Combined range of all rodent species (not including introduced populations)
Rodents (fromLatinrōdēns, 'gnawing') are a group ofmammals characterized by a single pair of continuously growingincisors in each of the upper andlower jaws. About 40% of all mammal species belong to theorderRodentia (/roʊˈdɛn(t)ʃə/roh-DEN-shə orroh-DEN-chə). They are native to all major land masses except forAntarctica, and several oceanic islands, though they have subsequently been introduced to most of these land masses by human activity.
Rodents are extremely diverse in their ecology and lifestyles and can be found in almost every terrestrial habitat, including human-made environments. Species can bearboreal,fossorial (burrowing),saltatorial/ricochetal (leaping on their hind legs), or semiaquatic. However, all rodents share several morphological features, including having only a single upper and lower pair of ever-growing incisors. Well-known rodents includemice,rats,squirrels,prairie dogs,porcupines,beavers,guinea pigs, andhamsters. Rodentia andLagomorpha (rabbits,hares, andpikas) aresister groups, sharing asingle common ancestor and forming theclade ofGlires. Lagomorphs also have incisors that grow continuously, but are distinguished by an extra pair of incisors on the upper jaw.
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 frommonogamy, topolygyny, topromiscuity. Many havelitters of underdeveloped,altricial young, while others areprecocial (relatively well developed) at birth.
The rodentfossil record dates back to thePaleocene ofAsia. Rodents greatly diversified in theEocene, as they spread across continents, sometimes evencrossing oceans. Rodents reached both South America and Madagascar from Africa and, until the arrival ofHomo sapiens, were the only terrestrialplacental mammals to reach and colonize Australia.
Rodents have been used as food, for clothing, aspets, and aslaboratory animals in research. Some species, in particular, thebrown rat, theblack rat, and thehouse mouse, are seriouspests, eating and spoiling food stored by humans and spreading diseases. Accidentallyintroduced species of rodents are often considered to beinvasive and have caused the extinction of numerous species, such as island birds, thedodo being an example, previously isolated from land-based predators.
The distinguishing feature of the rodents is their pairs of continuously growing, sharpenedincisors.[1] Enamel covers the front of the incisor back the back is uncovered.[2] 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 achisel.[3][4] Rodent species have 12–28 teeth total, usually less than 22, with no canines. A gap, ordiastema, is present between the incisors the cheek teeth. This allows rodents to suck in and block out inedible material as the incisors chip them off.[1] Chinchillas and guinea pigs have a high-fiber diet; their molars have no roots and grow continuously like their incisors.[5]
The complexly ridged surface of the molars are well equipped for grinding food into small particles.[1] The jaw musculature is strong. The lower jaw is thrust forward while gnawing and shifts backwards during chewing.[2] The incisors do the gnawing while molars chew, however, due to the cranial anatomy of rodents these feeding methods cannot be used at the same time.[6] Among rodents, themasseter muscle plays a key role in chewing, making up 60% – 80% of the total muscle mass among masticatory muscles.[7][8]
In rodents, masseter muscles are attached behind the eyes and contribute to eye boggling that occurs during gnawing, where the quick contraction and relaxation of the muscle causes the eyeballs to move up and down.[4] Variations in thezygomasseteric system are associated with different specializations in the chewing apparatus. Sciuromorphous rodents, such as theeastern grey squirrel, have a large deepmasseter, making them efficient at biting with the incisors. Hystricomorphous rodents, such as theguinea 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. Myomorphous rodents, such as thebrown rat, have enlargedtemporalis and masseter muscles, making them efficient with both gnawing and chewing.[6][8]
While the largest species, thecapybara, can reach 66 kg (146 lb), most species weigh less than 100 g (3.5 oz). Rodents have wide-ranging morphologies, but typically have stout bodies and short limbs.[1] The forefeet usually have five digits, including an opposable thumb, while the hind feet have three to five digits. The elbow gives the forearms great flexibility.[3] The majority of species areplantigrade, walking on the whole foot and have claw-likenails which vary in size. Rodents have nails on their first digit which they use in food handling. Such a nail combined withdexterous feeding movement with incisors which allow them to eat hard seeds and nuts, a niche that they presently dominate. This thumbnail is argued to be ancestorial with exceptions being linked to its replacement by claws in burrowing and for oral-only feeding.[9]
