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Rodents Temporal range: Early Paleocene – Present, 61.7–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N

Clockwise from top, capybara, spring hare, golden-mantled ground squirrel, house mouse and beaver representing the suborders Hystricomorpha, Anomaluromorpha, Sciuromorpha, Myomorpha, and Castorimorpha
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Mirorder:	Simplicidentata
Order:	Rodentia Bowdich, 1821
Suborders
Anomaluromorpha Castorimorpha Hystricomorpha (inc. Caviomorpha) Myomorpha Sciuromorpha

Combined range of all rodent species

Rodents are mammals of the order Rodentia, characterised by a single pair of continuously growing incisors in each of the upper and lower jaws. About forty percent of all species of mammal are rodents, and they are found in vast numbers on all continents except Antarctica. They can be found in a variety of terrestrial habitats including human-made environments. There are species that are arboreal, fossorial, and even semi-aquatic. Well known rodents include mice, rats, squirrels, porcupines, beavers, guinea pigs, and hamsters, but rabbits and hares are no longer considered to be rodents.

Most are small animals with robust bodies, short limbs and long tails, but there are exceptions to this. Rodents use their sharp incisors to gnaw food, excavate burrows and defend themselves. Most rodents eat seeds or other plant material, and some have more varied diets. Rodents tend to be social animals and many species live in societies with complex ways of communicating. Mating among rodents can vary from monogamy, to polygyny, to promiscuity. Many have litters of underdeveloped, altricial young; others have precocial young which are relatively well developed at birth.

The earliest fossil record of rodents dates to the Paleocene on the supercontinent of Laurasia. They greatly diversified in the Eocene, causing different groups to migrate across continents, sometimes even breaching oceans. Unlike almost all other placental mammals, a subfamily of rodents, the Murinae (including rats and mice), reached and colonized Australia.

Rodents interact with humans in various ways, and have been put to use as food, in clothing, as pets and as laboratory animals in research. Some species are serious pests, eating and spoiling food stored by humans, and spreading diseases. Accidentally introduced rodents are often considered to be invasive as they sometimes threaten the survival of native species such as island birds previously isolated from land-based predators.

Characteristics

Rodents are generally small animals; most species weigh less than 100 g (3.5 oz). However, the largest living species, the capybara, can weigh as much as 66 kg (146 lb). Rodents typically have squat bodies and short limbs. The fore-limbs usually have five digits, including a thumb that is sometimes vestigial, 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. In general, rodents are not well adapted for running but a few species, like the agouti are fleet-footed, being digitigrade and having hoof-like nails. Kangaroo mice and jerboas can hop at 48 km/h (30 mph). Semi-aquatic species like the beaver have webbed feet. Squirrels have well-developed dexterity in their front paws. The majority of species have tails, which can be of many shapes and sizes. Some tails are even prehensile, as in the Eurasian harvest mouse. The pelage of the tails can vary from bushy to completely bald. 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.

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 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 re-ingests the food from its anus so that the nutrients can be absorbed by the gut. Rodents therefore often produce a hard and dry fecal pellet. In males, the penis contains a bone and 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 and 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 it may be that males selected larger females due to their greater reproductive success. In some species, like voles, sexual dimorphism can vary from population to population. In bank voles, females are typically larger than males. Male-bias sexual dimorphism occurs in alpine populations, possibly because of the lack of predators and greater competition between males.

Dentition and jaw musculature

The most defining feature of the rodents is their teeth, particularly their razor sharp incisors which have thick layers of enamel on the front and little enamel on the back. Because the incisors do not stop growing, the animal must continue to wear them down so that they do not grow far enough to reach or even 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 like the blade of a chisel. Most species have up to 22 teeth (an exception being the silvery mole rat which has 28 teeth) with no canines or anterior premolars. There is a gap, or diastemata, between the incisors and the molars. This lets rodents suck in their cheeks or lips to shield their mouths and throats from wood shavings or other inedible material removed from whatever they are gnawing. The molars are relatively large, intricately structured and covered with convoluted ridges of enamel which are arranged transversely. They 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. Rodents are capable of gnawing though even the toughest husks, pods and seed shells.

