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The mammalian order consisting of the rodents, often known as the gnawing mammals. This is the most diverse group of mammals in the world, consisting of over 2000 species, more than 40% of the known species of mammals on Earth today. Rodents range in size from mice, weighing only a few grams, to the Central American capybara, which is up to 130 cm (4 ft) in length and weighs up to 79 kg (170 lb). Rodents have been found in virtually every habitat and on every continent except Antarctica. Rodents have adapted to nearly every mode of life, including semiaquatic swimming (beavers and muskrats), gliding (“flying” squirrels), burrowing (gophers and African mole rats), arboreal (dormice and tree squirrels), and hopping (kangaroo rats and jerboas). Nearly all rodents are herbivorous, with a few exceptions that are partially insectivorous to totally omnivorous, such as the domestic rat. The great adaptability and rapid evolution and diversity of rodents are mainly due to their short gestation periods (only 3 weeks in some mice) and rapid turnover of generations.
The most diagnostic feature of the Rodentia is the presence of two pair of ever-growing incisors (one pair above and one below) at the front of the jaws. These teeth have enamel only on the front surface, which allows them to wear into a chisellike shape, giving rodents the ability to gnaw. Associated with these unique teeth are a number of other anatomical features that enhance this ability. Behind the incisors is a gap in the jaws where no teeth grow, called a diastema. The diastema of the upper jaw is longer than that of the lower jaw, which allows rodents to engage their gnawing incisors while their chewing teeth (molars and premolars) are not being used. The reverse is also true; rodents can use their chewing teeth (also called cheek teeth) while their incisors are disengaged.
The entire skull structure of rodents is designed to accommodate this task of separating the use of the different types of teeth. Rodent skulls have long snouts; the articulation of the lower jaw with the skull is oriented front to back rather than sideways as in other mammals; the jaw muscles (masseter complex) are extended well forward into the snout; and the number of cheek teeth is less than in most other mammals—all features unique to rodents.
The classification of rodents has always been difficult because of the great diversity of both Recent and fossil species. Traditionally, there are two ways that rodents have been divided: into three major groups based on the structure of the attachment of the jaw muscle on the skull (Sciuromorpha, Hystricomorpha, Myomorpha); or into two groups based on the structure of the lower jaw (Sciurognathi, Hystricognathi). The difficulty in using these groups (usually considered suborders or infraorders) is that the distinctive adaptations of one group of rodents are also present in others, derived in completely separate ways.
the most numerous order of class Mammalia, embracing more than one-third of all mammalian species and about one-half the species of all fauna in the USSR.
Fossil remains of Rodentia have been found from as early as the Paleocene, although the order is supposed to have originated in the Cretaceous period—probably from ancestors in common with Insectívora. The most closely related extant group is the order Lagomorpha. Rodents are small (more rarely, medium-sized) animals (from 5 cm long in the harvest mouse to 1.25 m in the capybara). The cerebral hemispheres are small or underdeveloped; their surface is usually smooth. The teeth are adapted for tough vegetable foods. The highly developed incisors (one pair in each jaw) are curved, archlike, and grow throughout the animal’s life; their cutting edges sharpen themselves through detrition (associated with the varying hardness of the enamel and dentine). In some rodents the molars also grow constantly. There are no canine teeth; between the incisors and the molars there is a large toothless gap, or diastema. The grinding surfaces of the molars have tubercles, ridges, or flat folds; the edges usually are also self-sharpening under detrition (because of the differing thicknesses of enamel in different areas).
Rodents are found throughout the world. There are about 30 extant families, comprising more than 400 genera (as many as 2,000 species). In the USSR there are 11 families (50 genera, represented by over 130 species); in addition, as many as 70 extinct species are known from the Quaternary period alone, no less than 15 of which belong to extinct genera. The present-day species of Rodentia are usually grouped into three suborders: the Sciuromorpha, including five extant families—beavers (Castoridae), American mountain beavers, squirrels (Sciuridae), pocket gophers (Geomyidae), and pocket mice (Heteromyidae), and five extinct ones; the Hys-tricomorpha, represented by 12 extant families, including the true porcupines, arboreal porcupines, guinea pigs, capybaras, agoutis, coypus, and chinchillas, and three extinct ones; and the Myomorpha, consisting of ten extant families, which include the dormice, Seleviniinae, birch mice, jerboas, mole rats, mice, and hamsters, and three extinct ones. The suborder most abundant in species is Myomorpha, especially in the hamster family, to which belong the voles and lemmings (Microtinae), the hamsters, and the gerbils (Gerbillinae), and which includes more than one-half the species of fauna in the USSR.
Rodentia inhabit all natural zones from the arctic tundra to the mountain snow line. Characteristic of the tundra in the USSR are the arctic and common lemmings; of the forest zones, the squirrels, chipmunks, red-backed voles, and mice; of the steppe, the marmots, ground squirrels, common voles, and steppe lemmings; and of the desert zones, the jerboas and gerbils. The greatest diversity of species is found in the steppe and forest steppe.
The majority of rodents are semiunderground, living in burrows that they have dug themselves and that are often deep and intricate; however, they feed on the surface. There are also forms that are predominantly aboveground (mice and rats), completely underground (zokors and mole voles), semiaquatic (beavers and muskrats), and arboreal (squirrels and dormice).
Distinct adaptations are found in limb structure for digging, swimming, bipedal running (on the hind legs; jerboas), climbing, and even gliding in air (flying squirrels). Because they often live in burrows and hibernate, rodents can withstand unfavorable environmental conditions more readily than other mammals, and because they reproduce quickly, they are the most numerous of the class. The number of rodents in the tundra and of the “mouselike” rodents in the steppes may vary sharply, increasing 100 or 200 times during mass reproductive periods. Many species are serious agricultural pests (especially of grain farming); they are also harmful to forestry and do damage to produce and commodities in warehouses. A number of species are simultaneously the source of helminthic infections in man and domestic animals and the causative agents of infectious diseases transmitted by parasites (ticks, fleas), infected water, produce, and direct contact; among these are such especially dangerous infections as plague (sources of infection include rats, marmots, gophers, and gerbils), encephalitides (chipmunks and red-backed voles), and rabies (rats). In the USSR the number of rodents is being limited and the harmful species exterminated by mechanical, chemical, and biological means. There are fur-bearing species such as the squirrel, among the rodents, including a muskrat and a coypu that have been acclimatized to the USSR; many rodents also serve as the principal food of valuable predatory fur-bearers.
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I. M. GROMOV