Rodent species use a wide variety of methods of locomotion includingquadrupedal walking, running, burrowing, climbing,bipedal hopping (kangaroo rats andhopping mice), swimming and even gliding.[3]Flying squirrels can both glide from tree to tree usingparachute-like membranes that stretch from the fore to the hind limbs.[10] Theagouti is fleet-footed andantelope-like, beingdigitigrade with hoof-like nails. The majority of rodents have tails, which can be of many shapes and sizes. Some tails areprehensile, as in theEurasian harvest mouse, and the fur on the tails can vary from bushy to mostly bald. Some species have vestigial or non-visible tails.[1] In some species, the tail is capable of regeneration if a part is broken off.[3]
Rodents generally have well-developed senses ofsmell and hearing and while the eyes are enlarged in species that are active at night. Many species have long, sensitivewhiskers or vibrissae for touch or"whisking".[1] Whisker action is mostly driven by the brain stem, which is itself provoked by the cortex, though alternative routes have been found.[11] Many species havecheek pouches for storing food. In squirrels and species ofMuroidea, these structures are extensions of the oral cavity while in gophers and theHeteromyidae, they are separate. Both types reach to the shoulders.[12] The digestive system is efficient enough to absorb nearly 80% of the energy in their food. When eatingcellulose, the food is softened in the stomach and directed to thececum, where bacteria reduce it to itscarbohydrate elements. The rodent then practicescoprophagy, eating its own fecal pellets, so the nutrients can be absorbed by the gut. The final fecal pellet is hard and dry.[1] They may entirely lack the ability to vomit.[13][14] In many species, the penis contains a bone, thebaculum; the testes can be located either abdominally or at the groin.[3]
Sexual dimorphism occurs in many rodent species. In some rodents, males are larger than females, while in others the reverse is true. The former is typical forground squirrels, kangaroo rats, solitarymole rats andpocket gophers; it likely developed due tosexual selection with larger males being more reproductively successful. Female-bias sexual dimorphism exists amongchipmunks andjumping mice. Its function is not understood, but in the case ofyellow-pine chipmunks, males may choose larger females due to their greater fitness. In some species, such asvoles, sexual dimorphism can vary from population to population. Inbank voles, females are usually the larger sex, but males may be larger in certain alpine populations, possibly because of the lack of predators and greater competition between them.[15]
Drawing of typical rodent tooth system: The front surface of theincisors is hardenamel, whereas the rear is softerdentine. The act of chewing wears down the dentine, leaving a sharp, chisel-like edge.
Rodents have a near global distribution. They are the only terrestrial placental mammals to have colonized Australia andNew Guinea without human intervention. Humans have also allowed the animals to spread to many remote oceanic islands.[3] Rodents have adapted to almost every terrestrial habitat, from coldtundra (under snow) to hot deserts. Species range fromarboreal (tree-dwelling), tofossorial (underground) andsemiaquatic.[1] Rodents have also thrived in human-created environments such asagricultural andurban areas.[1][16]
Though some species are common pests for humans, rodents also play important ecological roles.[1] Burrowing rodents may eat the fruiting bodies offungi and spreadspores through their feces, thereby allowing the fungi to disperse and formsymbiotic relationships with the roots of plants (which usually cannot thrive without them). As such, these rodents may help preserve healthy forests.[17] Some rodents are consideredkeystone species andecosystem engineers in their respective habitats. In theGreat Plains of North America, the burrowing activities ofprairie dogs an important role in grassland habitats, contributing to soilaeration, which leads to more organic material and water absorption.[18] Similarly in many temperate regions, beavers play an essentialhydrological role. When building their dams and lodges, beavers alter the paths of streams and rivers and allow for the creation of wetland habitats.[19]
Most rodents areherbivorous, feeding exclusively on plant material while others areomnivorous and a few are predators.[1][2] Thefield 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.[20] Theplains pocket gopher eats grasses, roots, and tubers in its cheek pouches andcaches them in underground larder chambers.[21] TheTexas pocket gopher avoids emerging onto the surface to feed by pulling plants above them into their burrow by the roots. It also practices coprophagy.[22] TheAfrican pouched rat forages on the surface, gathering anything that might be edible into its capacious cheek pouches until they fully stretch. It then returns to its burrow to sort through the material it has gathered and eats the nutritious items.[23]
