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rodent : ウィキペディア英語版
rodent

Rodents (from Latin ''rodere'', "to gnaw") are mammals of the order Rodentia, which are characterized by a single pair of unremittingly growing incisors in each of the upper and lower jaws. About forty percent 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 human-made environments. There are species that are arboreal, fossorial (burrowing), and semiaquatic. Well-known rodents include mice, rats, squirrels, prairie dogs, porcupines, beavers, guinea pigs, hamsters, and capybaras. Other animals such as rabbits, hares and pikas were once included with them, but are now considered to be in a separate order, Lagomorpha.
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 have precocial young that are relatively well developed at birth.
The rodent fossil record dates back to the Paleocene on the supercontinent of Laurasia. They greatly diversified in the Eocene, as they spread across continents, sometimes even finding means to cross oceans. Rodents reached both South America and Madagascar from Africa, and were the only terrestrial placental mammals to reach and colonize Australia.
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, as they sometimes threaten the survival of native species, such as island birds, previously isolated from land-based predators.
== Characteristics ==

The distinguishing feature of the rodents is their single pair 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 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 shaped like the blade of a chisel.〔 Most species have up to 22 teeth with no canines or anterior premolars. There is a gap, or diastema, between the incisors and the cheek teeth in most species. This allows them 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 mouth. Chinchillas and guinea pigs have a high fiber diet; their molars have no roots and grow continuously like their incisors.
In many species, the molars are relatively large, intricately structured and highly cusped or ridged, though some, such as ''Pseudohydromys'', have smaller and simpler ones. 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 an enlarged temporalis muscle, making them able to chew powerfully with the molars. The Hystricomorpha, such as the guinea pig, have a larger superficial masseter muscle and a smaller deep masseter muscle than rats or squirrels, possibly making them less efficient at biting with the incisors, but their enlarged internal pterygoid muscle may allow them to move the jaw further sideways when chewing.
While the largest species, the capybara, can weigh as much as , most rodents weigh less than . The smallest rodent is the Baluchistan pygmy jerboa, which averages only in head and body length, with adult females weighing only . Rodents have wide-ranging morphologies, but typically have squat bodies and short limbs.〔 The forelimbs usually have five digits, including an opposable thumb, while the hindlimbs have three to five digits. The elbow gives the forearms 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 nails.〔 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 forelimbs to the hindlimbs. 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.〔
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 percent 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 that 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 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, however male-bias sexual dimorphism occurs in alpine populations, possibly because of the lack of predators and greater competition between males.
The diversity of characteristics of rodents is great, sometimes even in closely related species. Characteristics of several rodents are given in the table below.〔(【引用サイトリンク】url=http://genomics.senescence.info/species/ )

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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