Ratite

A ratite is any of a diverse group of large, flightless birds of Gondwanan origin, most of them now extinct. Unlike other flightless birds, the ratites have no keel on their sternum—hence the name from the Latin ratis (for raft). Without this to anchor their wing muscles, they could not fly even if they were to develop suitable wings.

Most parts of the former Gondwana have ratites, or did have until the fairly recent past. Their closest living relatives are the tinamous of South America.

Some taxonomical systems consider the various families of ratites to be orders, but the system used here uses the order "Struthioniformes" to refer to all ratites.

Living forms
The African Ostrich is the largest living ratite. A large member of this species can be nearly 3 metres (9.8 ft) tall, weigh as much as 159 kilograms (350 lb), and can outrun a horse.

Of the living species, the Australian emu is next in height, reaching up to 2 metres (6.6 ft) tall and about 60 kilograms (130 lb). Like the ostrich, 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 very dangerous when surprised or cornered because of their razor sharp talons.

South America has two species of rhea, mid-sized, fast-running birds of the Pampas. The larger American rhea grows to about 1.5 metres (4.9 ft) tall and usually weighs 20 to 25 kilograms (44–55 lb). (South America also has 47 species of the small and ground-dwelling but not flightless tinamou family, which is closely related to the ratite group.)

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 1.2 kilograms (2.6 lb) and 40 centimetres (16 in).

Extinct forms
At least 11 species of moa lived in New Zealand before the arrival of humans, ranging from turkey-sized to the Giant Moa Dinornis giganteus with a height of 3.3 metres (11 ft) and weighing about 250 kilograms (550 lb). They went extinct by A.D. 1500 due to hunting by Māori settlers, who arrived by A.D. 1300.

Aepyornis, the "elephant bird" of Madagascar, was the largest bird ever known. Although shorter than the tallest moa, a large Aepyornis could weigh over 450 kilograms (990 lb) and stand up to 3 metres (9.8 ft) tall.

Eggshell fragments similar to those of Aepyornis were found on the Canary Islands. The fragments date to the middle or late Miocene, and no satisfying theory has been proposed as to how they got there due to uncertainties about whether these islands were ever connected to the mainland.

Evolution and systematics
There are two taxonomic approaches to ratite classification: the one applied here 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 (e.g. Rheiformes, Casuariformes etc.).

Some studies based on morphology, immunology and DNA sequencing had indicated that ratites are monophyletic. The traditional account of ratite evolution has the group emerging in flightless form in Gondwana in the Cretaceous, then evolving in their separate directions as the continents drifted apart.

However, recent analysis of genetic variation between the ratites conflicts with this. DNA analysis appears to show that the ratites diverged from one another too recently to share a common Gondwanian ancestor. Also, the Middle Eocene fossil "proto-ostrich" Palaeotis from Central Europe may imply that the "out-of-Gondwana" hypothesis is wrong. Furthermore, recent analysis of twenty nuclear genes has drawn into question the monophyly of the group, suggesting that the flighted tinamous cluster within the ratite lineage. The authors say the data "unequivocally places tinamous within ratites".

A comparative study of the full mitochondrial DNA sequences of living ratites plus two moas places moas in the basal position, followed by rheas, followed by ostriches, followed by kiwis, with emus and cassowaries being closest relatives. Another study has reversed the relative positions of moas and rheas, and indicated that elephant birds are not close relatives of ostriches or other ratites, while a study of nuclear genes shows ostriches branching first, followed by rheas and tinamous, then kiwis splitting from emus and cassowaries. These studies share branching dates which suggest that, while ancestral moas may have been present in New Zealand since it split off from Gondwana, the ancestors of kiwis appear to have arrived there from Australia more recently, perhaps via a land bridge or by island-hopping. A recent extensive morphological comparison makes South American and Australian ratites a clade, with three successively more distant sister groups consisting of ostriches, elephant birds, and a clade of New Zealand ratites.

All analyses but the most recent morphological study show that rheas and extant Australo-Pacific ratites are monophyletic. The DNA data showing the ostriches branching first would match the sequence of Gondwana's plate tectonic breakup. Other, but not all, aspects of ratite paleobiogeography were found to be consistent with the vicariance (plate tectonic split-up of Gondwana) hypothesis. The sister-group relationship of New Zealand ratities to other ratites proposed on morphological grounds would not be consistent with vicariance, while the proposed South American-Australian clade would be.

Recent phylogenomic studies suggest that tinamous may in fact belong to this group. If so, this would make 'ratites' paraphyletic rather than monophyletic. Since tinamous are weak fliers, this raises interesting questions about the evolution of flightlessness in this group. While the ratites were traditionally thought of as an ancestrally flightless, monophyletic group, the branching of the tinamous within the ratite lineage suggests that ratites evolved flightlessness at least three times. Re-evolution of flight in the tinamous would be an alternative explanation, but such a development is without precedent in avian history, while loss of flight is commonplace.

Physical characteristics
Ratites in general have many physical characteristics in common, which are often not shared by the family Tinamidae, or tinamous. First, the breast muscles are underdeveloped. They do not have keeled sterna. Their wishbones (furculae) are almost absent. They have a simplified wing skeletons and musculature. Their legs are stronger and do not have air chambers, except the femurs. Their tail and flight feathers have retrogressed or have become decorative plumes. They have no feather vanes, which means they do not need to oil their feathers, hence they have no preen glands. They have no separation of pterylae and apteria, and finally, they have palaeognathous palates.

Ostriches have the greatest dimorphism, rheas show some dichromatism during the breeding season. Emus, cassowaries, and kiwis show some dimorphism, predominately in size.

While the ratites share a lot of similarities, they also have major differences. Ostriches have only two toes, with one being much larger than the other. Cassowaries have developed long inner toenails, used defensively. Ostriches and rheas have prominent wings; although they don't use them to fly, they do use them in courtship and predator distraction.

Feeding and diet
Ratite chicks tend to be more omnivorous or insectivorous; similarities in adults end with feeding, as they all vary in diet and length of digestive tract, which is indicative of diet. Ostriches, with the longest tracts at 14 metres (46 ft), are primarily vegetarian. Rheas' tracts are next longest at between 8–9 metres (26–30 ft), and they also have caeca. They are also mainly herbivores, concentrating on broad-leafed plants. However, they will eat insects if the opportunity arises. Emus have tracts of 7 metres (23 ft) length, and have a more omnivorous diet, including insects and other small animals. Cassowaries have nearly the shortest tracts at 4 metres (13 ft). Finally, kiwis have the shortest tracts and eat earthworms, insects, and other similar creatures. Finally, kiwis have the shortest tracts and eat earthworms, insects, and other similar creatures.

Reproduction
Ratites are different from the flying birds in that they needed to adapt or evolve certain features to protect their young. First and foremost is the thickness of the shells of their eggs. Their young are hatched more developed than most and they can run or walk soon thereafter. Also, most ratites have communal nests, where they share the incubating duties with others. Ostriches are the only ratites where the female incubates; they share the duties, with the males incubating at night. Kiwis stand out as the exception with a monogamous relationship.