Agouti species are one of the few animal groups that can break open the large capsules of theBrazil 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. Desert-dwelling seed eaters like kangaroo rats must gather as many as they can since they are only available for a limited time.[23] Some rodents eat as much as possible to store fat reserves for the long winterhibernation.Marmots do this, and may be 50% heavier in the autumn than in the spring.[23] Beavers, which feed on tree bark and vegatation, store their winter food in "rafts", piles of wood immersed in water.[24]
Although rodents have been regarded traditionally as herbivores, most small rodents opportunistically include insects, worms, snails, mussels, and even vertebrates in their diets and a few have become specialized to rely on a diet of animal matter, such as theshrewlike rats, therakali and thegrasshopper mouse. 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 Americanwhite-footed mouse showed 34% animal matter.[25] The grasshopper mouse, which feeds on insects, scorpions, and other small mice, can kill prey as large as itself.[26]
Prairie dog "town". Prairie dogs are extremely social.
The majority of rodent species are social orgregarious, notably prairie dogs and brown rats while more solitary species include hamsters and red squirrels.[1] Thepocket gophers are also solitary outside the breeding season, each individual digging a complex tunnel system and maintaining a territory.[27] Solitary behavior may be linked to the presence of resources that can be claimed and defended.[1] Brown rats usually live in smallcolonies with up to six females sharing a burrow and one male defending aterritory around the burrow. At high population densities, this system breaks down and males show a hierarchical system ofdominance with overlapping ranges. Female offspring remain in the colony while male young disperse.[28]Beavers live in extended family units typically with a pair of adults and their young, including those born in current and previous year or even older.[29]Prairie vole societies consist both of monogamous pairs and communal groups where only a dominant female breeds. Pairs will join with communal groups during the winter.[30]
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, protecting nesting areas, and preventing infanticide.[31] The prairie dogs live in colonies or 'towns' which can stretch for kilometers all around and number in the thousands.[1][32] These consist of territorial family groups known as coteries which occupy separate burrows. A coterie often consists of an adult male, three or four adult females, and offspring including yearlings and juveniles. Individuals within coteries are friendly with each other, but hostile towards outsiders.[32]
Perhaps the most extreme examples of colonial behavior in rodents are theeusocial naked mole rat andDamaraland mole rat. These species live in underground colonies, which can number in the hundreds in the case of the naked mole rat. In both species, colonies consists of one breeding female and a few males, while the rest are non-reproductive, their fertility being suppressed. The non-reproductive members dig, maintain and seal tunnels, as well as gather food and help with the young.[33][34]
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. Much can be learned about an individual from their urine, including their species, individual identity, sex, reproductive status, health and social rank. Compounds derived from themajor histocompatibility complex (MHC) bind to several urinary proteins. The odor of a predator reduces scent-marking behavior.[35]
Rodents are able to recognize close relatives by smell and this allows them to shownepotism (preferential behavior toward their kin) and also avoid inbreeding. Thiskin recognition is byolfactory 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-volatilemajor urinary proteins (MUPs), which function aspheromone transporters, may also be used. MUPs may also advertise individual identity, with each malehouse mouse (Mus musculus) having uniquely encoded MUPs in their urine.[36]
House mice urinate to mark territory and advertise their individual and group identity.[37] Territorial beavers andred squirrels react more to the scents of strangers than those of their neighbors. This is known as the "dear enemy effect".[38][39]
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 allowconspecifics or related individuals to take evasive action.[40] Prairie dogs in particular, 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.[41][42] The urgency of the threat can also be conveyed by the acoustic properties of the call.[43]
