Hystricomorpha (incl. Caviomorpha)
Combined range of all rodent species (not including introduced
Latin rodere, "to gnaw") are mammals of the order
Rodentia, which are characterized by a single pair of continuously
growing incisors in each of the upper and lower jaws. About 40% of all
mammal species are rodents; they are found in vast numbers on all
continents except Antarctica. They are the most diversified mammalian
order and live in a variety of terrestrial habitats, including
Species can be arboreal, fossorial (burrowing), or semiaquatic.
Well-known rodents include mice, rats, squirrels, prairie dogs,
porcupines, beavers, guinea pigs, hamsters, gerbils and capybaras.
Other animals such as rabbits, hares, and pikas, whose incisors also
grow continually, were once included with them, but are now considered
to be in a separate order, the Lagomorpha. Nonetheless, Rodentia and
Lagomorpha are sister groups, sharing a most recent common ancestor
and forming the clade of Glires.
Most rodents are small animals with robust bodies, short limbs, and
long tails. They use their sharp incisors to gnaw food, excavate
burrows, and defend themselves. Most eat seeds or other plant
material, but some have more varied diets. They tend to be social
animals and many species live in societies with complex ways of
communicating with each other. Mating among rodents can vary from
monogamy, to polygyny, to promiscuity. Many have litters of
underdeveloped, altricial young, while others are precocial
(relatively well developed) at birth.
The rodent fossil record dates back to the
Paleocene on the
supercontinent of Laurasia. Rodents greatly diversified in the Eocene,
as they spread across continents, sometimes even crossing oceans.
Rodents reached both South America and Madagascar from Africa, and
were the only terrestrial placental mammals to reach and colonize
Rodents have been used as food, for clothing, as pets, and as
laboratory animals in research. Some species, in particular, the brown
rat, the black rat, and the house mouse, are serious pests, eating and
spoiling food stored by humans, and spreading diseases. Accidentally
introduced species of rodents are often considered to be invasive, and
have caused the extinction of numerous species, such as island birds,
previously isolated from land-based predators.
2 Distribution and habitat
3 Behavior and life history
3.2 Social behavior
3.4 Mating strategies
3.5 Birth and parenting
4 Classification and evolution
4.1 Evolutionary history
4.2 Standard classification
5 Interaction with humans
5.3 As pests and disease vectors
6 See also
8 Further reading
9 External links
9.1 Zoology, osteology, comparative anatomy
Drawing of typical rodent tooth system: The front surface of the
incisors is hard enamel, whereas the rear is softer dentine. The act
of chewing wears down the dentine, leaving a sharp, chisel-like edge.
Lingual view of top incisor from
Rattus rattus. Top incisor outlined
in yellow. Molars circled in blue.
The distinguishing feature of the rodents is their pairs of
continuously growing, razor-sharp incisors. These incisors have
thick layers of enamel on the front and little enamel on the back.
Because they do not stop growing, the animal must continue to wear
them down so that they do not reach and pierce the skull. As the
incisors grind against each other, the softer dentine on the rear of
the teeth wears away, leaving the sharp enamel edge shaped like the
blade of a chisel. Most species have up to 22 teeth with no canines
or anterior premolars. A gap, or diastema, occurs between the incisors
and the cheek teeth in most species. This allows rodents to suck in
their cheeks or lips to shield their mouth and throat from wood
shavings and other inedible material, discarding this waste from the
sides of their mouths. Chinchillas and guinea pigs have a
high-fiber diet; their molars have no roots and grow continuously like
In many species, the molars are relatively large, intricately
structured, and highly cusped or ridged.
Rodent molars are well
equipped to grind food into small particles. The jaw musculature is
strong. The lower jaw is thrust forward while gnawing and is pulled
backwards during chewing.
Rodent groups differ in the arrangement
of the jaw muscles and associated skull structures, both from other
mammals and amongst themselves. The Sciuromorpha, such as the eastern
grey squirrel, have a large deep masseter, making them efficient at
biting with the incisors. The Myomorpha, such as the brown rat, have
enlarged temporalis muscles, making them able to chew powerfully with
their molars. The Hystricomorpha, such as the guinea pig, have larger
superficial masseter muscles and smaller deep masseter muscles than
rats or squirrels, possibly making them less efficient at biting with
the incisors, but their enlarged internal pterygoid muscles may allow
them to move the jaw further sideways when chewing. The cheek pouch
is a specific morphological feature used for storing food and is
evident in particular subgroups of rodents like kangaroo rats,
hamsters, chipmunks and gophers which have two bags that may range
from the mouth to the front of the shoulders. True mice and rats do
not contain this structure but their cheeks are elastic due to a high
degree of musculature and innervation in the region.
A lingual view of the lower incisor from the right dentary of a Rattus
rattus. This image shows the extended view of the incisor that is
normally hidden within the dentary.
While the largest species, the capybara, can weigh as much as
66 kg (146 lb), most rodents weigh less than 100 g
(3.5 oz). The smallest rodent is the Baluchistan pygmy jerboa,
which averages only 4.4 cm (1.7 in) in head and body length,
with adult females weighing only 3.75 g (0.132 oz). Rodents
have wide-ranging morphologies, but typically have squat bodies and
short limbs. The fore limbs usually have five digits, including an
opposable thumb, while the hind limbs have three to five digits. The
elbow gives the fore arms great flexibility. The majority of
species are plantigrade, walking on both the palms and soles of their
feet, and have claw-like nails. The nails of burrowing species tend to
be long and strong, while arboreal rodents have shorter, sharper
Rodent species use a wide variety of methods of locomotion
including quadrupedal walking, running, burrowing, and climbing,
bipedal hopping (kangaroo rats and hopping mice), swimming, and even
gliding. Scaly-tailed squirrels and flying squirrels, although not
closely related, can both glide from tree to tree using parachute-like
membranes that stretch from the fore to the hind limbs. The agouti
is fleet-footed and antelope-like, being digitigrade and having
hoof-like nails. The majority of rodents have tails, which can be of
many shapes and sizes. Some tails are prehensile, as in the Eurasian
harvest mouse, and the fur on the tails can vary from bushy to
completely bald. The tail is sometimes used for communication, as when
beavers slap their tails on the water surface or house mice rattle
their tails to indicate alarm. Some species have vestigial tails or no
tails at all. In some species, the tail is capable of regeneration
if a part is broken off.