Rodents groups differ in the arrangement of the jaw muscles and associated skull structures. The Sciuromorpha or squirrel-like rodents have a very simple jaw muscle that extends onto the snout in front of the eye. The Myomorpha or mouse-like rodents have jaw muscles that anchor on the side of the nose - these are the most efficient amongst the rodents. The Caviomorpha or cavy-like rodents have very large cheekbones and muscles that anchor to the side of the face.

Distribution and habitat

Rodents are one of the most widespread group of mammals. They can be found on every continent expect Antarctica and, along with bats, are the only terrestrial placental mammals that have colonised Australia and New Guinea without human intervention. Humans have also allowed the animals to spread to some oceanic islands. Rodents have adapted to almost every terrestrial habitat, from cold tundra (where they live under snow) to hot deserts. Some species are arboreal, some live underground where they build complex burrow systems and others dwell on the surface. Beavers and muskrats are known for being semi-aquatic. Rodents have also thrived in human-created environments such as agricultural and urban areas.

Though some species are common pests for humans, rodents also play important ecological roles. Burrowing rodents may eat the fruiting bodies of fungi and spread spores through their feces, thereby allowing the fungi to form symbiotic relationships with the roots of plants (which usually cannot survive without them). As such, these rodents may play a role in maintaining healthy forests.

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, nutrient redistribution, organic matter levels and water infiltration. They maintain these grassland habitats, and some large herbivores like bison and pronghorn prefer to graze near prairie dog colonies due to the increased nutritional quality of forage. Prairie dogs can also lead to regional and local biodiversity loss, increased seed depredation and the establishment and spread of invasive shrubs.

In many temperate regions, beavers play an essential hydrological role. When building their dams and lounges, 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 populations.

Behavior and life history

Feeding

Most rodents are herbivorous and feed exclusively on plant material including seeds, stems, leaves, flowers and roots. Some are omnivorous and others are carnivorous. 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. The 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, eating its own fecal pellets. 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.

The agouti is one of the few animals that can break open the large capsules of the Brazil nut fruit. There are too many seeds 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 only available for short periods. The kangaroo rat collects all it can find and stores them in larder chambers in its burrow.

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 percent 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 end into the mud to anchor them. Here they can access their food supply underwater even when their pond is frozen over.

One of the few largely carnivorous rodents is the grasshopper mouse found in dry regions of North America. This mouse feeds on insects, scorpions, other small mice and only a small portion of plant material. It has a chunky body with short legs and tail, but is agile and can easily overpower prey as large as itself. Another is the Australian water rat which feeds on crustaceans, insects, small fish, frogs and mollusks. Some plant matter is also eaten, especially in the winter.

Social behavior

Rodents exhibit a wide range of types of social behavior ranging from the only known mammalian caste systems of some mole rats, the extensive "town" of the colonial prairie dog, through family groups to the independent, solitary life of the edible dormouse.

In the case of the dormouse, males and females live independently, only coming together briefly in the breeding season to mate. Their feeding ranges may overlap but they live in individual nests and feed separately. The female raises the young without any assistance from the male. 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 youngsters 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 huddle together, groom one another, and share nesting and pup-raising responsibilities.

Among the most social of rodents are the ground squirrels, which typically form colonies based on female kinship, males dispersing after weaning and becoming nomadic as adults. Co-operation 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 non-breeding yearlings and this 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. These are considered to be the only two eusocial mammals. The naked mole rat lives completely underground and can form colonies of up to eighty individuals. Only one female and up to three males in the colony reproduce, while the rest of the members are smaller, 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 is characterized by having a single reproductively active male and female in a colony where the remaining animals are not truly sterile, but only become fertile if they establish a colony of their own.

Communication

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, degus, another social, burrowing rodent, exhibit a wide array of communication modalities and have 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.

Marmot whistling

Rats emit short, high frequency, ultrasonic, vocalizations during purportedly pleasurable experiences such as rough-and-tumble play, before receiving 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 that are too high for humans to hear without special equipment. Bat detectors are often used by pet owners for this purpose.