Social rodents have a wider range of vocalizations than do solitary species. At least fifteen separate call-types have been recorded in adultKataba mole rats and four in juveniles.[44] Similarly, thecommon degu, another social, burrowing rodent, exhibits an elaborate vocal range comprising fifteen different vocalizations.[45] Ultrasonic calls play a part in social communication betweendormice and are used when the individuals are out of sight of each other.[46][47]
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 nest.[37]
Laboratory rats (which are brown rats,Rattus norvegicus) emit short, high frequency, ultrasonic vocalizations during purportedly pleasurable experiences such as rough-and-tumble play, during mating, and when tickled. The vocalization, described as a distinct "chirping", has been likened tolaughter, 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.[48][49]
Rodents, are typical of placental mammals in having only two types of light receptive cones in their retina (dichromacy),[50] in their case, a short wavelength "blue-UV" type and a middle wavelength "green" type. However, they are visually sensitive into the ultraviolet (UV) spectrum and therefore can see light that humans cannot. The functions of this UV sensitivity are not always clear. Indegus, for example, more UV light reflects from the belly fur, allowing it to signal to other degus when it stands on its hind legs. The back fur is less reflective, so the degu stands on all fours when a predator is near. Ultraviolet light is more visible during the day making UV vision more advantageous for diurnal species.[51]
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.[51][52] However, it can also be detected by powers; thecommon kestrel can see fresh vole urine via UV light and determine their abundance.[53]
Some rodents communicate with substrate vibrations, known asseismic communication. The fossorialMiddle East blind mole rat communicates over long distances by head thumping.[54] Footdrumming is used widely as a predator warning or defensive action. It is used primarily by fossorial or semi-fossorial rodents.[55] 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, forcing the snake to not pursue.[56] Several studies have indicated intentional use ofground vibrations as a means of intra-specific communication during courtship among theCape mole rat.[57]
TheCape ground squirrel is an example of both a promiscuous and non-defense polygynous rodent.[58]
Some species of rodent are monogamous, with an adult male and female forming a lastingpair 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 asCalifornia deermice,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 increases the chances of having a mate, particularly a fertile one. In facultative monogamy, the males do not provide direct parental care and are forced to mate with one female due to more dispersed social spacing. Prairie voles appear to be an example of this form of monogamy, with males guarding and defending females within their vicinity.[58]
Inpolygynous 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 defend areas where females are spatially clumped as territories. This occurs in ground squirrels likeyellow-bellied marmots,California ground squirrels,Columbian ground squirrels andRichardson'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 repel invading males. Some species are also known to directly defend their resident females and the ensuing fights can lead to serious injuries. In species with non-defense polygyny, males are not territorial and wander widely in search of females. These males establish dominance hierarchies, with the high-ranking males having access to the most females. This occurs in species likeBelding's ground squirrels and some tree squirrel species.[58]
Promiscuity, in which both males and females mate with multiple partners, also occurs in rodents. In species such asGunnison's prairie dogs, females give birth to litters with multiple paternities. Promiscuity leads to increasedsperm competition and males tend to have larger testicles. In theCape 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 and promiscuity.[58]
Female rodents play an active role in choosing their mates. Factors that contribute to female preference may include the fitness, dominance andspatial ability of the male.[59] In the eusocial naked mole rats, a single female monopolizes mating from the breeding males.[60] Reproductively active female naked mole-rats are more discriminating in the of males they associate with, preferring non-relatives.[61] Likely a protective measure against inbreeding.[62]
In most rodent species,ovulation is cyclical while a minority of species have itinduced by mating. Males of some rodent species leave behind amating plug, which blocks both sperm leakage and other males from inseminating the female. Females can remove the plug and may do so right after mating or after several hours.[59]
Youngbank voles in their nest beneath a wood pile.
