they had developed suitable wings. Ratites are a polyphyletic group; tinamous fall within them, and are the sister group of the extinct moa. This implies that flightlessness is a trait that evolved independently multiple times in different ratite lineages. Most parts of the former supercontinent Gondwana have ratites, or did have until the fairly recent past. So did Europe in the Paleocene and Eocene, from where the first flightless paleognaths are known. Ostriches were present in Asia as recently as the Holocene, although the genus is thought to have originated in Africa. However, the ostrich order may have evolved in Eurasia. A recent study posits a Laurasian origin for the clade. Geranoidids, which may have been ratites, existed in North America.
Species
Living forms The African ostriches are the largest living ratites. A large member of these species can be nearly 2.8 metres (9 ft 2 in) tall, weigh as much as 156 kilograms (344 lb), and can outrun a horse. Of the living species, the Australian emu is next in height, reaching up to 1.9 metres (6 ft 3 in) tall and about 50 kilograms (110 lb). Like the ostriches, it is a fast-running, powerful bird of the open plains and woodlands. Also native to Australia and the islands to the north are the three species of cassowary. Shorter than an emu, but heavier and solidly built, cassowaries prefer thickly vegetated tropical forest. They can be dangerous when surprised or cornered because of their razor-sharp talons. In New Guinea, cassowary eggs are brought back to villages and the chicks raised for eating as a much-prized delicacy, despite (or perhaps because of) the risk they pose to life and limb. They reach up to 1.8 metres (5 ft 11 in) tall and weigh as much as 85 kilograms (187 lb). South America has two species of rhea, large fast-running birds of the Pampas. The larger American rhea grows to about 1.4 metres (4 ft 7 in) tall and usually weighs 15 to 40 kilograms (33–88 lb). The smallest ratites are the five species of kiwi from New Zealand. Kiwi are chicken-sized, shy, and nocturnal. They nest in deep burrows and use a highly developed sense of smell to find small insects and grubs in the soil. Kiwi are notable for laying eggs that are very large in relation to their body size. A kiwi egg may equal 15 to 20 percent of the body mass of a female kiwi. The smallest species of kiwi is the little spotted kiwi, at 0.9 to 1.9 kilograms (2.0–4.2 lb) and 35 to 45 centimetres (14–18 in) tall.
Holocene extinct forms At least nine species of moa lived in New Zealand before the arrival of humans, ranging from turkey-sized to the giant moa Dinornis robustus with a height of 3.7 metres (12 ft 2 in) and weighing about 230 kilograms (510 lb). They became extinct by A.D. 1400 due to hunting by Māori settlers, who arrived around A.D. 1280. Aepyornis maximus, the "elephant bird" of Madagascar, was the heaviest bird ever known. Although shorter than the tallest moa, a large A. maximus could weigh over 400 kilograms (880 lb) and stand up to 3 metres (9 ft 10 in) tall. Accompanying it were three other species of Aepyornis as well as three species of the smaller genus Mullerornis. All these species went into decline following the arrival of humans on Madagascar around 2,000 years ago, and were extinct by the 17th or 18th century if not earlier.
Classification
There are two taxonomic approaches to ratite classification: one combines the groups as families in the order Struthioniformes, while the other supposes that the lineages evolved mostly independently and thus elevates the families to order rank (Rheiformes, Casuariformes etc.).
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