Everything about Rodents totally explained
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.
Forty percent of mammal species are rodents, and they're 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. 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)
Rodents are found in vast numbers on all continents except Antarctica, most islands, and in all habitats except oceans. They are the only
placental order, other than
bats (Chiroptera) and
Pinnipeds, to reach
Australia without human introduction.
Characteristics
Many 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
65 kg (140 pounds) and the extinct
Phoberomys pattersoni is believed to have weighed 700 kg. The extinct
Josephoartigasia monesi weighed about 1 ton, and the biggest could have been more than 2.5 tons.
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 pets, as
model organisms in animal testing, for food, and even in detecting
landmines.
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 with rodents.
Evolution
The
fossil record of rodent-like mammals begins shortly after the extinction of the non-avian
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. 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
Josephoartigasia monesi, the largest known rodent » Phoberomys pattersoni, the second 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
- Suborder Castorimorpha
- Superfamily Castoroidea
- Superfamily Geomyoidea
- Suborder Hystricomorpha
- Family incertae sedis Diatomyidae: Laotian rock rat
- Infraorder Ctenodactylomorphi
- Infraorder Hystricognathi
- Family Bathyergidae: African mole rats
- Family Hystricidae: Old World porcupines
- Family Petromuridae: dassie rat
- Family Thryonomyidae: cane rats
- Parvorder Caviomorpha
- Suborder Myomorpha
- Superfamily Dipodoidea
- 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
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:
An unnnamed clade contains:
Myodonta includes:
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. 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 Information
Get more info on 'Rodents'.
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