Rodents may be born eitheraltricial (blind, hairless and relatively underdeveloped) orprecocial (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 construct complex nests for their young, lasting from before birth and until they areweaned. The female gives birth sitting or lying down and the young emerge in the direction she is facing. Starting at a few days old, when their eyes first open, offspring can periodically venture outside.[63]
In precocial species, the mothers invest little in nest building and some do not build nests at all. The female gives birth standing as the offspring emerge behind her. Mothers maintain contact with their highly mobile young with 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 with smaller litters having longer maternal care than larger ones.[63]
TwoPatagonian maras with young, an example of a monogamous and communal nesting species
Mother rodents provide both direct parental care, such as nursing, feeding, grooming, warmth, transportation, socialization and expelling, and indirect parenting, such as food storing, nest building and protection to their offspring.[63] In many social species, young may be cared for by individuals other than their parents, a practice known asalloparenting orcooperative 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 thePatagonian mara, young are also placed in communal warrens, but mothers do not permit youngsters other than their own to nurse.[36]
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 nutrition, resource competition, avoiding caring for alien young and, in the case of males, attempting to make the mother sexually receptive. The latter reason is more studied inprimates andlions.[64] Infanticide appears to be widespread in black-tailed prairie dogs, accounting for around half of litter deaths and usually done by resident lactating females.[65] To protect against infanticide from other adults, female rodents may employ avoidance or direct aggression (including in groups) against potential perpetrators, multiple mating, territoriality or early abortions.[64] Inalpine marmots, dominant females tend to suppress the reproduction of subordinates by harassing them while they are pregnant. The resulting stress decreases their reproductive health.[66]
Kangaroo rats can locate food caches by spatial memory.
Rodents have advancedcognitive abilities. They can recognize poisoned baits, making pest control difficult.[1] Guinea pigs can learn and remember complex pathways to food.[67] Squirrels and kangaroo rats are able to locate caches of food byspatial memory, rather than just by smell.[68][69]
Because laboratory mice (house mice) and rats (brown rats) are widely used asscientific models to further our understanding of biology, a great deal has come to be known about their cognitive capacities. Brown rats exhibitcognitive bias, where information processing is biased by whether they are in a positive or negative affective state.[70] 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 model."[71]
Laboratory (brown) rats may have the capacity formetacognition—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 known to have this capacity,[72][73] but these findings are disputed, since the rats may have been following simpleoperant conditioning principles,[74] or abehavioral economic model.[75] Brown rats use social learning in a wide range of situations, but perhaps especially so in acquiring food preferences.[76][77]
The earliest fossil record of mammals with the characteristic rodentdentition comes from thePaleocene, shortly after the extinction of the non-aviandinosaurs some 66 million years ago. These fossils are found inLaurasia,[78] the supercontinent composed of modern-day North America, Europe, and Asia. The divergence ofGlires, a clade consisting of rodents andlagomorphs (rabbits, hares and pikas), from other placental mammals occurred within a few million years after the Cretaceous-Paleogene boundary; rodents and lagomorphs thenspread during theCenozoic.[79][80] Somemolecular clock data suggest modern rodents (members of the order Rodentia) had appeared by the lateCretaceous,[81] although other molecular divergence estimations are in agreement with the fossil record.[82][83] Rodents may have outcompeted as replacedmultituberculates, but this is debated.[84][85]
Fossils of an early ancestor of the sciuromorph rodents,Acritoparamys atavus have been found in the deposits of earlyPalaeocene North America,[86] and sciuromorphs were widespread by the late Ecocene.[87] TheCastoridae which includes modern beavers, first appeared in North America in the lateEocene and colonized Eurasia via theBering Land Bridge in the earlyOligocene.[88][89] Late in the Eocene,hystricognaths invaded Africa, most probably having originated in Asia at least 39.5 million years ago.[90] 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 tocross the Atlantic on floating debris.[91] Caviomorphs had arrived in South America by 41 million years ago (implying a date at least as early as this for hystricognaths in Africa),[90] andreached the Greater Antilles by the earlyOligocene.[92]