Wood mouse with its long whiskers
Rodents generally have well-developed senses of smell, hearing, and
vision. Nocturnal species often have enlarged eyes and some are
sensitive to ultraviolet light. Many species have long, sensitive
whiskers or vibrissae for touch or "whisking". Some rodents have cheek
pouches, which may be lined with fur. These can be turned inside out
for cleaning. In many species, the tongue cannot reach past the
incisors. Rodents have efficient digestive systems, absorbing nearly
80% of ingested energy. When eating cellulose, the food is softened in
the stomach and passed to the cecum, where bacteria reduce it to its
carbohydrate elements. The rodent then practices coprophagy, eating
its own fecal pellets, so the nutrients can be absorbed by the gut.
Rodents therefore often produce a hard and dry fecal pellet. In
many species, the penis contains a bone, the baculum; the testes can
be located either abdominally or at the groin.
Sexual dimorphism occurs in many rodent species. In some rodents,
males are larger than females, while in others the reverse is true.
Male-bias sexual dimorphism is typical for ground squirrels, kangaroo
rats, solitary mole rats, and pocket gophers; it likely developed due
to sexual selection and greater male-male combat. Female-bias sexual
dimorphism exists among chipmunks and jumping mice. It is not
understood why this pattern occurs, but in the case of yellow-pine
chipmunks, males may have selected larger females due to their greater
reproductive success. In some species, such as voles, sexual
dimorphism can vary from population to population. In bank voles,
females are typically larger than males, but male-bias sexual
dimorphism occurs in alpine populations, possibly because of the lack
of predators and greater competition between males.
Distribution and habitat
Brown rat in a flowerbox: Some rodents thrive in human habitats.
One of the most widespread groups of mammals, rodents can be found on
every continent except Antarctica. They are the only terrestrial
placental mammals to have colonized Australia and
New Guinea without
human intervention. Humans have also allowed the animals to spread to
many remote oceanic islands (e.g., the Polynesian rat). Rodents
have adapted to almost every terrestrial habitat, from cold tundra
(where they can live under snow) to hot deserts.
Some species such as tree squirrels and New World porcupines are
arboreal, while some, such as gophers, tuco-tucos, and mole rats, live
almost completely underground, where they build complex burrow
systems. Others dwell on the surface of the ground, but may have a
burrow into which they can retreat. Beavers and muskrats are known for
being semiaquatic, but the rodent best-adapted for aquatic life is
probably the earless water rat from New Guinea. Rodents have also
thrived in human-created environments such as agricultural and urban
Some rodents, like this
North American beaver
North American beaver with its dam of gnawed
tree trunks and the lake it has created, are considered ecosystem
Though some species are common pests for humans, rodents also play
important ecological roles. Some rodents are considered keystone
species and ecosystem engineers in their respective habitats. In the
Great Plains of North America, the burrowing activities of prairie
dogs play important roles in soil aeration and nutrient
redistribution, raising the organic content of the soil and increasing
the absorption of water. They maintain these grassland habitats,
and some large herbivores such as bison and pronghorn prefer to graze
near prairie dog colonies due to the increased nutritional quality of
Extirpation of prairie dogs can also contribute to regional and local
biodiversity loss, increased seed depredation, and the establishment
and spread of invasive shrubs. Burrowing rodents may eat the
fruiting bodies of fungi and spread spores through their feces,
thereby allowing the fungi to disperse and form symbiotic
relationships with the roots of plants (which usually cannot thrive
without them). As such, these rodents may play a role in maintaining
In many temperate regions, beavers play an essential hydrological
role. When building their dams and lodges, beavers alter the paths of
streams and rivers and allow for the creation of extensive wetland
habitats. One study found that engineering by beavers leads to a 33
percent increase in the number of herbaceous plant species in riparian
areas. Another study found that beavers increase wild salmon
Behavior and life history
Eastern chipmunk carrying food in cheek pouches
Most rodents are herbivorous, feeding exclusively on plant material
such as seeds, stems, leaves, flowers, and roots. Some are omnivorous
and a few are predators. The field vole is a typical herbivorous
rodent and feeds on grasses, herbs, root tubers, moss, and other
vegetation, and gnaws on bark during the winter. It occasionally eats
invertebrates such as insect larvae. The plains pocket gopher eats
plant material found underground during tunneling, and also collects
grasses, roots, and tubers in its cheek pouches and caches them in
underground larder chambers.
Texas pocket gopher
Texas pocket gopher avoids emerging onto the surface to feed by
seizing the roots of plants with its jaws and pulling them downwards
into its burrow. It also practices coprophagy. The African pouched
rat forages on the surface, gathering anything that might be edible
into its capacious cheek pouches until its face bulges out sideways.
It then returns to its burrow to sort through the material it has
gathered and eats the nutritious items.
Agouti species are one of the few animal groups that can break open
the large capsules of the
Brazil nut fruit. Too many seeds are inside
to be consumed in one meal, so the agouti carries some off and caches
them. This helps dispersal of the seeds as any that the agouti fails
to retrieve are distant from the parent tree when they germinate.
Other nut-bearing trees tend to bear a glut of fruits in the autumn.
These are too numerous to be eaten in one meal and squirrels gather
and store the surplus in crevices and hollow trees. In desert regions,
seeds are often available only for short periods. The kangaroo rat
collects all it can find and stores them in larder chambers in its
A strategy for dealing with seasonal plenty is to eat as much as
possible and store the surplus nutrients as fat. Marmots do this, and
may be 50% heavier in the autumn than in the spring. They rely on
their fat reserves during their long winter hibernation. Beavers
feed on the leaves, buds, and inner bark of growing trees, as well as
aquatic plants. They store food for winter use by felling small trees
and leafy branches in the autumn and immersing them in their pond,
sticking the ends into the mud to anchor them. Here, they can access
their food supply underwater even when their pond is frozen over.
