Ratite
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 Various ratite birds
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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 - hence their name which comes 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 have had until the fairly recent past. Their closest living relatives are the tinamous of South America.[1][2]
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Living forms
The African Ostrich is the largest living ratite. A large member of this species can be nearly 3 meters (9.9 feet) tall, weigh as much as 159 kg (350 lb), and can outrun a horse.
Of the living species, the Australian emu is next in height, reaching up to 2 m (6.6 feet) tall and about 60 kg (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. 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 meters(5 feet) tall and usually weighs 20 to 25 kg (44 to 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 kg (2.7 lb) and 40 cm (16 inches).
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 over 450 kg (1,000 lb) and stand up to 3 meters (10 feet) tall.
At least eleven species of moa lived in New Zealand prior to the arrival of humans, ranging from turkey-sized to the Giant Moa Dinornis giganteus with a height of 3.3 meters (11 feet) and weighing about 250 kg (550 lb)[1]. They went extinct by A.D. 1500 due to hunting by Māori settlers, who arrived by A.D. 1300.
In addition, eggshell fragments similar to those of Aepyornis 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
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.).
Numerous studies based on morphology, immunology and DNA sequencing indicate that ratites are monophyletic.[1][3] 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. Further, recent analysis of twenty nuclear genes has drawn into question not only the continental drift mechanism, but the monophyly of the group, suggesting that the flighted tinamous cluster within the ratite lineage.[4]
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.[1] 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,[2] while a study of nuclear genes shows ostriches branching first, followed by rheas and tinamous, then kiwis split from emus and cassowaries.[4] Studies share branching dates that imply that while the ancestors of moas may have been present in New Zealand since it split off from other parts of Gondwana, the ancestors of kiwis appear to have somehow dispersed there from Australia more recently, perhaps via a land bridge or by island-hopping. By earlier analyses, ostriches seemed to have arrived in Africa by some route after it detached from South America (such as by invading Eurasia and then Africa out of India), but the nuclear data showing the ostriches branching first would match the sequence of Gondwana's plate tectonic breakup.[4] Other, but not all, aspects of ratite paleobiogeography were found to be consistent with the vicariance (plate tectonic split-up of Gondwana) hypothesis.
References and Notes
- ↑ 1.0 1.1 1.2 Haddrath, O.; Baker, A. J. (2001). "Complete mitochondrial DNA genome sequences of extinct birds: ratite phylogenetics and the vicariance biogeography hypothesis". Proc. R. Soc. Lond. B 268: 939–945. doi:. http://journals.royalsociety.org/content/597tbumbn1wr76k3/.
- ↑ 2.0 2.1 Cooper, A.; Lalueza-Fox, C.; Anderson, S.; Rambaut, A.; Austin, J; Ward, R. (2001-02-08). "Complete mitochondrial genome sequences of two extinct moas clarify ratite evolution". Nature 409: 704–707. doi:. http://www.nature.com/nature/journal/v409/n6821/abs/409704a0.html. Retrieved on 2008-04-05.
- ↑ Hackett, Shannon J.; et al. (2008-06-27). "A Phylogenomic Study of Birds Reveals Their Evolutionary History". Science 320 (5884): 1763 - 1768. doi:. http://www.sciencemag.org/cgi/content/abstract/320/5884/1763. Retrieved on 2008-10-18.
- ↑ 4.0 4.1 4.2 Harshman, J.; Braun, E.; Braun, M.; Huddleston, C.; Bowie, R.; Chojnowski, J.; Hackett, S.; Han, K.-L.; Kimball, R.; Marks, B.; Miglia, K.; Moore, W.; Reddy, S.; Sheldon, F.; Steadman, D.; Steppan, S.; Witt, C.; Yuri, T. (2008-09-09). "Phylogenetic evidence for multiple losses of flight in ratite birds". Proceedings of the National Academy of Sciences (USA) 105 (36): 13462-13467. doi:. http://www.pnas.org/content/105/36/13462. Retrieved on 2008-10-17.
Gallery of Living Species
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