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Rodents Facts: The Most Successful of All Mammal Species

About Rodents

Rodents, order Rodentia, are among the most successful of all mammal orders, constituting about 40 percent of all mammal species. The actual number of species has been estimated at somewhere around 2,000.

Although rodents are a diverse group in appearance and habits, they all share the characteristic of being gnawing animals. They are generally small—in the size range of their most familiar representatives, the mouse and the rat—although the largest of all rodents, the South and Central American capybara, may weigh up to 50 kg (110 lb).

Some rodents feed entirely on various kinds of plant foods. This may range from fruits and nuts to fungi. A number of rodents have more specialized diets, such as the beaver, which specializes in feeding on bark.

Some species add insects and other small organisms to their plant diet. The grasshopper mouse, for example, is specialized for eating invertebrates. Rodents that have long lived in close association with humans, primarily the rats and mice, have learned to adopt an omnivorous diet.

Many rodents are nocturnal animals. They are found in nearly every terrestrial habitat, and they exist at higher altitudes in the mountains than any other mammals. Other than the rats and mice, some of the most familiar rodents include the various squirrel species, the guinea pig, the hamster, the porcupine, the prairie dog, and the groundhog.

In 2005 the discovery of a new species of rodent, Laonastes aenigmamus, or Laotian rock rat, was announced. This unique animal, looking somewhat like a cross between a squirrel and guinea pig, diverged from the rest of the rodent order millions of years ago.

Specialized Teeth

The common feature of all rodents is a pair of chisel-shaped teeth, or incisors, at the front of both the upper and lower jaws. These teeth continue to grow throughout the lives of the animals and must be worn down by gnawing. The bases of the incisors extend far back into the jaws. The front face of each incisor consists of hard enamel. The rest of the tooth is of softer dentine.

This results in differential wear, always leaving a sharp cutting edge on the tooth. If for some reason a rodent loses either the upper or lower pair of incisors, the other pair continues to grow uncontrollably, and the animal soon dies.

Rodents lack other incisor teeth, and the first and (in the lower jaw) second premolars are also lacking. This leaves a gap, called the diastema, between the incisors and the cheek teeth (premolars and molars). The front teeth and cheek teeth are offset in relation to each other. Thus when the upper and lower incisors are brought together in gnawing, the cheek teeth do not need to meet and wear uselessly, and vice versa.

A skull depression at the rear of the skull that acts as a socket for the rearward projecting hinge of the lower jaw is shallow and elongated from front to rear. This permits the lower jaw to move back and forth as well as from side to side. The different forms of skull attachment of the muscle involved in this movement have been used in classifying suborders of rodents.

Some rodents do not fit into these groupings and instead commonly are grouped in a fourth suborder. Consequently, four groups -protogomorphs (example, the mountain beaver), sciuromorphs (example, the squirrel), hystricomorphs or caviomorphs (example, the guinea pig), and myomorphs (example, the rat)- are often recognized and are now thought of as convenience groups for dealing with the very many species of rodent.

Other Characteristics

Physically, most rodents are like the familiar mouse or squirrel types. Some, however, such as the Eurasian mole rats, Spalax, resemble moles, and the South American maras, Dolichotis, suggest nonhopping rabbits.

Most rodents are fully haired, except for the tail. A number, such as the chinchilla, have especially thick coats, whereas the East African naked mole rat, Heterocephalus, is practically hairless.

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Some rodents, such as the beaver, have webbed hind feet or are otherwise modified for swimming. The North and Central American flying squirrels are so named because they have a flap of skin along each side of the body, between the front and hind legs. These flaps are extended for gliding from branch to branch through the air.

The many rodent species that live in temperate or colder regions of the world generally have life patterns in which the seasonal length of day determines their degree of activity at that time of year. Many of these rodents do not breed at all during the winter. Some of them, such as the dormouse and woodchuck, hibernate instead (see hibernation).

Rodents that dwell in more strenuous habitats exhibit various specialized adaptations. For example, the desert-dwelling kangaroo rats are able to survive without drinking water. The water they obtain is instead derived through the digestion of the foods in their diet. This water is maintained in the body for long periods because the urine of the kangaroo rat is highly concentrated. The animals also are only active at night, when temperatures are low and relative humidity is higher.

The species that are found in Arctic regions, such as the lemmings and some of the voles, are commonly larger than related species found in warmer regions. This is a form of heat conservation. The collared lemming also has broad furred feet that enable it to run swiftly over snow, and it is hidden from predators by a white coat in wintertime. In addition, the young are reared under cover of snow before they are exposed to predators during spring melting.

Reproductive Behavior

Rodents are generally social animals, although some are solitary. Breeding cycles vary considerably among species. Female rodents have a duplex type of uterus, in which the two uteri remain separate but are joined to a single vagina. Males commonly have a penis bone. After mating, gestation varies from about 16 days (as in the golden hamster) to about 4 1/2 months (in the capybara).

Litter size ranges from one to more than 18 newborn. Usually only the female raises the young, but in pygmy mice, for example, the male parent also contributes to their care. Reproductive age may be attained at 60 days or earlier (as in the Eurasian steppe lemmings), and reproductive life may continue into old age. Most rodents are short-lived, rarely surviving naturally beyond 2–3 years. Old World porcupines, however, may live for more than 20 years.

Evolution

The earliest known rodent, Paramys, lived in North America from the late Paleocene to the middle Eocene Epoch, about 56 million to 45 million years ago. Its fossil remains were unquestionably those of a rodent and not a transitional form, but they provided no definite information as to its origins.

Indeed, no other fossil has yet been found that definitely links the rodents to any other mammal group. (Some scientists maintain that rodents themselves are genetically too diverse to be considered a single order. They claim, for example, that according to DNA evidence guinea pigs definitely should not be grouped as rodents. Other scientists disagree with this claim.)

It is believed that the rodents evolved from early insectivores, as did the primates, and that the ancestors of the rodents and the ancestors of the primates were themselves closely related.

Rodents and Humans

Rodents as a group tend to have a bad reputation because many species cause serious economic damage by feeding on and damaging crops and stored foods. Some rat species are also notorious for transmitting disease.

Rat-borne epidemics such as the bubonic plague, for instance, have caused many millions of human deaths throughout recorded history, and house mice can also be transmitters of various illnesses. Humans have contributed to these harmful effects by inadvertently transporting disease-carrying rat and mice species to all parts of the world.

Despite this, rodents as a whole are of considerable value to humans. They are beneficial in destroying large quantities of harmful weeds and insects. Because of their usually large numbers in a region, and the consequent effects of these numbers on food sources -as well as the fact that they are all preyed upon by other animals- rodents are a vital element in the ecology of most regions.

Beavers, as a result of their practice of damming up streams, even create their own local environments. Humans have used some rodents -for example, the beaver, chinchilla, muskrat, and nutria (coypu)- as sources of furs, and in some regions a number of different rodents are used as food.

Finally, humans owe a large debt to rodents because many of the significant achievements in biological research were made possible only through the use of laboratory rats and mice.

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