Rodents use scent marking in many social contexts including for inter-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. The chemicals involved are the major histocompatibility complex and several urinary proteins which are detected and interpreted by two olfactory bulbs. The odor of a predator depresses scent marking behavior. The urine of many rodents strongly reflects ultraviolet light but the amount reflected decreases with time. This may in some instances be disadvantageous; kestrels in Finland have been shown to use it and have greater hunting success when they assess whether vole trails are currently in use. Ultraviolet reflectivity is of dubious value for nocturnal rodents.

Using olfaction, rodents are able to recognize close relatives. This allows them to express 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 major histocompatibility complex (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 excreting urine containing about a dozen genetically encoded MUPs.

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 was the first mammal for which vibrational communication was documented. These fossorial rodents bang their head against the walls of their tunnels, which was initially interpreted as part of their tunnel building behavior. It was eventually realized that they generate temporally patterned vibrational 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 where the dominant male indicates its resource holding potential by drumming, thus minimizing physical contact with potential rivals.

Mating strategies

Several different mating systems exist among rodents. Some species practice monogamy where an adult male and female form a 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 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 monopolise and mate with multiple females. As with monogamy, polygyny in rodent can come in two forms; defense and non-defense polygyny. Defense polygyny involves males controlling territories which 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 cause of marmots, resident males do not appear to ever lose their territories and always wins 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 monopolise. These males establish dominance hierarchies with the high ranking males having access to the most females. This occurs in Belding's ground squirrels and some tree squirrel species.

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 which can vary between monogamy, polygyny and promiscuity. Females 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 mole rats, a single female monopolizes mating from at least three males.

In most rodent species, for example rats and 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

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 altrical state is typical for squirrels and mice; while the precocial state usually occurs in species like guinea pigs and porcupines. Females with altrical 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 front of their mother. 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.

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 then 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 alien 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 alien young 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 the 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 which causes stress and causes the young to abort.

Classification and evolution

Evolutionary history

The fossil record of rodent-like mammals begins shortly after the extinction of the non-avian dinosaurs 66 million years ago, in Laurasia, the supercontinent composed of today's North America, Europe, and Asia, during the Paleocene. 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 record. Some fossil species, such as the giant beavers, Castoroides, and a giant dormouse, Leithia, attained great size. The largest known rodent was Josephoartigasia monesi, a giant pacarana.

The history of the colonization of the world's continents by rodents is complex. The movements of the large Muroidea superfamily (including hamsters, gerbils, true mice and rats) may have involved up to seven colonizations of Africa, five colonizations of North America, four of Southeast Asia, two of South America and up to ten re-colonizations of Eurasia.

During the Eocene, rodents began to diversify; for example beavers appear in North America in the late Eocene before spreading to Eurasia. By 20 million years ago (in the Miocene) fossils recognizably belonging to the current families such as Muridae appear. Late in the Eocene, hystricognaths colonized Africa, most probably having originated in Asia, at least 39.5 million years ago. From Africa, fossil evidence shows that some hystricognaths (caviomorphs) migrated to South America, which had been an isolated continent during the Oligocene and Miocene epochs, possibly making use of ocean currents and the Ceara and Sierra Leone Rises in the Atlantic. Caviomorphs had arrived in South America by 41 million years ago (implying a date at least as early as this for hystricognaths in Africa). By the Miocene, when Africa had collided with Asia, rodents such as porcupines began to spread into Eurasia.

During the Pliocene, rodent fossils appeared in Australia. Although marsupials are the most prominent mammals in Australia, many rodents, all belonging to the Murinae, are among the continent's mammal species, with about 50 'old endemics' and ten true rats (Rattus) of which eight are 'new endemics' and two were introduced by Europeans. Meanwhile, the Americas became joined by the Isthmus of Panama, and some rodents participated in the resulting Great American Interchange; sigmodontines surged southward and a small number of species such as New World porcupines (Erethizontidae) headed north.

