AskDefine | Define rodent

Dictionary Definition

rodent n : relatively small gnawing animals having a single pair of constantly growing incisor teeth specialized for gnawing [syn: gnawer, gnawing animal]

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English

Etymology

Latin rodens (stem rodent-), present participle of rodere ‘to gnaw’.

Noun

  1. A mammal of the order Rodentia, characterized by long incisors that grow continuously and are worn down by gnawing.

Synonyms

Translations

See also

Extensive Definition

Rodentia is an order of mammals also known as rodents, characterised by two continuously-growing incisors in the upper and lower jaws which must be kept short by gnawing.
Forty percent of mammal species are rodents, and they are found in vast numbers on all continents other than Antarctica. Common rodents include mice, rats, squirrels, chipmunks, gophers, porcupines, beavers, hamsters, gerbils, and guinea pigs. Their success is probably due to their small size, short breeding cycle, and ability to gnaw and eat a wide variety of foods. (Lambert, 2000)
Rodents are found in vast numbers on all continents except Antarctica, most islands, and in all habitats except oceans. They are the only placental order, other than bats (Chiroptera) and Pinnipeds, to reach Australia without human introduction.

Characteristics

Many rodents are small; the tiny African pygmy mouse is only 6 cm in length and 7 grams in weight. On the other hand, the capybara can weigh up to , and the largest known rodent, the extinct Josephoartigasia monesi, is estimated to weigh about , and possibly up to or .
Rodents have two incisors in the upper as well as in the lower jaw which grow continuously and must be kept worn down by gnawing; this is the origin of the name, from the Latin rodere, to gnaw, and dens, dentis, tooth. These teeth are used for cutting wood, biting through the skin of fruit, or for defense. The teeth have enamel on the outside and exposed dentine on the inside, so they self-sharpen during gnawing. Rodents lack canines, and have a space between their incisors and premolars. Nearly all rodents feed on plants, seeds in particular, but there are a few exceptions which eat insects or fish. Some squirrels are known to eat passerine birds like cardinals and blue jays. Rodents are important in many ecosystems because they reproduce rapidly, and can function as food sources for predators, mechanisms for seed dispersal, and as disease vectors. Humans use rodents as a source of fur, as pets, as model organisms in animal testing, for food, and even in detecting landmines.
Members of non-rodent orders such as Chiroptera (bats), Scandentia (treeshrews), Insectivora (moles, shrews and hedgehogs), Lagomorpha (hares, rabbits and pikas) and mustelid carnivores such as weasels and mink are sometimes confused with rodents.

Evolution

The fossil record of rodent-like mammals begins shortly after the extinction of the non-avian dinosaurs 65 million years ago, as early as the Paleocene. Some molecular clock data, however, suggests that modern rodents (members of the order Rodentia) already appeared in the late Cretaceous, although other molecular divergence estimations are in agreement with the fossil record. By the end of the Eocene epoch, relatives of beavers, dormouse, squirrels, and other groups appeared in the fossil record. They originated in Laurasia, the formerly joined continents of North America, Europe, and Asia. Some species colonized Africa, giving rise to the earliest hystricognaths. There is, however, a minority belief in the scientific community that evidence from mitochondrial DNA indicates that the Hystricognathi may belong to a different evolutionary offshoot and therefore a different order. From there hystricognaths rafted to South America, an isolated continent during the Oligocene and Miocene epochs. By the Miocene, Africa collided with Asia, allowing rodents such as the porcupine to spread into Eurasia. During the Pliocene, rodent fossils appeared in Australia. Even though marsupials are the prominent mammals in Australia, rodents make up almost 25% of the mammals on the continent. Meanwhile, the Americas became joined and some rodents expanded into new territory; mice headed south and porcupines headed north.
Castoroides, a giant beaver
Ceratogaulus, a horned burrowing rodent
Spelaeomys, a rat that grew to a large size on the island of Flores
Giant hutias, a group of rodents once found in the West Indies
Ischyromys, a primitive squirrel-like rodent
Leithia, a giant dormouse
Neochoerus pinckneyi, a giant North American Capybara that weighed 50 kg
Josephoartigasia monesi, the largest known rodent
Phoberomys pattersoni, the second largest known rodent
Telicomys, a giant South American rodent

Classification

Standard classification

The rodents are part of the clades: Glires (along with lagomorphs), Euarchontoglires (along with lagomorphs, primates, treeshrews, and colugos), and Boreoeutheria (along with most other placental mammals). The order Rodentia may be divided into suborders, infraorders, superfamilies and families.
Classification scheme:
ORDER RODENTIA (from Latin, rodere, to gnaw)

Alternate classifications

The above taxonomy uses the shape of the lower jaw (sciurognath or hystricognath) as the primary character. This is the most commonly used approach for dividing the order into suborders. Many older references emphasize the zygomasseteric system (suborders Protrogomorpha, Sciuromorpha, Hystricomorpha, and Myomorpha).
Several molecular phylogenetic studies have used gene sequences to determine the relationships among rodents, but these studies are yet to produce a single consistent and well-supported taxonomy. Some clades have been consistently produced such as:
The positions of the Castoridae, Geomyoidea, Anomaluridae, and Pedetidae are still being debated.