Although rodents have been regarded traditionally as herbivores, a
number of species opportunistically include insects, fish, or meat in
their diets and more specialized forms rely on such foods. A
functional-morphological study of the rodent tooth system supports the
idea that primitive rodents were omnivores rather than herbivores.
Studies of the literature show that numerous members of the
Sciuromorpha and Myomorpha, and a few members of the Hystricomorpha,
have either included animal matter in their diets or been prepared to
eat such food when offered it in captivity. Examination of the stomach
contents of the North American white-footed mouse, normally considered
to be herbivorous, showed 34% animal matter.
More specialized carnivores include the shrewlike rats of the
Philippines, which feed on insects and soft-bodied invertebrates, and
the Australian water rat, which devours aquatic insects, fish,
crustaceans, mussels, snails, frogs, birds' eggs, and water
birds. The grasshopper mouse from dry regions of North America
feeds on insects, scorpions, and other small mice, and only a small
part of its diet is plant material. It has a chunky body with short
legs and tail, but is agile and can easily overpower prey as large as
Prairie dog "town"
Rodents exhibit a wide range of types of social behavior ranging from
the mammalian caste system of the naked mole-rat, the extensive
"town" of the colonial prairie dog, through family groups to the
independent, solitary life of the edible dormouse. Adult dormice may
have overlapping feeding ranges, but they live in individual nests and
feed separately, coming together briefly in the breeding season to
mate. The pocket gopher is also a solitary animal outside the breeding
season, each individual digging a complex tunnel system and
maintaining a territory.
Larger rodents tend to live in family units where parents and their
offspring live together until the young disperse. Beavers live in
extended family units typically with a pair of adults, this year's
kits, the previous year's offspring, and sometimes older young.
Brown rats usually live in small colonies with up to six females
sharing a burrow and one male defending a territory around the burrow.
At high population densities, this system breaks down and males show a
hierarchical system of dominance with overlapping ranges. Female
offspring remain in the colony while male young disperse. The
prairie vole is monogamous and forms a lifelong pair bond. Outside the
breeding season, prairie voles live in close proximity with others in
small colonies. A male is not aggressive towards other males until he
has mated, after which time he defends a territory, a female, and a
nest against other males. The pair huddles together, grooms one
another, and shares nesting and pup-raising responsibilities.
A nest of naked mole rats
Among the most social of rodents are the ground squirrels, which
typically form colonies based on female kinship, with males dispersing
after weaning and becoming nomadic as adults. Cooperation in ground
squirrels varies between species and typically includes making alarm
calls, defending territories, sharing food, protecting nesting areas,
and preventing infanticide. The black-tailed prairie dog forms
large towns that may cover many hectares. The burrows do not
interconnect, but are excavated and occupied by territorial family
groups known as coteries. A coterie often consists of an adult male,
three or four adult females, several nonbreeding yearlings, and the
current year's offspring. Individuals within coteries are friendly
with each other, but hostile towards outsiders.
Perhaps the most extreme examples of colonial behavior in rodents are
the eusocial naked mole rat and Damaraland mole rat. The naked mole
rat lives completely underground and can form colonies of up to 80
individuals. Only one female and up to three males in the colony
reproduce, while the rest of the members are smaller and sterile, and
function as workers. Some individuals are of intermediate size. They
help with the rearing of the young and can take the place of a
reproductive if one dies. The
Damaraland mole rat
Damaraland mole rat is characterized
by having a single reproductively active male and female in a colony
where the remaining animals are not truly sterile, but become fertile
only if they establish a colony of their own.
Nepotistic species such as house mice rely on urine, feces and
glandular secretions to recognize their kin.
Rodents use scent marking in many social contexts including inter- and
intra-species communication, the marking of trails and the
establishment of territories. Their urine provides genetic information
about individuals including the species, the sex and individual
identity, and metabolic information on dominance, reproductive status
and health. Compounds derived from the major histocompatibility
complex (MHC) are bound to several urinary proteins. The odor of a
predator depresses scent-marking behavior.
Rodents are able to recognize close relatives by smell and this allows
them to show nepotism (preferential behavior toward their kin) and
also avoid inbreeding. This kin recognition is by olfactory cues from
urine, feces and glandular secretions. The main assessment may involve
the MHC, where the degree of relatedness of two individuals is
correlated to the MHC genes they have in common. In non-kin
communication, where more permanent odor markers are required, as at
territorial borders, then non-volatile major urinary proteins (MUPs),
which function as pheromone transporters, may also be used. MUPs may
also signal individual identity, with each male house mouse (Mus
musculus) excreting urine containing about a dozen genetically encoded
House mice deposit urine, which contains pheromones, for territorial
marking, individual and group recognition, and social organization
Territorial beavers and red squirrels investigate and become familiar
with the scents of their neighbors and respond less aggressively to
intrusions by them than to those made by non-territorial "floaters" or
strangers. This is known as the "dear enemy effect".
Common degus have a complex vocal repertoire.
Many rodent species, particularly those that are diurnal and social,
have a wide range of alarm calls that are emitted when they perceive
threats. There are both direct and indirect benefits of doing this. A
potential predator may stop when it knows it has been detected, or an
alarm call can allow conspecifics or related individuals to take
evasive action. Several species, for example prairie dogs, have
complex anti-predator alarm call systems. These species may have
different calls for different predators (e.g. aerial predators or
ground-based predators) and each call contains information about the
nature of the precise threat. The urgency of the threat is also
conveyed by the acoustic properties of the call.
Social rodents have a wider range of vocalizations than do solitary
species. Fifteen different call-types have been recognized in adult
Kataba mole rats and four in juveniles. Similarly, the common
degu, another social, burrowing rodent, exhibits a wide array of
communication methods and has an elaborate vocal repertoire comprising
fifteen different categories of sound. Ultrasonic calls play a
part in social communication between dormice and are used when the
individuals are out of sight of each other.
House mice use both audible and ultrasonic calls in a variety of
contexts. Audible vocalizations can often be heard during agonistic or
aggressive encounters, whereas ultrasound is used in sexual
communication and also by pups when they have fallen out of the
Laboratory rats (which are brown rats,
Rattus norvegicus) emit short,
high frequency, ultrasonic vocalizations during purportedly
pleasurable experiences such as rough-and-tumble play, when
anticipating routine doses of morphine, during mating, and when
tickled. The vocalization, described as a distinct "chirping", has
been likened to laughter, and is interpreted as an expectation of
something rewarding. In clinical studies, the chirping is associated
with positive emotional feelings, and social bonding occurs with the
tickler, resulting in the rats becoming conditioned to seek the
tickling. However, as the rats age, the tendency to chirp declines.