The horned gopherCeratogaulus hatcheri, a burrowing mammal of the late Miocene to early Pleistocene, is the only known horned rodent.[93]
By 20 million years ago, fossils recognizably belonging to the current families such as Muridae can be found.[78] Muroids may have originated in Eurasia, and recolonized it up to ten more times. There is also evidence fo up to seven colonizations of Africa, five of North America, four of Southeast Asia, two of South America.[94]Nesomyid rodents are thought to have rafted from Africa to Madagascar 20–24 million years ago.[95] Some fossil rodents were very large in comparison to modern species; the largest known rodent wasJosephoartigasia monesi, apacarana which may have reached a length of 3 m (10 ft) and weight of 1,000 kg (2,200 lb). This animals lived 4–2 mya.[96]
Althoughmarsupials are the most prominent mammals in Australia, manyrodents, all belonging to Muridae, are among thecontinent's mammal species.[97] Fossil evidence suggests that rodents inhabited Australia as early as 4.5 million years. Species include theHydromyini 'old endemics', the first wave of rodents to colonize the country in the Miocene and earlyPliocene, and the true rat (Rattus) 'new endemics', arriving in a subsequent wave in the late Pliocene or earlyPleistocene.[98] Molecular evidence supports one origin for 'old endemics' rodents in both Australia and New Guinea.[99]
Rodents participated in theGreat American Interchange that resulted from the joining of the Americas by formation of theIsthmus of Panama, around 5 million years ago in thePiacenzian age.[78][100] In this exchange, asmall number of species such as the New World porcupines (Erethizontidae) headed north while murids migrated south.[78] However, the main southward invasion ofsigmodontines preceded formation of the land bridge by at least several million years, probably occurring via rafting.[101][102][103] Sigmodontines diversified explosively once in South America, although some degree of diversification may have already occurred in Central America before the colonization.[102][103]
The use of theorder name "Rodentia" is attributed to the English traveler and naturalistThomas Edward Bowdich (1821).[104] TheModern Latin wordRodentia is derived fromrōdēns, present participle ofrōdere,rōdō'to gnaw, eat away'.[105] The phylogeny of the rodents places them with thehares,rabbits andpikas (order Lagomorpha) in theclades Glires,, inEuarchontoglires andBoreoeutheria.[106] Lagomorphs share with rodents continuously growing incisors. However, they have an additional pair of incisors in the upper jaw.[107] The cladogram below shows some of the inner and outer relations of Rodentia based on a 2012 attempt by Wu et al. to align the molecular clock with paleontological data:[106]
The order Rodentia may be divided intosuborders,infraorders,superfamilies andfamilies. There is a great deal ofparallelism andconvergence 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.[109][110]Brandt (1855) was the first to propose dividing Rodentia into three suborders, Sciuromorpha, Hystricomorpha and 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 aphylogenetic 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.[109]
These disagreements remain unresolved, nor havemolecular studies fully resolved the situation though they have confirmed themonophyly of the group and that the clade has descended from a common Paleocene ancestor. Carleton and Musser (2005) inMammal Species of the World have provisionally adopted a five suborder system: Sciuromorpha, Castorimorpha, Myomorpha, Anomaluromorpha, and Hystricomorpha. As of 2021 theAmerican Society of Mammalogists recognizes 34 recent families containing more than 481 genera and 2277 species.[111][112][113]
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[114] 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.[114] Several species ofrice rat have become extinct since the 19th century, probably through habitat loss and the introduction of alien species.[115] In Colombia, thebrown hairy dwarf porcupine was recorded from only two mountain localities in the 1920s, while thered crested soft-furred spiny rat is known only from itstype locality on the Caribbean coast, so these species are considered vulnerable.[116] 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".[117]
The "three now cosmopolitan commensal rodent pest species"[118] (the brown rat, theblack rat and the house mouse) have been dispersed in association with humans, partly on sailing ships in theAge of Exploration, and with a fourth species in the Pacific, the Polynesian rat (Rattus exulans), have severely damaged islandbiotas around the world. For example, when the black rat reachedLord Howe Island in 1918, over 40 percent of the terrestrial bird species of the island, including theLord Howe fantail,[119] became extinct within ten years. Similar destruction has been seen onMidway Island (1943) andBig South Cape Island (1962). Conservation projects can with careful planning completely eradicate these pest rodents from islands using ananticoagulant rodenticide such asbrodifacoum.[118] This approach has been successful on the island ofLundy in the United Kingdom, where the eradication of an estimated 40,000 brown rats is giving populations ofManx shearwater andAtlantic puffin a chance to recover from near-extinction.[120][121]