Some examples of extinct rodents
Genus	Species	Notes	Location	Approx. max. weight	Stratigraphy
Castoroides		giant beavers	North America	up to 100 kg (220 lb)	Pleistocene
Ceratogaulus		horned gophers	North America	(smallest horned mammal)	Late Miocene to Pleistocene
Spelaeomys		a large cave rat	Flores	-	extinct by 1500
"Giant hutias"		a paraphyletic group of rodents resembling large guinea pigs	West Indies	up to 200 kg (440 lb)	Pleistocene
Leithia		a giant dormouse	Europe (Malta, Sicily)	113 kg (249 lb)	Pleistocene
Neochoerus	N. pinckneyi	a large capybara	North America	100 kg (220 lb)	Pleistocene
Josephoartigasia	J. monesi	'giant pacarana', largest known rodent	South America	1,500 kg (3,300 lb)	Pliocene to early Pleistocene
Phoberomys	P. pattersoni	a horse-sized rodent	North America	probably under 280 kg (620 lb); earlier estimates up to 700 kg (1,500 lb)	Miocene
Telicomys		a giant rodent, to 2 metres (6 ft 7 in) long	South American	perhaps 70% of size of P. pattersoni	Late Miocene to early Pleistocene

Standard classification

The use of the name "Rodentia" is attributed to the English traveler and naturalist Thomas Edward Bowdich (1821). The hares, rabbits and pikas (order Lagomorpha) have continuously growing incisors 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, Euarchontoglires and Boreoeutheria. Below is a cladogram showing the inner and outer relations of Rodentia based on an attempt (Wu et al., 2012) to align the molecular clock with paleontological data:

Boreoeutheria

Laurasiatheria

	Perissodactyla

	Carnivora

	Ochotona

	Sylvilagus

Rodentia

	Erethizon

	Cavia

Sciuromorpha

Aplodontia

	Glaucomys

	Tamias

Castorimorpha

Castor

	Dipodomys

	Thomomys

Myodonta

Muroidea

Peromyscus

	Mus

	Rattus

	Allactaginae

	Dipodinae

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 makes classification difficult as similar traits may not be due to common ancestry. Brandt (1855) was the first to propose dividing the Simplicidentata (rodents and their closest extinct relatives) into three suborders, Sciuromorpha, Hystricomorpha and Myomorpha, based on the development of certain muscles in the jaw. This system was widely accepted and is known as the "classical" arrangement. Schlosser (1884) performed a comprehensive review of rodent fossils, mainly using the cheek teeth, and found 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.

While these disagreements have been going on, the rodent families themselves have remained relatively stable, it being just the higher clades that are in dispute. 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)

Suborder Anomaluromorpha
Family Anomaluridae: scaly-tailed squirrels

Family Pedetidae: springhares
Suborder Castorimorpha
- Superfamily Castoroidea
  - Family Castoridae: beavers
- Superfamily Geomyoidea
  - Family Geomyidae: pocket gophers (true gophers)
  - Family Heteromyidae: kangaroo rats and kangaroo mice
Suborder Hystricomorpha
Family incertae sedis Diatomyidae: Laotian rock rat
- Infraorder Ctenodactylomorphi
  - Family Ctenodactylidae: gundis
- Infraorder Hystricognathi
  - Family Bathyergidae: African mole rats
  - Family Hystricidae: Old World porcupines
  - Family Petromuridae: dassie rat
  - Family Thryonomyidae: cane rats
  - Parvorder Caviomorpha
    - Family †Heptaxodontidae: giant hutias
    - Family Abrocomidae: chinchilla rats
    - Family Capromyidae: hutias
    - Family Caviidae: cavies, including Guinea pigs and the capybara
    - Family Chinchillidae: chinchillas and 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: nutria, coypu
    - Family Octodontidae: octodonts
Suborder Myomorpha
- Superfamily Dipodoidea
  - Family Dipodidae: jerboas and jumping mice
- Superfamily Muroidea
  - 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, and zokors
Suborder Sciuromorpha
Family Aplodontiidae: mountain beaver

Family Gliridae (also Myoxidae, Muscardinidae): dormice

Family Sciuridae: squirrels, including chipmunks, prairie dogs, and marmots

Monophyly versus polyphyly

In 1991, a paper published by 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 pig proteins. 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 argued 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 a majority opinion among mammalian biologists that the order Rodentia is monophyletic, although there is not a complete consensus.