Like most rat vocalizations, the chirping is at frequencies too high
for humans to hear without special equipment, so bat detectors have
been used for this purpose.
Rodents, like all placental mammals except primates, have just two
types of light receptive cones in their retina, a short wavelength
"blue-UV" type and a middle wavelength "green" type. They are
therefore classified as dichromats; however, they are visually
sensitive into the ultraviolet (UV) spectrum and therefore can see
light that humans can not. The functions of this UV sensitivity are
not always clear. In degus, for example, the belly reflects more UV
light than the back. Therefore, when a degu stands up on its hind
legs, which it does when alarmed, it exposes its belly to other degus
and ultraviolet vision may serve a purpose in communicating the alarm.
When it stands on all fours, its low UV-reflectance back could help
make the degu less visible to predators.
Ultraviolet light is
abundant during the day but not at night. There is a large increase in
the ratio of ultraviolet to visible light in the morning and evening
twilight hours. Many rodents are active during twilight hours
(crepuscular activity), and UV-sensitivity would be advantageous at
Ultraviolet reflectivity is of dubious value for
The urine of many rodents (e.g. voles, degus, mice, rats) strongly
reflects UV light and this may be used in communication by leaving
visible as well as olfactory markings. However, the amount of UV
that is reflected decreases with time, which in some circumstances can
be disadvantageous; the common kestrel can distinguish between old and
fresh rodent trails and has greater success hunting over more recently
Middle East blind mole rat
Middle East blind mole rat uses seismic communication.
Vibrations can provide cues to conspecifics about specific behaviors
being performed, predator warning and avoidance, herd or group
maintenance, and courtship. The
Middle East blind mole rat
Middle East blind mole rat was the
first mammal for which seismic communication was documented. These
fossorial rodents bang their head against the walls of their tunnels.
This behavior was initially interpreted as part of their tunnel
building behavior, but it was eventually realized that they generate
temporally patterned seismic signals for long-distance communication
with neighboring mole rats.
Footdrumming is used widely as a predator warning or defensive action.
It is used primarily by fossorial or semi-fossorial rodents. The
banner-tailed kangaroo rat produces several complex footdrumming
patterns in a number of different contexts, one of which is when it
encounters a snake. The footdrumming may alert nearby offspring but
most likely conveys that the rat is too alert for a successful attack,
thus preventing the snake's predatory pursuit. Several studies
have indicated intentional use of ground vibrations as a means of
intra-specific communication during courtship among the Cape mole
rat. Footdrumming has been reported to be involved in male-male
competition; the dominant male indicates its resource holding
potential by drumming, thus minimizing physical contact with potential
Cape ground squirrel
Cape ground squirrel is an example of a promiscuous rodent.
Some species of rodent are monogamous, with an adult male and female
forming a lasting pair bond. Monogamy can come in two forms; obligate
and facultative. In obligate monogamy, both parents care for the
offspring and play an important part in their survival. This occurs in
species such as California mice, oldfield mice, Malagasy giant rats
and beavers. In these species, males usually mate only with their
partners. In addition to increased care for young, obligate monogamy
can also be beneficial to the adult male as it decreases the chances
of never finding a mate or mating with an infertile female. In
facultative monogamy, the males do not provide direct parental care
and stay with one female because they cannot access others due to
being spatially dispersed. Prairie voles appear to be an example of
this form of monogamy, with males guarding and defending females
within their vicinity.
In polygynous species, males will try to monopolize and mate with
multiple females. As with monogamy, polygyny in rodents can come in
two forms; defense and non-defense. Defense polygyny involves males
controlling territories that contain resources that attract females.
This occurs in ground squirrels like yellow-bellied marmots,
California ground squirrels, Columbian ground squirrels and
Richardson's ground squirrels. Males with territories are known as
"resident" males and the females that live within the territories are
known as "resident" females. In the case of marmots, resident males do
not appear to ever lose their territories and always win encounters
with invading males. Some species are also known to directly defend
their resident females and the ensuing fights can lead to severe
wounding. In species with non-defense polygyny, males are not
territorial and wander widely in search of females to monopolize.
These males establish dominance hierarchies, with the high-ranking
males having access to the most females. This occurs in species like
Belding's ground squirrels and some tree squirrel species.
A mating plug in a female Richardson's ground squirrel
Promiscuity, in which both males and females mate with multiple
partners, also occurs in rodents. In species such as the white-footed
mouse, females give birth to litters with multiple paternities.
Promiscuity leads to increased sperm competition and males tend to
have larger testicles. In the Cape ground squirrel, the male's testes
can be 20 percent of its head-body length. Several rodent species
have flexible mating systems that can vary between monogamy, polygyny
Female rodents play an active role in choosing their mates. Factors
that contribute to female preference may include the size, dominance
and spatial ability of the male. In the eusocial naked mole rats,
a single female monopolizes mating from at least three males.
In most rodent species, such as brown rats and house mice, ovulation
occurs on a regular cycle while in others, such as voles, it is
induced by mating. During copulation, males of some rodent species
deposit a mating plug in the female's genital opening, both to prevent
sperm leakage and to protect against other males inseminating the
female. Females can remove the plug and may do so either immediately
or after several hours.
Birth and parenting
Young bank voles in their nest beneath a wood pile
Rodents may be born either altricial (blind, hairless and relatively
underdeveloped) or precocial (mostly furred, eyes open and fairly
developed) depending on the species. The altricial state is typical
for squirrels and mice, while the precocial state usually occurs in
species like guinea pigs and porcupines. Females with altricial young
typically build elaborate nests before they give birth and maintain
them until their offspring are weaned. The female gives birth sitting
or lying down and the young emerge in the direction she is facing. The
newborns first venture out of the nest a few days after they have
opened their eyes and initially keep returning regularly. As they get
older and more developed, they visit the nest less often and leave
permanently when weaned.