Chinchillafur coat, exhibited at the 1900Exposition Universelle, Paris
Humans globally have used rodent fur for clothing.[2] 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 theNorth 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.[123][124] Later, thecoypu 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.[125] Thechinchilla 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.[125] The quills and guardhairs of porcupines are used for traditional decorative clothing. For example, their guardhairs are used in the creation of theNative 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 forquillwork by throwing a blanket over a porcupine and retrieving the quills it left stuck in the blanket.[126]
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.[127] Guinea pigs were first raised for food around 2500 B.C. and by 1500 B.C. had become the main source of meat for theInca 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.[127]
Guinea pigs are used in the cuisine ofCuzco, Peru, in dishes such ascuy al horno, baked guinea pig.[128] The traditional Andean stove, known as aqoncha or afogón, is made from mud and clay reinforced with straw and hair from animals such as guinea pigs.[129] 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.[127] In the United States, mostly squirrels, but also muskrats, porcupines, andgroundhogs are eaten by humans. TheNavajo people ate prairie dog baked in mud, while thePaiute ate gophers, squirrels, and rats.[127]
Rodents are used widely as in animal testing.[2] Albino mutantrats were first used for research in 1828 and later became the first animaldomesticated for purely scientific purposes.[130] 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 intogenetics,developmental biology,cell biology,oncology andimmunology.[131] 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,[132] but that total decreased to about 375,000 by the mid-1990s.[133] In 2007, they constituted about 2% of all laboratory animals.[132] Guinea pigs played a major role in the establishment ofgerm theory in the late 19th century, through the experiments ofLouis Pasteur,Émile Roux, andRobert Koch.[134] They have beenlaunched into orbital space flight several times—first by theUSSR on theSputnik 9biosatellite of 9 March 1961, with a successful recovery.[135] The naked mole rat is the only known mammal that ispoikilothermic; it is used in studies onthermoregulation. It is also unusual in not producing the neurotransmittersubstance P, a fact which researchers find useful in studies onpain.[136]
Rodents have sensitive olfactory abilities, which have been used by humans to detect odors or chemicals of interest.[137] TheGambian pouched rat is able to detecttuberculosis 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 detectland mines.[138][139] 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 avoid.[140][141][142]
Rodents including guinea pigs,[143] mice, rats, hamsters, gerbils, chinchillas, degus and chipmunks make convenient pets able to live in small spaces, each species with its own qualities.[144] 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.[145] 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.[146]
Rodents cause significant losses to crops, such as these potatoes damaged byvoles.
Some rodent species are serious agriculturalpests, eating large quantities of food stored by humans.[147] For example, in 2003, the amount ofrice 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.[148] InIndonesia andTanzania, rodents reduce crop yields by around fifteen percent, while in some instances in South America losses have reached ninety percent. Across Africa, rodents includingMastomys andArvicanthis damage cereals, groundnuts, vegetables and cacao. In Asia, rats, mice and species such asMicrotus brandti,Meriones unguiculatus andEospalax baileyi damage crops of rice,sorghum, tubers, vegetables and nuts. In Europe, as well as rats and mice, species ofApodemus,Microtus and in occasional outbreaksArvicola terrestris cause damage to orchards, vegetables and pasture as well as cereals. In South America, a wider range of rodent species, such asHolochilus,Akodon,Calomys,Oligoryzomys,Phyllotis,Sigmodon andZygodontomys, damage many crops including sugar cane, fruits, vegetables, and tubers.[148]
Because rodents are a nuisance and endangerpublic 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, andbiological control usingpathogens orpredators, as well aspoisoning and trapping.[153] The use of pathogens such asSalmonella has the drawback that they can infect man and domestic animals, and rodents often become resistant. The use of predators includingferrets,mongooses andmonitor lizards has been found unsatisfactory. Domestic and feralcats are able to control rodents effectively, provided the rodent population is not too large.[154] 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.[155]
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