Ratite
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 Male Masai Ostrich
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Struthionidae (ostriches) |
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 and, lacking a strong anchor for their wing muscles, could not fly even were they to develop suitable wings. The name ratite comes from the Latin word for raft (ratis), because their breastbone looks like a raft.
Most parts of the former Gondwana have ratites, or have had until the fairly recent past.
[edit] Living forms
- The African Ostrich is the largest living ratite. A large member of this species can be nearly 3 m tall, weigh 135 kg or more, and outrun a horse.
- Of the living species, the Australian emu is next in size, reaching up to 2 m tall and about 60 kg. 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 of the pampas. The larger American rhea grows to about 1.5 m tall and weighs 20 to 25 kg. (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.
[edit] Extinct forms
- Aepyornis, the "elephant bird" of Madagascar, was the largest bird ever known. Although shorter than the tallest moa, a large aepyornis could weigh 450 kg.
- Moa - at least ten species in New Zealand, ranging from just over turkey-sized, to the Giant Moa Dinornis robustus (formerly known as Dinornis giganteus) with a height of 3 m and weighing about 250 kg[1]. Extinct by 1500 due to hunting by human settlers, who arrived around 1000, although at least one species may have survived past this date and maybe was seen by early European settlers.
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.
[edit] 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.).
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[citation needed]. 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 suggest that the kiwi are more closely related to the cassowaries than the moa[citation needed]. 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 is tentatively supporting a splitting of the group, with the Struthioniformes sensu stricto being one of the last ratite lineages to emerge.