In precocial species, the mothers invest little in nest building and
some do not build nests at all. The female gives birth standing and
the young emerge behind her. Mothers of these species maintain contact
with their highly mobile young with maternal contact calls. Though
relatively independent and weaned within days, precocial young may
continue to nurse and be groomed by their mothers.
Rodent litter sizes
also vary and females with smaller litters spend more time in the nest
than those with larger litters.
Two Patagonian maras with young, an example of a monogamous and
communal nesting species
Mother rodents provide both direct parental care, such as nursing,
grooming, retrieving and huddling, and indirect parenting, such as
food caching, nest building and protection to their offspring. In
many social species, young may be cared for by individuals other than
their parents, a practice known as alloparenting or cooperative
breeding. This is known to occur in black-tailed prairie dogs and
Belding's ground squirrels, where mothers have communal nests and
nurse unrelated young along with their own. There is some question as
to whether these mothers can distinguish which young are theirs. In
the Patagonian mara, young are also placed in communal warrens, but
mothers do not permit youngsters other than their own to nurse.
Infanticide exists in numerous rodent species and may be practiced by
adult conspecifics of either sex. Several reasons have been proposed
for this behavior, including nutritional stress, resource competition,
avoiding misdirecting parental care and, in the case of males,
attempting to make the mother sexually receptive. The latter reason is
well supported in primates and lions but less so in rodents.
Infanticide appears to be widespread in black-tailed prairie dogs,
including infanticide from invading males and immigrant females, as
well as occasional cannibalism of an individual's own offspring.
To protect against infanticide from other adults, female rodents may
employ avoidance or direct aggression against potential perpetrators,
multiple mating, territoriality or early termination of pregnancy.
Feticide can also occur among rodents; in Alpine marmots, dominant
females tend to suppress the reproduction of subordinates by being
antagonistic towards them while they are pregnant. The resulting
stress causes the fetuses to abort.
Kangaroo rats can locate food caches by spatial memory.
Rodents have advanced cognitive abilities. They can quickly learn to
avoid poisoned baits, which makes them difficult pests to deal
with. Guinea pigs can learn and remember complex pathways to
food. Squirrels and kangaroo rats are able to locate caches of
food by spatial memory, rather than just by smell.
Because laboratory mice (house mice) and rats (brown rats) are widely
used as scientific models to further our understanding of biology, a
great deal has come to be known about their cognitive capacities.
Brown rats exhibit cognitive bias, where information processing is
biased by whether they are in a positive or negative affective
state. For example, laboratory rats trained to respond to a
specific tone by pressing a lever to receive a reward, and to press
another lever in response to a different tone so as to avoid receiving
an electric shock, are more likely to respond to an intermediate tone
by choosing the reward lever if they have just been tickled (something
they enjoy), indicating "a link between the directly measured positive
affective state and decision making under uncertainty in an animal
Laboratory (brown) rats may have the capacity for metacognition—to
consider their own learning and then make decisions based on what they
know, or do not know, as indicated by choices they make apparently
trading off difficulty of tasks and expected rewards, making them the
first animals other than primates to have this capacity, but
these findings are disputed, since the rats may have been following
simple operant conditioning principles, or a behavioral economic
model. Brown rats use social learning in a wide range of
situations, but perhaps especially so in acquiring food
Classification and evolution
Masillamys sp. fossil from the
Eocene Messel Pit fossil site, Germany
See also: List of extinct rodents
Dentition is the key feature by which fossil rodents are recognized
and the earliest record of such mammals comes from the Paleocene,
shortly after the extinction of the non-avian dinosaurs some 66
million years ago. These fossils are found in Laurasia, the
supercontinent composed of modern-day North America, Europe, and Asia.
The divergence of Glires, a clade consisting of rodents and lagomorphs
(rabbits, hares and pikas), from other placental mammals occurred
within a few million years after the Cretaceous-Tertiary boundary;
rodents and lagomorphs then radiated during the Cenozoic. Some
molecular clock data suggest modern rodents (members of the order
Rodentia) had appeared by the late Cretaceous, although other
molecular divergence estimations are in agreement with the fossil
Rodents are thought to have evolved in Asia, where local
multituberculate faunas were severely affected by the
Paleogene extinction event and never fully recovered,
unlike their North American and European relatives. In the resulting
ecological vacuum, rodents and other
Glires were able to evolve and
diversify, taking the niches left by extinct multituberculates. The
correlation between the spread of rodents and the demise of
multituberculates is a controversial topic, not fully resolved.
American and European multituberculate assemblages do decline in
diversity in correlation with the introduction of rodents in these
areas, but the remaining Asian multituberculates co-existed with
rodents with no observable replacement taking place, and ultimately
both clades co-existed for at least 15 million years.
The history of the colonization of the world's continents by rodents
is complex. The movements of the large superfamily
hamsters, gerbils, true mice and rats) may have involved up to seven
colonizations of Africa, five of North America, four of Southeast
Asia, two of South America and up to ten of Eurasia.
The horned gopher Ceratogaulus hatcheri, a burrowing mammal of the
Miocene to early Pleistocene, was the only horned rodent.
During the Eocene, rodents began to diversify. Beavers appeared in
North America in the late
Eocene before spreading to Eurasia. Late
in the Eocene, hystricognaths invaded Africa, most probably having
originated in Asia at least 39.5 million years ago. From Africa,
fossil evidence shows that some hystricognaths (caviomorphs) colonized
South America, which was an isolated continent at the time, evidently
making use of ocean currents to cross the Atlantic on floating
debris. Caviomorphs had arrived in South America by 41 million
years ago (implying a date at least as early as this for
hystricognaths in Africa), and had reached the Greater Antilles by
the early Oligocene, suggesting that they must have dispersed rapidly
across South America.
Nesomyid rodents are thought to have rafted from Africa to Madagascar
20–24 million years ago. All 27 species of native Malagasy
rodents appear to be descendents of a single colonization event.