Monophyly or polyphyly?

In 1991, a paper submitted to Nature proposed that caviomorphs should be reclassified as a separate order (similar to Lagomorpha), based on an analysis of the amino acid sequences of guinea pigs. This hypothesis was refined in a 1992 paper, which asserted the possibility that caviomorphs may have diverged from myomorphs prior to later divergences of Myomorpha; this would mean caviomorphs, or possibly hystricomorphs, would be moved out of the rodent classification into a separate order. A minority scientific opinion briefly emerged arguing that guinea pigs, degus, and other caviomorphs are not rodents, while several papers were put forward in support of rodent monophyly. Subsequent studies published since 2002, using wider taxon and gene samples, have restored consensus among mammalian biologists that the order Rodentia is monophyletic.

Notes

  • Adkins, R. M. E. L. Gelke, D. Rowe, and R. L. Honeycutt. 2001. Molecular phylogeny and divergence time estimates for major rodent groups: Evidence from multiple genes. Molecular Biology and Evolution, 18:777-791.
  • Carleton, M. D. and G. G. Musser. 2005. Order Rodentia. Pp 745-752 in Mammal Species of the World A Taxonomic and Geographic Reference. Johns Hopkins University Press, Baltimore.
  • David Lambert and the Diagram Group. The Field Guide to Prehistoric Life. New York: Facts on File Publications, 1985. ISBN 0-8160-1125-7
  • Leung LKP, Peter G. Cox, Gary C. Jahn and Robert Nugent. 2002. Evaluating rodent management with Cambodian rice farmers. Cambodian Journal of Agriculture Vol. 5, pp. 21-26.
  • McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN 0-231-11013-8
  • Nowak, R. M. 1999. Walker's Mammals of the World, Vol. 2. Johns Hopkins University Press, London.
  • Steppan, S. J., R. A. Adkins, and J. Anderson. 2004. Phylogeny and divergence date estimates of rapid radiations in muroid rodents based on multiple nuclear genes. Systematic Biology, 53:533-553.
  • Wilson, D. E. and D. M. Reeder, eds. 2005. Mammal Species of the World A Taxonomic and Geographic Reference. Johns Hopkins University Press, Baltimore.

Links

References

rodent in Arabic: قوارض
rodent in Bavarian: Fieslfiecha
rodent in Bulgarian: Гризачи
rodent in Catalan: Rosegador
rodent in Czech: Hlodavci
rodent in Danish: Gnavere
rodent in German: Nagetiere
rodent in Modern Greek (1453-): Τρωκτικά
rodent in Spanish: Rodentia
rodent in Esperanto: Ronĝuloj
rodent in Basque: Karraskari
rodent in Persian: جوندگان
rodent in Faroese: Gnagdýr
rodent in French: Rodentia
rodent in Galician: Roedor
rodent in Korean: 설치류
rodent in Hindi: गिलहरी
rodent in Upper Sorbian: Hrymzaki
rodent in Croatian: Glodavci
rodent in Ido: Rodero
rodent in Indonesian: Hewan pengerat
rodent in Icelandic: Nagdýr
rodent in Italian: Rodentia
rodent in Hebrew: מכרסמים
rodent in Georgian: მღრღნელები
rodent in Latin: Rodentia
rodent in Latvian: Grauzēji
rodent in Luxembourgish: Knabberdéieren
rodent in Lithuanian: Graužikai
rodent in Ligurian: Rodentia
rodent in Limburgan: Knaagdiere
rodent in Hungarian: Rágcsálók
rodent in Macedonian: Глодари
rodent in Dutch: Knaagdieren
rodent in Japanese: ネズミ目
rodent in Norwegian: Gnagere
rodent in Norwegian Nynorsk: Gnagarar
rodent in Narom: Grugeux
rodent in Novial: Rodentia
rodent in Occitan (post 1500): Rodentia
rodent in Low German: Gnaagdeerter
rodent in Polish: Gryzonie
rodent in Portuguese: Roedores
rodent in Romanian: Rozătoare
rodent in Quechua: Khankiq
rodent in Russian: Грызуны
rodent in Simple English: Rodent
rodent in Slovak: Hlodavce
rodent in Slovenian: Glodavci
rodent in Serbian: Глодари
rodent in Finnish: Jyrsijät
rodent in Swedish: Gnagare
rodent in Tamil: கொறிணி
rodent in Thai: สัตว์ฟันแทะ
rodent in Vietnamese: Bộ Gặm nhấm
rodent in Turkish: Kemiriciler
rodent in Ukrainian: Гризуни
rodent in Chinese: 啮齿目

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