member of the mammalian order Rodentia, characterized by front teeth adapted for gnawing and cheek teeth adapted for chewing. The Rodentia is by far the largest mammalian order; nearly half of all mammal species are rodents. They are worldwide in distribution and are found in almost every terrestrial and freshwater habitat, from the shores of the Arctic Ocean to the hottest deserts. They are variously adapted for running, jumping, climbing, burrowing, swimming, and gliding. Many of them have dexterous forepaws, which they use as hands while sitting on their haunches in a position characteristic of many rodents. The great majority are under a few inches in length; the largest, the capybara, is about 4 ft (120 cm) long and 20 in. (50 cm) high at the shoulder.

Characteristics of Rodents

Rodents have enlarged, chisel-shaped upper and lower front incisors that grow throughout their lives. These have hard enamel on the front surface and soft dentine on the back surface, so that unequal wear keeps the chisel edge sharp. There is a gap between the front teeth and the cheek teeth. When the lower jaw is in a forward position, for gnawing, the upper and lower incisors are in contact but the upper and lower cheek teeth are not; thus, wear on the cheek teeth is avoided. The cheeks are drawn in behind the incisors when the animal is gnawing, so that bits of hard material cannot be swallowed. When the lower jaw is pulled back into the chewing position, only the cheek teeth make contact.

Types of Rodents

The approximately 4,000 rodent species are divided on the basis of their anatomy into three well-defined groups, or suborders, and more than 30 families. The Sciuromorpha, or squirrellike rodents, include the various species of squirrel, chipmunk, marmot, woodchuck (or ground hog), prairie dog, gopher (or pocket gopher), pocket mouse, kangaroo rat, and beaver. The Myomorpha, or mouselike rodents, include a great variety of mouse and rat species, as well as species of hamster, lemming, vole, muskrat, gerbil, dormouse, and jerboa. This is the largest rodent group. The Hystricomorpha, or porcupinelike rodents, include the porcupine, capybara, nutria (or coypu), agouti, cavy (including the domestic guinea pig), mara, and chinchilla, as well as many species whose common names include the term rat (e.g., the South American bush rat).

Rabbits and hares were once classified as rodents because of their large, chisel-shaped incisors. However, they are quite distinct anatomically and have a long, separate evolutionary history; they are now classified in an order of their own, the Lagomorpha. Using DNA analyses as evidence, some scientists believe that the some other groups of rodents have descended from different ancestors and should thus be placed in orders of their own.

See also mountain beaver, ground squirrel, flying squirrel, pack rat, bandicoot rat, and jumping mouse.

Bibliography

See Sir J. R. Ellerman, The Families and Genera of Living Rodents (2 vol., 1940, repr. 1965); B. S. Vindgradov and A. I. Argiropulo, Key to Rodents (tr. 1968).

Rodents Fossil range: Late Paleocene - Recent
Ammospermophilus leucurus
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Infraclass: Eutheria Superorder: Euarchontoglires Order: Rodentia Bowdich, 1821
Suborders
Sciuromorpha Castorimorpha Myomorpha Anomaluromorpha Hystricomorpha

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.^{[citation needed]}

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. Rodents have sharp incisors that they use to gnaw wood, break into food, and bite predators. Most eat seeds or plants, though some have more varied diets. They have historically been pests, eating human seed stores and spreading disease.

Rodents evolved some time around the end of the Cretaceous period c 65 million years ago.

Size and range of order

In terms of number of species — although not necessarily in terms of number of organisms (population) or biomass — rodents make up the largest order of mammals, with over 40 percent of mammalian species belonging to the order.^[1] 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)

There are about 2,277 species of rodents, about 42% of all mammal species (Wilson and Reeder, 2005). Rodents are found in vast numbers on all continents except Antarctica, most islands, and in all habitats except for oceans. They are the only placental order, other than bats (Chiroptera) and Pinnipeds, to reach Australia without human introduction.

Characteristics

Most 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 45 kg (100 pounds) and the extinct Phoberomys pattersoni is believed to have weighed 700 kg.

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 model organisms in animal testing, for food, and even in detecting landmines.^[2]

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 for rodents. Rodents have a carrier-immunity to the rabies virus, making them immune to the potentially infectious and lethal disease.^{[citation needed]} They are carriers for most other animal-to-human illnesses, however, and still should not be agitated.^{[citation needed]}

Evolution

The fossil record of rodent-like mammals begins shortly after the extinction of the 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.^[3][4] 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.

Some Prehistoric Rodents

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

Phoberomys pattersoni, the 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)

• Suborder Anomaluromorpha
  • Family Anomaluridae: scaly-tailed squirrels
  • Family Pedetidae: springhares
• Suborder Castorimorpha
  • Superfamily Castoroidea
    • Family Castoridae: beavers
  • Superfamily Geomyoidea
    • Family Geomyidae: pocket gophers (true gophers)
    • Family Heteromyidae: kangaroo rats and kangaroo mice
• Suborder Hystricomorpha
  • Family incertae sedis Diatomyidae: Laotian rock rat
  • Infraorder Ctenodactylomorphi
    • Family Ctenodactylidae: gundis
  • Infraorder Hystricognathi
    • Family Bathyergidae: African mole rats
    • Family Erethizontidae: New World porcupines
    • Family Hystricidae: Old World porcupines
    • Family Petromuridae: dassie rat
    • Family Thryonomyidae: cane rats
    • Parvorder Caviomorpha
      • Family †Heptaxodontidae: giant hutias
      • Family Abrocomidae: chinchilla rats
      • Family Capromyidae: hutias
      • Family Caviidae: cavies, including guinea pigs
      • Family Chinchillidae: chinchillas and viscachas
      • Family Ctenomyidae: tuco-tucos
      • Family Dasyproctidae: agoutis
      • Family Dinomyidae: pacaranas
      • Family Echimyidae: spiny rats
      • Family Hydrochoeridae: capybara
      • Family Myocastoridae: nutria
      • Family Octodontidae: octodonts
• Suborder Myomorpha
  • Superfamily Dipodoidea
    • Family Dipodidae: jerboas and jumping mice
  • Superfamily Muroidea
    • Family Calomyscidae: mouse-like hamsters
    • Family Cricetidae: hamsters, New World rats and mice, voles
    • Family Muridae: true mice and rats, gerbils, spiny mice, crested rat
    • Family Nesomyidae: climbing mice, rock mice, white-tailed rat, Malagasy rats and mice
    • Family Platacanthomyidae: spiny dormice
    • Family Spalacidae: mole rats, bamboo rats, and zokors
• Suborder Sciuromorpha
  • Family Aplodontiidae: mountain beaver
  • Family Gliridae (also Myoxidae, Muscardinidae): dormice
  • Family Sciuridae: squirrels, including chipmunks, prairie dogs, & marmots