By 20 million years ago, fossils recognizably belonging to the current
families such as
Muridae had emerged. By the Miocene, when Africa
had collided with Asia, African rodents such as the porcupine began to
spread into Eurasia. Some fossil species were very large in
comparison to modern rodents and included the giant beaver,
Castoroides ohioensis, which grew to a length of 2.5 m (8 ft
2 in) and weight of 100 kg (220 lb). The largest
known rodent was Josephoartigasia monesi, a pacarana with an estimated
body length of 3 m (10 ft).
The first rodents arrived in Australia via Indonesia around 5 million
years ago. Although marsupials are the most prominent mammals in
Australia, many rodents, all belonging to the subfamily Murinae, are
among the continent's mammal species. There are about fifty
species of 'old endemics', the first wave of rodents to colonize the
country in the
Miocene and early Pliocene, and eight true rat (Rattus)
species of 'new endemics', arriving in a subsequent wave in the late
Pliocene or early Pleistocene. The earliest fossil rodents in
Australia have a maximum age of 4.5 million years, and molecular
data is consistent with the colonization of
New Guinea from the west
during the late
Miocene or early
Pliocene followed by rapid
diversification. A further wave of adaptive radiation occurred after
one or more colonizations of Australia some 2 to 3 million years
Rodents participated in the
Great American Interchange
Great American Interchange that resulted
from the joining of the Americas by formation of the Isthmus of
Panama, around 3 million years ago in the
Piacenzian age. In this
exchange, a small number of species such as the New World porcupines
(Erethizontidae) headed north. However, the main southward
invasion of sigmodontines preceded formation of the land bridge by at
least several million years, probably occurring via
rafting. Sigmodontines diversified explosively once in
South America, although some degree of diversification may have
already occurred in Central America before the colonization.
Their "head start" has relegated other North American rodent groups
(sciurids, geomyids, heteromyids and nonsigmodontine cricetids) to a
minor presence in the contemporary South American fauna.
The use of the order name "Rodentia" is attributed to the English
traveler and naturalist
Thomas Edward Bowdich
Thomas Edward Bowdich (1821). The Modern
Latin word "Rodentia" is derived from "rodens", present participle of
"rodere" – "to gnaw", "eat away". The hares, rabbits and pikas
(order Lagomorpha) have continuously growing incisors, as do rodents,
and were at one time included in the order. However, they have an
additional pair of incisors in the upper jaw and the two orders have
quite separate evolutionary histories. The phylogeny of the
rodents places them in the clades Glires,
Boreoeutheria. The cladogram below shows the inner and outer relations
of Rodentia based on a 2012 attempt by Wu et al. to align the
molecular clock with paleontological data:
Ochotona (Old World rabbits)
Sylvilagus (New World rabbits)
Atherurus (brush-tailed porcupines)
Octodontomys (mountain degus)
Erethizon (North American porcupines)
Cavia (guinea pigs)
Aplodontia (mountain beavers)
Glaucomys (New World flying squirrels)
Dipodomys (kangaroo rats)
Thomomys (pocket gophers)
Peromyscus (deer mice)
Mus ([true] mice)
Sicista (birch mice)
Zapus (jumping mice)
Cardiocranius (pygmy jerboas)
The living rodent families based on the study done by Fabre et al.
The order Rodentia may be divided into suborders, infraorders,
superfamilies and families. There is a great deal of parallelism and
convergence among rodents caused by the fact that they have tended to
evolve to fill largely similar niches. This parallel evolution
includes not only the structure of the teeth, but also the
infraorbital region of the skull (below the eye socket) and makes
classification difficult as similar traits may not be due to common
ancestry. Brandt (1855) was the first to propose dividing
Rodentia into three suborders, Sciuromorpha,
Myomorpha, based on the development of certain muscles in the jaw and
this system was widely accepted. Schlosser (1884) performed a
comprehensive review of rodent fossils, mainly using the cheek teeth,
and found that they fitted into the classical system, but Tullborg
(1899) proposed just two sub-orders, Sciurognathi and Hystricognathi.
These were based on the degree of inflection of the lower jaw and were
to be further subdivided into Sciuromorpha, Myomorpha, Hystricomorpha
and Bathyergomorpha. Matthew (1910) created a phylogenetic tree of New
World rodents but did not include the more problematic Old World
species. Further attempts at classification continued without
agreement, with some authors adopting the classical three suborder
system and others Tullborg's two suborders.
These disagreements remain unresolved, nor have molecular studies
fully resolved the situation though they have confirmed the monophyly
of the group and that the clade has descended from a common Paleocene
ancestor. Carleton and Musser (2005) in
Mammal Species of the World
have provisionally adopted a five suborder system: Sciuromorpha,
Castorimorpha, Myomorpha, Anomaluromorpha, and Hystricomorpha. These
include 33 families, 481 genera and 2277 species:
Order Rodentia (from Latin, rodere, to gnaw)
Anomaluromorpha: East African springhare
Castorimorpha: Botta's pocket gopher
Caviomorpha: North American porcupine
Myomorpha: Golden or Syrian hamster
Sciuromorpha: African dormouse
Family Anomaluridae: scaly-tailed squirrels
Family Pedetidae: springhares
Family Castoridae: beavers
Family Geomyidae: pocket gophers (true gophers)
Family Heteromyidae: kangaroo rats, kangaroo mice
Family incertae sedis Diatomyidae: Laotian rock rat
Family Ctenodactylidae: gundis
Family Bathyergidae: African mole rats
Family Hystricidae: Old World porcupines
Family Petromuridae: dassie rat
Family Thryonomyidae: cane rats
Family †Heptaxodontidae: giant hutias
Family Abrocomidae: chinchilla rats
Family Capromyidae: hutias
Family Caviidae: cavies, including Guinea pigs and the capybara
Family Chinchillidae: chinchillas, viscachas
Family Ctenomyidae: tuco-tucos
Family Dasyproctidae: agoutis
Family Cuniculidae: pacas
Family Dinomyidae: pacaranas
Family Echimyidae: spiny rats
Family Erethizontidae: New World porcupines
Family Myocastoridae: coypu (nutria)
Family Octodontidae: octodonts
Family Dipodidae: jerboas and jumping mice
Family Calomyscidae: mouse-like hamsters
Family Cricetidae: hamsters, New World rats and mice, muskrats, 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, zokors
Family Aplodontiidae: mountain beaver
Gliridae (also Myoxidae, Muscardinidae): dormice
Family Sciuridae: squirrels, including chipmunks, prairie dogs,
Interaction with humans
Drawing of the critically endangered red crested soft-furred spiny rat
While rodents are not the most seriously threatened order of mammals,
there are 168 species in 126 genera that are said to warrant
conservation attention in the face of limited appreciation by the
public. Since 76 percent of rodent genera contain only one species,
much phylogenetic diversity could be lost with a comparatively small
number of extinctions. In the absence of more detailed knowledge of
species at risk and accurate taxonomy, conservation must be based
mainly on higher taxa (such as families rather than species) and
geographical hot spots. Several species of rice rat have become
extinct since the 19th century, probably through habitat loss and the
introduction of alien species. In Colombia, the brown hairy dwarf
porcupine was recorded from only two mountain localities in the 1920s,
while the red crested soft-furred spiny rat is known only from its
type locality on the Caribbean coast, so these species are considered
vulnerable. The IUCN Species Survival Commission writes "We can
safely conclude that many South American rodents are seriously
threatened, mainly by environmental disturbance and intensive
The "three now cosmopolitan commensal rodent pest species" (the
brown rat, the black rat and the house mouse) have been dispersed in
association with humans, partly on sailing ships in the Age of
Exploration, and with a fourth species in the Pacific, the Polynesian
Rattus exulans), have severely damaged island biotas around the
world. For example, when the black rat reached
Lord Howe Island
Lord Howe Island in
1918, over 40 percent of the terrestrial bird species of the island,
including the Lord Howe fantail, became extinct within ten years.
