The order Rodentia is the most numerous of all the branches on the mammal family tree. Currently there are, depending on the authority consulted, between 2000 and 3000 species of rodent—roughly half of all mammal species. Rodents are found in vast numbers on all continents (they are the only placental order other than the bats to reach Australia without human introduction), most islands, and in all habitats bar the oceans.

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. These teeth are used for cutting wood, biting through the skin of fruit, or for defence. Nearly all rodents feed on plants, seeds in particular, but there are a few exceptions which eat insects or even fish.

Classification

The rodents are part of the clades: Glires (along with lagomorphs), Euarchontoglires (along with lagomorphs, primates, tree shrews, and colugos), and Laurasiatheria (along with most other placental mammals). The order Rodentia may be divided into suborders, infraorders, superfamilies and families. This is a common classification scheme:

• ORDER RODENTIA

• Suborder Sciurognathi
  • Infraorder Sciurida
    • Family Sciuridae: squirrels, including chipmunks & prairie dogs
    • Family Aplodontidae: mountain beaver
  • Infraorder Castorimorpha
    • Family Castoridae: beavers
  • Infraorder Anomaluromorpha
    • Family Anomaluridae: scaly-tailed squirrels
    • Family Pedetidae: springhares
  • Infraorder Ctenodactylomorpha
    • Family Ctenodactylidae: gundis
  • Infraorder Glirimorpha
    • Family Gliridae (also Myoxidae, Muscardinidae): dormice
  • Infraorder Geomorpha
    • Family Geomyidae: pocket gophers
    • Family Heteromyidae: kangaroo mice
  • Infraorder Myodonta
    • Superfamily Dipodoidea
      • Family Zapodidae: jumping mice
      • Family Dipodidae: jerboas
    • Superfamily Muroidea
      • Family Platacanthomyidae: spiny dormice
      • Family Spalacidae: mole rats, bamboo rats, and zokors
      • Family Calomyscidae: mouse-like hamsters
      • Family Nesomyidae: climbing mice, rock mice, white-tailed rat, Malagasy rats and mice
      • Family Cricetidae: hamsters, New World rats and mice, voles
      • Family Muridae: true mice and rats, gerbils, spiny mice, crested rat

• Suborder Hystricognathi
  • Infraorder Hystricomorpha
    • Family Hystricidae: Old World porcupines
  • Infraorder Phiomorpha
    • Family Erethizontidae: New World porcupines
    • Family Thryonomyidae: cane rats
    • Family Bathyergidae: African mole rats
  • Infraorder Caviomorpha
    • Family Octodontidae: octodonts
    • Family Echimyidae: spiny rats, including nutrias
    • Family Capromyidae: hutias
    • Family Dasyproctidae: agoutis
    • Family Dinomyidae: pacaranas
    • Family Caviidae: cavies, including guinea pigs
    • Family Hydrochoeridae: Capybara
    • Family Chinchillidae: chinchillas and viscachas
    • Family Abrocomidae: chinchilla rats

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:
    • Aplodontidae
    • Sciuridae

• Myodonta includes:
  • Dipodoidea
  • Muroidea

The positions of the Castoridae, Geomyoidea, Anomaluridae, and Pedetidae are still being debated.

References

• 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.