Rodents are
of the
order Rodentia, characterised by a single pair of continuously growing
incisors in each of the upper and lower jaws that must be kept short by gnawing.
About 40% of mammal species are rodents, and they are found in vast numbers on all continents other than Antarctica. Common rodents include mice, , , , , , and .
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. With about 2,277
species of rodents,
[Wilson, D. E. and D. M. Reeder, eds. 2005. Mammal Species of the World: A Taxonomic and Geographic Reference. Johns Hopkins University Press, Baltimore.] over 40% of mammalian species belong 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.
[David Lambert and the Diagram Group. The Field Guide to Prehistoric Life. New York: Facts on File Publications, 1985. ISBN 0-8160-1125-7
]
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 besides bats and to have reached Australia without human introduction. Members of non-rodent orders, such as Chiroptera (bats), Scandentia (), Soricomorpha ( and mole), Lagomorpha (, and ), and mustelid carnivore, such as and mink, are sometimes confused with rodents.
Characteristics
Dentition
all rodents share the characteristic of
dentition highly specialized for gnawing. This specialization gives rodents their name from the Latin,
rodere, to gnaw.
[ ξ1 ] 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;
Paucidentomys vermidax is unique among the rodents in that it possesses no molars or premolars, and its incisors are so specialised, they are not for gnawing.
[Esselstyn, J.A., Achmadi, A.S. and Rowe, K.C., (2012). Evolutionary novelty in a rat with no molars. Biology Letters, published online 22 August 2012. doi:10.1098/rsbl.2012.0574 1744-957X]
Typical 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 like the blade of a chisel.
[Hurst, J.L., (1999). Introduction to rodents. In: The UFAW Handbook on the Care and Management of Laboratory Animals, Vol. 1, Terrestrial Vertebrates, 7th edn. Ed. Poole, T., pp. 262–273. Blackwell Publishing, Oxford] 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. Their incisors are highly versatile and can be used for a range of functions, such as cutting wood, biting through the skin of fruit, prey capture, or defense, depending on the species. Nearly all rodents feed on plants, seeds in particular, but a number of species eat insects (grasshopper mouse,
Onychomys leucogaster) or fish (fish-eating rats,
Ichthyomyini). Some squirrels are known to eat
passerine , such as
cardinal and
. One species,
Paucidentomys vermidax, feeds primarily on
worms and lacks the ability to gnaw or even chew, possessing bladelike, forked upper incisors and no molars.
Size
, the largest living rodent, can weigh up to .]] Many rodents are small; the tiny
African pygmy mouse,
Mus minutoides, can be as small as in length and in weight at maturity, and the
Baluchistan pygmy jerboa,
Salpingotulus michaelis, is of roughly similar or slightly smaller dimensions. Conversely, the largest extant rodent, the capybara,
Hydrochoerus hydrochaeris, usually weighs up to , with exceptional specimens weighing up to .
[http://www.science.smith.edu/msi/pdf/i0076-3519-264-01-0001.pdf] Several enormous rodents are known from the fossil record, the largest known being
Josephoartigasia monesi, which is estimated to have typically weighed about , and possibly up to
or 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
in animal testing, for food, and even for detecting
.
[ "A rat with a nose for landmines is doing its bit for humanity" Cited as coming from the New York Times in the article.] Due to the wide diversity of their characteristics, some of which are considered uncommon or unique amongst mammals, rodents are used widely in research.
[Sherwin, C.M., (2010). The Husbandry and Welfare of Non-traditional Laboratory Rodents. In “UFAW Handbook on the Care and Management of Laboratory Animals”, R. Hubrecht and J. Kirkwood (Eds). Wiley-Blackwell. Chapter 25, pp. 359–369] 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 and history
The
fossil record of rodent-like mammals begins shortly after the extinction of the non-avian
66 million years ago, as early as the
Paleocene. Some
molecular clock data, however, suggest modern rodents (members of the order Rodentia) had 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
, to spread into
Eurasia. During the
Pliocene, rodent fossils appeared in Australia. Although
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
- , 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
- Josephoartigasia monesi, the largest known rodent, with an estimated weight of very roughly
- Phoberomys pattersoni, the second-largest known rodent, with an estimated weight of
- Telicomys, another giant South American rodent
Classification
Standard classification
The rodents are part of the
Glires (along with
lagomorphs),
Euarchontoglires (along with
lagomorphs,
,
, and
), and
Boreoeutheria (along with most other
placental ). The order Rodentia may be divided into
,
,
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:
-
:*Superfamily Geomyoidea
-
:**Family Geomyidae: pocket gophers (true gophers)
-
:**Family Heteromyidae: and kangaroo mice
-
Suborder Hystricomorpha
-
::*Family incertae sedis Diatomyidae: Laotian rock rat
-
Suborder Myomorpha
-
:*Superfamily Dipodoidea
-
:**Family Dipodidae: jerboas and jumping mice
-
:*Superfamily Muroidea
-
:**Family Calomyscidae: mouse-like hamsters
-
:**Family Cricetidae: , New World rats and mice, ,
-
:**Family Muridae: true mice and , , 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
-
Suborder Sciuromorpha
-
::*Family Aplodontiidae: mountain beaver
-
::*Family Gliridae (also Myoxidae, Muscardinidae): dormice
-
::*Family Sciuridae: , including , , &
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 have been consistently produced, such as:
-
An unnamed clade containing:
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
caviomorph 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
myomorph prior to later divergences of Myomorpha; this would mean caviomorphs, or possibly
hystricomorph, would be moved out of the rodent classification into a separate order.
A minority scientific opinion exists that argues that
,
, 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 a majority opinion among mammalian biologists that the order Rodentia is monophyletic, although there is not a complete consensus.
[Carleton, Michael D., and Musser, Guy G. "Order Rodentia". Mammal Species of the World, 3rd edition, 2005, vol. 2, p. 745. (Concise overview of the literature)]
Further reading
-
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.
-
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.
-
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
- Zoology, osteology, comparative anatomy
- Various
References