Similar destruction has been seen on
Midway Island (1943) and Big
South Cape Island (1962). Conservation projects can with careful
planning completely eradicate these pest rodents from islands using an
anticoagulant rodenticide such as brodifacoum. This approach has
been successful on the island of
Lundy in the United Kingdom, where
the eradication of an estimated 40,000 brown rats is giving
Manx shearwater and
Atlantic puffin a chance to recover
Chinchilla fur coat, exhibited at the 1900 Exposition Universal, Paris
Humanity has long used animal skins for clothing, as the leather is
durable and the fur provides extra insulation. The native people of
North America made much use of beaver pelts, tanning and sewing them
together to make robes. Europeans appreciated the quality of these and
North American fur trade
North American fur trade developed and became of prime importance
to early settlers. In Europe, the soft underfur known as "beaver wool"
was found to be ideal for felting and was made into beaver hats and
trimming for clothing. Later, the coypu took over as a
cheaper source of fur for felting and was farmed extensively in
America and Europe; however, fashions changed, new materials became
available and this area of the animal fur industry declined. The
chinchilla has a soft and silky coat and the demand for its fur was so
high that it was nearly wiped out in the wild before farming took over
as the main source of pelts. The quills and guardhairs of
porcupines are used for traditional decorative clothing. For example,
their guardhairs are used in the creation of the Native American
"porky roach" headdress. The main quills may be dyed, and then applied
in combination with thread to embellish leather accessories such as
knife sheaths and leather bags. Lakota women would harvest the quills
for quillwork by throwing a blanket over a porcupine and retrieving
the quills it left stuck in the blanket.
At least 89 species of rodent, mostly
Hystricomorpha such as guinea
pigs, agoutis and capybaras, are eaten by humans; in 1985, there were
at least 42 different societies in which people eat rats. Guinea
pigs were first raised for food around 2500 B.C. and by 1500 B.C. had
become the main source of meat for the Inca Empire. Dormice were
raised by the Romans in special pots called "gliraria", or in large
outdoor enclosures, where they were fattened on walnuts, chestnuts,
and acorns. The dormice were also caught from the wild in autumn when
they were fattest, and either roasted and dipped into honey or baked
while stuffed with a mixture of pork, pine nuts, and other flavorings.
Researchers found that in Amazonia, where large mammals were scarce,
pacas and common agoutis accounted for around 40 percent of the annual
game taken by the indigenous people, but in forested areas where
larger mammals were abundant, these rodents constituted only about 3
percent of the take.
Guinea pigs are used in the cuisine of Cuzco, Peru, in dishes such as
cuy al horno, baked guinea pig. The traditional Andean stove,
known as a qoncha or a fogón, is made from mud and clay reinforced
with straw and hair from animals such as guinea pigs. In Peru,
there are at any time 20 million domestic guinea pigs, which annually
produce 64 million edible carcasses. This animal is an excellent food
source since the flesh is 19% protein. In the United States,
mostly squirrels, but also muskrats, porcupines, and ground hogs are
eaten by humans. The
Navajo people ate prairie dog baked in mud, while
Paiute ate gophers, squirrels, and rats.
Rodents including guinea pigs, mice, rats, hamsters, gerbils,
chinchillas, degus and chipmunks make convenient pets able to live in
small spaces, each species with its own qualities. Most are
normally kept in cages of suitable sizes and have varied requirements
for space and social interaction. If handled from a young age, they
are usually docile and do not bite. Guinea pigs have a long lifespan
and need a large cage. Rats also need plenty of space and can
become very tame, can learn tricks and seem to enjoy human
companionship. Mice are short-lived but take up very little space.
Hamsters are solitary but tend to be nocturnal. They have interesting
behaviors, but unless handled regularly they may be defensive. Gerbils
are not usually aggressive, rarely bite and are sociable animals that
enjoy the company of humans and their own kind.
Laboratory house mouse
Rodents are used widely as model organisms in animal testing.
Albino mutant rats were first used for research in 1828 and later
became the first animal domesticated for purely scientific
purposes. Nowadays, the house mouse is the most commonly used
laboratory rodent, and in 1979 it was estimated that fifty million
were used annually worldwide. They are favored because of their small
size, fertility, short gestation period and ease of handling and
because they are susceptible to many of the conditions and infections
that afflict humans. They are used in research into genetics,
developmental biology, cell biology, oncology and immunology.
