| Ratites | ||||||||||
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 Various ratite birds
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Struthionidae (ostriches) |
Ratite is the common name for any of a group of flightless birds characterized by a flat, raft-like sternum (breastbone) lacking the keel for attachment of wing muscles that is typical of most flying birds and some other flightless birds. The name ratite comes from the Latin word for raft (ratis), because their breastbone looks like a raft. Among ratites are included ostriches, emus, rheas, kiwis, and elephant birds. The flightless penguins are not ratites since they lack the flat breastbone and actually have strong wings, albeit adapted for swimming.
The diversity in the world's fauna, as seen in the unique forms and behaviors of ratites, adds to the joy and fascination that people feel from nature.
The shared shape of the breastbone of ratites is considered by many authorities to be more a product of adaptation to living on the ground rather than shared ancestry. Based on this view, the ratites are differentiated into several orders of birds. Another taxonomic view assumes shared ancestry and places the ratites together. At one point, they were placed in one superorder, Palaeognathae, and one current approach is to combine them as different families within the order Struthioniformes.
Living forms
The African ostrich is the largest living ratite. A large member of this species can be nearly three meters (9.9 feet) tall, weigh as much as 159 kilograms (350 pounds), and can outrun a horse.
Of the living species, the Australian emu is next in size, reaching up to two meters (6.6 feet) tall and about 60 kilograms (132 pounds). 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 and very solidly built, cassowaries prefer thickly vegetated tropical forest. They can be very dangerous when surprised or cornered. 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.
South America has two species of rhea, mid-sized, fast-running birds. The larger American rhea grows to about 1.5 meters (five feet) tall and usually weighs 20 to 25 kilograms (44 to 55 pounds). (South America also has 73 species of the small and ground-dwelling, but not flightless tinamou family, which is distantly 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 (2.7 pounds) and 25 centimeters (10 inches).
Recent extinct forms
Elephant birds are an extinct family of ratites native to Madagascar that have been extinct since at least the sixteenth century. They are placed in the genera Aepyornis and Mullerornis. Aepyornis, was the largest bird ever known. Although shorter than the tallest moa, a large Aepyornis could weigh over 450 kg (1,000 lbs) and stand up to three meters (10 feet) tall.
Moa were giant flightless birds native to New Zealand. They were unique in having no wings, not even small wings, unlike other ratites. Fifteen species of varying sizes are known, with the largest species, the giant moa (Dinornis robustus and Dinornis novaezelandiae), reaching about 3.6 meters (12 feet) in height and about 250 kilograms (550 pounds) in weight. They were the dominant herbivores in the New Zealand forest ecosystem. Leaves, twigs and fruit played a big part in their diet. Moa are thought to have become extinct about 1500, due to hunting by human settlers who arrived around 1000, although some reports speculate that a few stragglers of Megalapteryx didinus may have persisted in remote corners of New Zealand until the eighteenth and even nineteenth centuries.
In addition, eggshell fragments similar to those of Aepyornis (though this is probably a symplesiomorphy) were found on the Canary Islands. The fragments apparently 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
Most parts of the former Gondwana have ratites, or have had until the fairly recent past.
There are two taxonomic approaches to ratite classification. One combines the groups as families in the order Struthioniformes. The other approach supposes that the lineages evolved mostly independently and thus elevates the families to order rank (e.g. Rheiformes, Casuariformes etc.).
The traditional account of ratite evolution has the group emerging in Gondwana in the Cretaceous, then evolving in their separate directions as the continents drifted apart. Cladistic analysis of morphology strongly supports this—Ratites share too many features for their current forms to be parsimoniously explained by convergent evolution.
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, and suggests that the kiwi are more closely related to the cassowaries than the moa.
At present, there is no generally accepted explanation. Also, there is the Middle Eocene fossil "proto-ostrich" Palaeotis from Central Europe, which either implies that the ancestral ratites had not yet lost flight when they were dispersing all over Gondwana—by the Middle Eocene, both Laurasia and Gondwana had separated into the continents of today—or that the "out-of-Gondwana" hypothesis is wrong. Research continues, but at present the ratites are perhaps the one group of modern birds for which no robust theory of their evolution and paleobiogeography exists. Current opinion tentatively is supporting a splitting of the group, with the Struthioniformes sensu stricto being one of the last ratite lineages to emerge.
| Birds |
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| Anatomy: Anatomy - Skeleton - Flight - Eggs - Feathers - Plumage |
| Evolution and extinction. Evolution - Archaeopteryx - Hybridisation - Late Quaternary prehistoric birds - Fossils - Taxonomy - Extinction |
| Behaviour: Singing - Intelligence - Migration - Reproduction- Brood parasites |
| Bird types: Seabirds - Shorebirds - Waterbirds - Song birds - Raptors - Poultry |
| Bird lists: Familes and orders - Lists by region |
| Birds and Humans: Ringing - Ornithology - Birdwatching - Birdfeeding - Conservation - Aviculture |
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