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Rodents are of the Rodentia, characterised by a single pair of continuously growing in each of the upper and lower jaws that must be kept short by gnawing.

About 40% of mammal species are rodents, and they are found in vast numbers on all continents other than . Common rodents include , , , , , , and . Rodents use their sharp incisors to gnaw wood, break into food, and bite predators. Most rodents eat seeds or plants, though some have more varied diets. Some species have historically been , eating seeds stored by people and spreading disease.


Size and range of order
In terms of number of —although not necessarily in terms of number of organisms (population) or —rodents make up the largest order of mammals. With about 2,277 of rodents,Wilson, D. E. and D. M. Reeder, eds. 2005. Mammal Species of the World: A Taxonomic and Geographic Reference. Johns Hopkins University Press, Baltimore. over 40% of mammalian species belong to the order. Their success is probably due to their small size, short breeding cycle, and ability to gnaw and eat a wide variety of foods.David Lambert and the Diagram Group. The Field Guide to Prehistoric Life. New York: Facts on File Publications, 1985. ISBN 0-8160-1125-7

Rodents are found in vast numbers on all continents except Antarctica, most islands, and in all habitats except oceans. They are the only order besides bats and to have reached without human introduction. Members of non-rodent orders, such as (bats), (), ( and ), (, and ), and , such as and , are sometimes confused with rodents.


Characteristics

Dentition
all rodents share the characteristic of highly specialized for gnawing. This specialization gives rodents their name from the Latin, rodere, to gnaw. ξ1 All rodents have a single pair of upper and a single pair of lower , followed by a gap (diastema), and then one or more molars or premolars; is unique among the rodents in that it possesses no molars or premolars, and its incisors are so specialised, they are not for gnawing.Esselstyn, J.A., Achmadi, A.S. and Rowe, K.C., (2012). Evolutionary novelty in a rat with no molars. Biology Letters, published online 22 August 2012. doi:10.1098/rsbl.2012.0574 1744-957X Typical rodent incisors grow continuously and must be kept worn down by gnawing. Their and surfaces are covered with , but the posterior surface is exposed . During gnawing, the incisors grind against each other, wearing away the softer dentine, leaving the enamel edge like the blade of a chisel.Hurst, J.L., (1999). Introduction to rodents. In: The UFAW Handbook on the Care and Management of Laboratory Animals, Vol. 1, Terrestrial Vertebrates, 7th edn. Ed. Poole, T., pp. 262–273. Blackwell Publishing, Oxford This ‘self-sharpening’ system is very effective and is one of the keys to the enormous success of rodents. Rodents lack , and have a between their incisors and premolars. Their incisors are highly versatile and can be used for a range of functions, such as cutting wood, biting through the skin of fruit, prey capture, or defense, depending on the species. Nearly all rodents feed on plants, seeds in particular, but a number of species eat insects (grasshopper mouse, ) or fish (fish-eating rats, ). Some squirrels are known to eat , such as and . One species, , feeds primarily on and lacks the ability to gnaw or even chew, possessing bladelike, forked upper incisors and no molars.


Size
, the largest living rodent, can weigh up to .]] Many rodents are small; the tiny , Mus minutoides, can be as small as in length and in weight at maturity, and the , Salpingotulus michaelis, is of roughly similar or slightly smaller dimensions. Conversely, the largest extant rodent, the capybara, Hydrochoerus hydrochaeris, usually weighs up to , with exceptional specimens weighing up to .http://www.science.smith.edu/msi/pdf/i0076-3519-264-01-0001.pdf Several enormous rodents are known from the fossil record, the largest known being , which is estimated to have typically weighed about , and possibly up to or in large individuals.


Ecology and use by humans
Rodents are important in many ecosystems because they reproduce rapidly, and can function as food sources for predators, mechanisms for , and . Humans use rodents as a source of , as pets, as in animal testing, for food, and even for detecting . "A rat with a nose for landmines is doing its bit for humanity" Cited as coming from the New York Times in the article. Due to the wide diversity of their characteristics, some of which are considered uncommon or unique amongst mammals, rodents are used widely in research.Sherwin, C.M., (2010). The Husbandry and Welfare of Non-traditional Laboratory Rodents. In “UFAW Handbook on the Care and Management of Laboratory Animals”, R. Hubrecht and J. Kirkwood (Eds). Wiley-Blackwell. Chapter 25, pp. 359–369 For example, the , Heterocephalus glaber, is the only known mammal that is and also does not produce the neurotransmitter ; it is therefore used in studies on and .


Evolution and history
The record of rodent-like mammals begins shortly after the extinction of the non-avian 66 million years ago, as early as the . Some data, however, suggest modern rodents (members of the order Rodentia) had appeared in the late , although other molecular divergence estimations are in agreement with the fossil record. By the end of the epoch, relatives of beavers, , squirrels, and other groups appeared in the fossil record. They originated in , the supercontinent composed of today's , , and Asia. Some species colonized , giving rise to the earliest . From Africa, hystricognaths to , an isolated during the and epochs. By the , Africa collided with Asia, allowing rodents, such as , to spread into . During the , rodent fossils appeared in Australia. Although are the most prominent mammals in Australia, rodents now make up almost 25% of the continent's mammal species. Meanwhile, the Americas became joined by the , and some rodents participated in the resulting ; surged southward and headed north.

Some prehistoric rodents
, a giant
, a horned burrowing rodent
, a rat that grew to a large size on the island of
, a group of rodents once found in the
, a primitive, -like rodent
, a giant
, a large North American that weighed
, the largest known rodent, with an estimated weight of very roughly
, the second-largest known rodent, with an estimated weight of
, another giant South American rodent


Classification


Standard classification
The rodents are part of the (along with ), (along with , , , and ), and (along with most other ). The order Rodentia may be divided into , , and .

Classification scheme:

ORDER RODENTIA (from Latin, rodere, to gnaw)


Alternative classifications
The above uses the shape of the ( or ) as the primary character. This is the most commonly used approach for dividing the into . Many older references emphasize the (suborders , , , and ).

Several studies have used sequences to determine the relationships among rodents, but these studies have yet to produce a single, consistent and well-supported . Some have been consistently produced, such as:

The positions of the , , , and are still being debated.


Monophyly or polyphyly?
In 1991, a paper submitted to proposed that should be reclassified as a separate order (similar to ), based on an analysis of the sequences of proteins. This hypothesis was refined in a 1992 paper, which asserted the possibility that caviomorphs may have diverged from prior to later divergences of Myomorpha; this would mean caviomorphs, or possibly , would be moved out of the rodent classification into a separate order. A minority scientific opinion exists that argues that , , and other caviomorphs are not rodents, while several papers were put forward in support of rodent . Subsequent studies published since 2002, using wider and samples, have restored a majority opinion among mammalian biologists that the order Rodentia is monophyletic, although there is not a complete consensus.Carleton, Michael D., and Musser, Guy G. "Order Rodentia". Mammal Species of the World, 3rd edition, 2005, vol. 2, p. 745. (Concise overview of the literature)


Further reading
  • 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.
  • 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.
  • University of California Museum of Paleontology (UCMP). 2007 "Rodentia". [1]
  • Wilson, D. E. and D. M. Reeder, eds. 2005. Mammal Species of the World: A Taxonomic and Geographic Reference. Johns Hopkins University Press, Baltimore.


External links
Zoology, osteology, comparative anatomy

Various


References
    ^ (2018). 9780198605720, Oxford University Press.

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