Rodent

 

Rodents Temporal range: Early Paleocene–Recent PreЄ Є O S D C P T J K Pg N
Indian palm squirrel (Funambulus palmarum)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Eutheria
Magnorder:	Boreoeutheria
Superorder:	Euarchontoglires
Order:	Rodentia Bowdich, 1821
Suborders
Sciuromorpha Castorimorpha Myomorpha Anomaluromorpha Hystricomorpha
Combined range of all rodent species

Rodentia is the order of mammals known as rodents, characterised by a single pair of continuously growing incisors in each of 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, porcupines, beavers, guinea pigs, and hamsters. Rodents have sharp incisors that they use 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 pests, eating seeds stored by people and spreading disease.

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. There are about 2,277 species of rodents (Wilson and Reeder, 2005), with over 40 percent of mammalian species belonging 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. (Lambert, 2000)

Members of nonrodent orders, such as Chiroptera (bats), Scandentia (treeshrews), Soricomorpha (shrews and moles), Lagomorpha (hares, rabbits and pikas) and mustelid carnivores such as weasels and mink, are sometimes confused with rodents.^{[citation needed]}

Rodents are found in vast numbers on all continents except Antarctica, most islands, and in all habitats except oceans. They are the only nonvolant, nonmarine placental order—and in particular are the only placental order besides bats and pinnipeds—to have reached Australia without human introduction.

Characteristics

Dentition

Typical rodent tooth system

All rodents share the characteristic of dentition highly specialised for gnawing. This specialisation gives rodents their name from the Latin, rodere, to gnaw. All rodents have a single pair of upper and a single pair of lower incisors, followed by a gap (diastema), and then one or more molars or premolars. Rodent incisors grow continuously and must be kept worn down by gnawing. Their anterior and lateral surfaces are covered with enamel, but the posterior surface is exposed dentine. During gnawing, the incisors grind against each other, wearing away the softer dentine, leaving the enamel edge as the blade of a chisel. This ‘self-sharpening’ system is very effective and is one of the keys to the enormous success of rodents. Rodents lack canines, and have a diastema between their incisors and premolars. They use their teeth for cutting wood, biting through the skin of fruit, or for defense. Nearly all rodents feed on plants, seeds in particular, but a few exceptions eat insects (grasshopper mouse, Onychomys leucogaster) or fish. Some squirrels are known to eat passerine birds, such as cardinals and blue jays.

Size

The capybara, the largest living rodent, can weigh up to 91 kg (200 lb).

Many rodents are small; the tiny African pygmy mouse, Mus minutoides, can be as small as 6 cm (2.4 in) in length and 7 g (0.25 oz) in weight at maturity, and the Baluchistan pygmy jerboa, Salpingotulus michaelis, is of roughly similar or slightly smaller dimensions. On the other hand, the largest extant rodent, the capybara, Hydrochoerus hydrochaeris, usually weighs up to 65 kg (140 lb), with exceptional specimens weighing up to 91 kg (200 lb). Several enormous rodents are known from the fossil record, the largest known being Josephoartigasia monesi, which is estimated to have typically weighed about 1,000 kg (2,200 lb), and possibly up to 1,534 kg (3,380 lb) or 2,586 kg (5,700 lb) 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 seed dispersal, and disease vectors. Humans use rodents as a source of fur, as pets, as model organisms in animal testing, for food, and even for detecting land mines. Due to the wide diversity of their characteristics, some of which are considered uncommon or unique amongst mammals, rodents are used widely in research. For example, the naked mole rat, Heterocephalus glaber, is the only known mammal that is poikilothermic and also does not produce the neurotransmitter substance P; it is therefore used in studies on thermoregulation and pain.

Evolution

Masillamys sp. fossil from the Messel Pit fossil site

The fossil record of rodent-like mammals begins shortly after the extinction of the nonavian dinosaurs 65 million years ago, as early as the Paleocene. Some molecular clock data, however, suggest 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, dormice, squirrels, and other groups appeared in the fossil record. They originated in Laurasia, the supercontinent composed of today's North America, Europe, and Asia. Some species colonized Africa, giving rise to the earliest hystricognaths. From Africa, 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 porcupines to spread into Eurasia. During the Pliocene, rodent fossils appeared in Australia. Although marsupials 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 Isthmus of Panama, and some rodents participated in the resulting Great American Interchange; sigmodontines surged southward and caviomorphs 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 large North American capybara that weighed 100 kg (220 lb)

Josephoartigasia monesi, the largest known rodent, with an estimated weight of very roughly 1,000 kg (2,200 lb)

Phoberomys pattersoni, the second-largest known rodent, with an estimated weight of 700 kg (1,500 lb)

Telicomys, another giant South American rodent

Classification

About 2/3 of rodent species are in the superfamily Muroidea (rats, mice, and related species). The families Muridae (blue) and Cricetidae (red) make up the bulk of the Muroidea.

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 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 and the capybara
      • Family Chinchillidae: chinchillas and viscachas
      • Family Ctenomyidae: tuco-tucos
      • Family Dasyproctidae: agoutis
      • Family Cuniculidae: pacas
      • Family Dinomyidae: pacaranas
      • Family Echimyidae: spiny rats
      • Family Erethizontidae: New World porcupines
      • Family Myocastoridae: nutria, coypu
      • 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

Alternative 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 have 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 unnamed clade containing:
  • 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 pig proteins. 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.

Further reading

• Adkins RM, Gelke R. M. E. L., Rowe D., Honeycutt R. L. (2001). "Molecular phylogeny and divergence time estimates for major rodent groups: Evidence from multiple genes". Molecular Biology and Evolution 18 (5): 777–791. PMID 11319262. 
• 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; Cox, Peter G.; Jahn, Gary C.; Nugent, Robert (2002). "Evaluating rodent management with Cambodian rice farmers". Cambodian Journal of Agriculture 5: 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.; Adkins, R. A.; Anderson, J. (2004). "Phylogeny and divergence date estimates of rapid radiations in muroid rodents based on multiple nuclear genes". Systematic Biology 53 (4): 533–553. doi:10.1080/10635150490468701. PMID 15371245. 
• 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

• Website on African rodentia
• Rodent Photos
• Rodent Pests chapter in United States Environmental Protection Agency and University of Florida/Institute of Food and Agricultural Sciences National Public Health Pesticide Applicator Training Manual
• Can Rodents be trained not to gnaw?