Rodent
Rodentia is an order of mammals . Members of the order Rodentia are called rodents.
Encyclopedia
Rodentia is an order of mammals . Members of the order Rodentia are called
rodents.
Size and range of order
In terms of number of species — although not necessarily in terms of number of organisms or
biomass — rodents make up the largest order of mammals, 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.
There are between 2000 and 3000 species of rodents, which 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 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 and the extinct
Phoberomys pattersoni was a rodent [i] that lived in the ancient Orinoco River [i] delta approximate ...
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.
Members of non-rodent orders such as
Chiroptera ,
Scandentia ,
Insectivora ,
Lagomorpha and
mustelid carnivores such as
weasels and
mink are sometimes confused for rodents. Rodents have a carrier-immunity to
rabies virus, making them immune to the potentially infectious and lethal disease. They are carriers for most other animal-to-human illnesses, however, and still should not be agitated.
Natural history
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 beaver
- Ceratogaulus, 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 Indies
- Ischyromys, a primitive squirrel-like rodent
- Leithia, a giant dormouse
- Neochoerus pinckneyi, a giant North American capybara that weighed 50 kg
- Phoberomys pattersoni was a rodent [i] that lived in the ancient Orinoco River [i] delta approximate ...
, the largest known rodent - Telicomys, a giant South American rodent
Classification
The rodents are part of the clades:
Glires ,
Euarchontoglires , and Boreoeutheria . The order Rodentia may be divided into suborders, infraorders, superfamilies and
families.
Classification scheme:
ORDER RODENTIA ...
and
rats,
gerbils, spiny mice, crested rat
...
: cavies, including
guinea pigs
Alternate classifications
The above taxonomy uses the shape of the lower jaw as the primary character. This is the most commonly used approach for dividing the order into suborders. Many older references emphasize the zygomasseteric system .
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:
- An unnnamed clade contains:
The positions of the
Castoridae,
Geomyoidea, Anomaluridae, and
Pedetidae are still being debated.
Notes
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. 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 0-8160-1125-7
- 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.