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:

• Ctenohystrica contains:
  • Ctenodactylidae (gundis)
  • Hystricognathi containing:
    • Hystricidae
    • An unnamed clade containing:
      • Phiomorpha
      • Caviomorpha

• An unnnamed clade contains:
  • Gliridae
  • Sciuroidea containing:
    • Aplodontiidae
    • Sciuridae

• Myodonta includes:
  • Dipodoidea
  • Muroidea

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.^[5] 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.^[6] A minority scientific opinion briefly emerged arguing that guinea pigs, degus, and other caviomorphs are not rodents,^[7][8] while several papers were put forward in support of rodent monophyly.^[9][10][11] Subsequent studies published since 2002, using wider taxon and gene samples, have restored consensus among mammalian biologists that the order Rodentia is monophyletic.^[12][13]

Notes

1. ^ Myers, Phil (2000). Rodentia. Animal Diversity Web. University of Michigan Museum of Zoology. Retrieved on 2006-05-25.
2. ^ Wines, Michael. "Gambian rodents risk death for bananas", The Age, The Age Company Ltd., 2004-05-19. Retrieved on 2006-05-25.  "A rat with a nose for landmines is doing its bit for humanity" Cited as coming from the New York Times in the article.
3. ^ Douzery, E.J.P., F. Delsuc, M.J. Stanhope, and D. Huchon (2003). "Local molecular clocks in three nuclear genes: divergence times for rodents and other mammals and incompatibility among fossil calibrations". Journal of Molecular Evolution 57: S201-S213. 
4. ^ Horner, D.S., K. Lefkimmiatis, A. Reyes, C. Gissi, C. Saccone, and G. Pesole (2007). "Phylogenetic analyses of complete mitochondrial genome sequences suggest a basal divergence of the enigmatic rodent Anomalurus". BMC Evolutionary Biology 7: 16. 
5. ^ Graur, D., Hide, W. and Li, W. (1991) 'Is the guinea-pig a rodent?' Nature, 351: 649-652.
6. ^ Li, W., Hide, W., Zharkikh, A., Ma, D. and Graur, D. (1992) 'The molecular taxonomy and evolution of the guinea pig.' Journal of Heredity, 83 (3): 174-81.
7. ^ D'Erchia, A., Gissi, C., Pesole, G., Saccone, C. and Arnason, U. (1996) 'The guinea-pig is not a rodent.' Nature, 381 (6583): 597-600.
8. ^ Reyes, A., Pesole, G. and Saccone, C. (2000) 'Long-branch attraction phenomenon and the impact of among-site rate variation on rodent phylogeny.' Gene, 259 (1-2): 177-87.
9. ^ Cao, Y., Adachi, J., Yano, T. and Hasegawa, M. (1994) 'Phylogenetic place of guinea pigs: No support of the rodent-polyphyly hypothesis from maximum-likelihood analyses of multiple protein sequences.' Molecular Biology and Evolution, 11: 593-604.
10. ^ Kuma, K. and Miyata, T. (1994) 'Mammalian phylogeny inferred from multiple protein data.' Japanese Journal of Genetics, 69 (5): 555-66.
11. ^ Robinson-Rechavi, M., Ponger, L. and Mouchiroud, D. (2000) 'Nuclear gene LCAT supports rodent monophyly.' Molecular Biology and Evolution, 17: 1410-1412.
12. ^ Lin, Y-H, et al. "Four new mitochondrial genomes and the increased stability of evolutionary trees of mammals from improved taxon sampling." Molecular Biology and Evolution 19 (2002): 2060-2070.
13. ^ 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)

References

• 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

• Jahn, G. C. 1998. “When Birds Sing at Midnight” War Against Rats Newsletter 6:10-11. [1]

• 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.

• University of California Museum of Paleontology (UCMP). 2007 "Rodentia". [2]

• Wilson, D. E. and D. M. Reeder, eds. 2005. Mammal Species of the World A Taxonomic and Geographic Reference. Johns Hopkins University Press, Baltimore.

Extant mammal orders by infraclass
Australosphenida	Monotremata
Metatheria	Didelphimorphia · Paucituberculata · Microbiotheria · Notoryctemorphia · Dasyuromorphia · Peramelemorphia · Diprotodontia
Eutheria	Afrosoricida · Macroscelidea · Tubulidentata · Hyracoidea · Proboscidea · Sirenia · Cingulata · Pilosa · Scandentia · Dermoptera · Primates · Rodentia · Lagomorpha · Erinaceomorpha · Soricomorpha · Chiroptera · Pholidota · Carnivora · Perissodactyla · Artiodactyla · Cetacea

bar:Fieslfiechahsb:Hrymzakilij:Rodentianrm:Grugeux

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