Guinea pigs were popular laboratory animals until the late 20th
century; about 2.5 million guinea pigs were used annually in the
United States for research in the 1960s, but that total decreased
to about 375,000 by the mid-1990s. In 2007, they constituted
about 2% of all laboratory animals. Guinea pigs played a major
role in the establishment of germ theory in the late 19th century,
through the experiments of Louis Pasteur, Émile Roux, and Robert
Koch. They have been launched into orbital space flight several
times—first by the USSR on the
Sputnik 9 biosatellite of March 9,
1961, with a successful recovery. The naked mole rat is the only
known mammal that is poikilothermic; it is used in studies on
thermoregulation. It is also unusual in not producing the
neurotransmitter substance P, a fact which researchers find useful in
studies on pain.
Rodents have sensitive olfactory abilities, which have been used by
humans to detect odors or chemicals of interest. The Gambian
pouched rat is able to detect tuberculosis bacilli with a sensitivity
of up to 86.6%, and specificity (detecting the absence of the bacilli)
of over 93%; the same species has been trained to detect land
mines. Rats have been studied for possible use in hazardous
situations such as in disaster zones. They can be trained to respond
to commands, which may be given remotely, and even persuaded to
venture into brightly lit areas, which rats usually
As pests and disease vectors
Rodents cause significant losses to crops, such as these potatoes
damaged by voles.
Some rodent species are serious agricultural pests, eating large
quantities of food stored by humans. For example, in 2003, the
amount of rice lost to mice and rats in Asia was estimated to be
enough to feed 200 million people. Most of the damage worldwide is
caused by a relatively small number of species, chiefly rats and
mice. In Indonesia and Tanzania, rodents reduce crop yields by
around fifteen percent, while in some instances in South America
losses have reached ninety percent. Across Africa, rodents including
Arvicanthis damage cereals, groundnuts, vegetables and
cacao. In Asia, rats, mice and species such as
Meriones unguiculatus and
Eospalax baileyi damage crops of rice,
sorghum, tubers, vegetables and nuts. In Europe, as well as rats and
mice, species of Apodemus,
Microtus and in occasional outbreaks
Arvicola terrestris cause damage to orchards, vegetables and pasture
as well as cereals. In South America, a wider range of rodent species,
such as Holochilus, Akodon, Calomys, Oligoryzomys, Phyllotis, Sigmodon
and Zygodontomys, damage many crops including sugar cane, fruits,
vegetables, and tubers.
Rodents are also significant vectors of disease. The black rat,
with the fleas that it carries, plays a primary role in spreading the
Yersinia pestis responsible for bubonic plague, and
carries the organisms responsible for typhus, Weil's disease,
toxoplasmosis and trichinosis. A number of rodents carry
hantaviruses, including the Puumala, Dobrava and Saaremaa viruses,
which can infect humans. Rodents also help to transmit diseases
including babesiosis, cutaneous leishmaniasis, human granulocytic
anaplasmosis, Lyme disease, Omsk hemorrhagic fever, Powassan virus,
rickettsialpox, relapsing fever, Rocky Mountain spotted fever, and
West Nile virus.
Rodent Bait Station, Chennai, India
Because rodents are a nuisance and endanger public health, human
societies often attempt to control them. Traditionally, this involved
poisoning and trapping, methods that were not always safe or
effective. More recently, integrated pest management attempts to
improve control with a combination of surveys to determine the size
and distribution of the pest population, the establishment of
tolerance limits (levels of pest activity at which to intervene),
interventions, and evaluation of effectiveness based on repeated
surveys. Interventions may include education, making and applying laws
and regulations, modifying the habitat, changing farming practices,
and biological control using pathogens or predators, as well as
poisoning and trapping. The use of pathogens such as Salmonella
has the drawback that they can infect man and domestic animals, and
rodents often become resistant. The use of predators including
ferrets, mongooses and monitor lizards has been found unsatisfactory.
Domestic and feral cats are able to control rodents effectively,
provided the rodent population is not too large. In the UK, two
species in particular, the house mouse and the brown rat, are actively
controlled to limit damage in growing crops, loss and contamination of
stored crops and structural damage to facilities, as well as to comply
with the law.
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Wikimedia Commons has media related to Rodentia.
Wikispecies has information related to Rodentia
Zoology, osteology, comparative anatomy
Rodent osteology (photos)
Rodent skeleton drawings
Rodent Species Fact Sheets from the National Pest Management
Association on Deer Mice, Norway Rats, and other rodent species
Extant mammal orders
Monotremata (Platypus and echidnas)
Paucituberculata (Shrew opossums)
Microbiotheria (Monito del monte)
Dasyuromorphia (Quolls and dunnarts)
Peramelemorphia (Bilbies and bandicoots)
Diprotodontia (Kangaroos and relatives)
Pilosa (Anteaters and sloths)
Afrosoricida (Tenrecs and golden moles)
Macroscelidea (Elephant shrews)
Sirenia (Dugongs and manatees)
Eulipotyphla (Hedgehogs, shrews, moles and relatives)
Carnivora (Dogs, cats and relatives)
Perissodactyla (Odd-toed ungulates)
Artiodactyla (Even-toed ungulates and cetaceans)
Lagomorpha (Rabbits and pikas)
Extant families in order Rodentia
Aplodontiidae (Mountain beaver)
Sciuridae (Squirrels, chipmunks, marmots, susliks and prairie dogs)
Geomyidae (Pocket gophers)
Heteromyidae (Kangaroo rats and mice, pocket mice)
Dipodidae (Jerboas, jumping mice and birch mice)
Platacanthomyidae (Oriental dormice)
Spalacidae (Zokors, bamboo rats, mole rats, blind mole rats)
Calomyscidae (Mouse-like hamsters)
Nesomyidae (Malagasy rats and relatives)
Cricetidae (Hamsters and relatives)
House mouse and relatives)
Diatomyidae (Laotian rock rat)
Hystricidae (Old World porcupines)
Petromuridae (Dassie rat)
Thryonomyidae (Cane rats)
Caviomorpha (New World hystricognaths)
Erethizontidae (New World porcupines)
Dasyproctidae (Agoutis and acouchis)
Echimyidae (Spiny rats and coypus)
Octodontidae (Degus and relatives)
Chinchillidae (Chinchillas and viscachas)
Prehistoric families in order Rodentia
See also: Category
Fauna Europaea: 12648
BNF: cb11977167j (d