Interaction with humans

Conservation

While rodents are not the most seriously threatened order of mammals, there are 168 species in 126 genera which "deserve 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. For example, 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 hunting".

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, Rattus exulans, have severely damaged island biotas around the world. For example, when the black rat reached Lord Howe Island in 1918, over 40 percent of the terrestrial bird species of the island 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 rats is giving populations of Manx shearwater and Atlantic puffin a chance to recover from near extinction.

Uses

Since they started wearing clothes, humans have used animal skins for this purpose, the leather being durable and the fur providing 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 the North American fur trade developed and became of prime importance to early settlers. In Europe, people wore beaver hats and trimmed their clothing with the fur. Later, the coypu took over as a cheaper source of pelts and was much farmed in America and Europe, but fashions change, new materials become available and the 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.

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. Romans raised dormice in special pots called "gliraria" and in large outdoor enclosures where they were fed walnuts, chestnuts, and acorns for fattening. They were also caught from the wild in autumn when it is fattest and either roasted and dipped into honey or baked while stuffed with a mixture of pork, pine nuts, and other flavorings. Among indigenous Amazonians, when large mammals are scarce, pacas and common agoutis can account for 39 percent of the annual game take by weight, but in forested areas where larger mammal species are abundant, these rodents comprised 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. In addition, 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 muskrat, porcupine, and ground hog are eaten by humans. Native American Navajo ate prairie dog baked in mud while the Paiute ate gophers, squirrels, and rats.

Rodents make convenient pets where space is limited, and the different types exhibit differing qualities as pets. Animals kept for this purpose include guinea pigs, mice, rats, hamsters, gerbils, chinchillas, degus and chipmunks. 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, need a large cage and have a remarkable ability to learn and remember complex pathways to food. 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.

Rodents are used widely as model organisms in animal testing. 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. The naked mole rat is the only known mammal that is poikilothermic and also does not produce the neurotransmitter substance P; it is used in studies on thermoregulation and pain.

Rodents have a sensitive olfactory sense which has 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, as they can be trained to respond to commands which may be given remotely, even in brightly lit areas, which rats usually avoid.

As pests and disease vectors

Some rodent species are 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.

Rodents are also vectors of disease. The black rat, with the fleas that it carries, plays a primary role in spreading the bacterium Yersinia pestis responsible for bubonic plague, and also carries the organisms responsible for typhus, Weil's disease, toxoplasmosis and trichinosis.

Because rodents are a nuisance and endanger public health, human societies often attempt to control them. Traditionally this involved poisoning and trapping, methods which 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, 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, while 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.

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External links

Zoology, osteology, comparative anatomy

Various

African rodentia
Rodent photos
Rodent Species Fact Sheets from the National Pest Management Association on Deer Mice, Norway Rats, and other rodent species
Rodent Pests chapter in EPA and University of Florida/IFAS manual

Extant mammal orders

Kingdom Animalia
Phylum Chordata
Subphylum Vertebrata
(unranked) Amniota

Yinotheria

Australosphenida	Monotremata (platypuses and echidnas)

Theria

Metatheria
(Marsupial inclusive)

Ameridelphia	Paucituberculata (shrew opossums) Didelphimorphia (opossums)
Australidelphia	Microbiotheria (monitos del monte) Notoryctemorphia (marsupial moles) Dasyuromorphia (quolls and dunnarts) Peramelemorphia (bilbies and bandicoots) Diprotodontia (kangaroos, koalas, and relatives)

Eutheria
(Placental inclusive)

Atlantogenata

Xenarthra	Cingulata (armadillos) Pilosa (anteaters and sloths)
Afrotheria	Afrosoricida (tenrecs, golden moles, and otter shrews) Macroscelidea (elephant shrews) Tubulidentata (aardvarks) Hyracoidea (hyraxes) Proboscidea (elephants) Sirenia (dugongs and manatees)