Rodent
Their success is probably due to their small size, short breeding cycle, and ability to gnaw and eat a wide variety of foods.(Lambert, 200)
There are between 2000 and 3000
species of rodents, which are found in vast numbers on all continents except
Antarctica (they are the only
placental order other than
bats, (Chiroptera), to reach
Australia without human introduction), most islands, and in all habitats except for 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, 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 even fish.
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[ "A rat with a nose for landmines is doing its bit for humanity" Cited as coming from the New York Times in the article.].
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.
The
fossil record of rodents began after the extinction of the
dinosaurs 65 million years ago. By the end of the
Eocene epoch,
beavers and
squirrels appeared in the fossil record. They originated in
Laurasia, the joined continents of
North America,
Europe, and
Asia. Some species colonized
Africa, giving rise to the earliest
hystricognaths. From there they 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. 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
beaverCeratogaulus, a horned burrowing rodent:
Flores Giant Rat, a rat that grew to a large size on the island of
Flores:
Giant hutias, a group of rodents once found in the
West IndiesIschyromys, a primitive
squirrel-like rodent
Leithia, a giant
dormouseNeochoerus pinckneyi, a giant North American
capybara that weighed 50 kg
Phoberomys pattersoni, the largest known rodent
Telicomys, a giant South American rodent
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
Sciuromorpha**Family
Aplodontiidae:
mountain beaver**Family
Sciuridae:
squirrels, including
chipmunks &
prairie dogs
**Family
Gliridae (also Myoxidae, Muscardinidae):
dormice*Suborder
Castorimorpha**Superfamily Castoroidea
*** Family
Castoridae:
beavers
**Superfamily
Geomyoidea***Family
Heteromyidae:
kangaroo rats and
kangaroo mice***Family
Geomyidae: pocket gophers (true gophers)
*Suborder
Myomorpha**Superfamily
Dipodoidea***Family
Dipodidae: jerboas and jumping mice
**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
Anomaluromorpha**Family
Anomaluridae: scaly-tailed squirrels
**Family
Pedetidae: springhares
*Suborder
Hystricomorpha**Family
incertae sedis Diatomyidae: Laotian rock rat
**Infraorder
Ctenodactylomorphi***Family
Ctenodactylidae: gundis
**Infraorder
Hystricognathi***Family
Hystricidae: Old World porcupines
***Family
Erethizontidae: New World porcupines
***Family
Thryonomyidae: cane rats
***Family
Petromuridae: dassie rat
***Family
Bathyergidae: African mole rats
***Parvorder
Caviomorpha****Family
Octodontidae: octodonts
****Family
Echimyidae: spiny rats
****Family
Capromyidae: hutias
****Family †
Heptaxodontidae: giant hutias
****Family
Myocastoridae: nutria
****Family
Dasyproctidae: agoutis
****Family
Dinomyidae: pacaranas
****Family
Caviidae: cavies, including
guinea pigs****Family
Hydrochoeridae: Capybara
****Family
Chinchillidae:
chinchillas and viscachas
****Family
Abrocomidae: chinchilla rats
****Family
Ctenomyidae:
tuco-tucos
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**
MuroideaThe positions of the
Castoridae,
Geomyoidea,
Anomaluridae, and
Pedetidae are still being debated.
*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. John Hopkins University Press, Baltimore.
*David Lambert and the Diagram Group.
The Field Guide to Prehistoric Life. New York: Facts on File Publications, 1985. ISBN 0816011257
*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.
