Rodents

About two thousand species of rodents, the gnawing mammals, are classified in the order Rodentia. They are found worldwide, in trees, on the ground, underground, and in partly aquatic environments. Rodents are fur sources, pets, laboratory animals, and pests.

Principal Terms cuspid: a tearing tooth found in the mouth of a carnivorous animal herbivore: any animal that subsists entirely on plant foods incisor: a cutting tooth which acts like scissors or a chisel molar: a flat tooth found at the back of the jaw and used to grind food omnivore: an animal which eats both plants and other animals

Rodents, comprising about two thousand species, form the largest, most abundant mammal order. They are found almost everywhere on the earth. Most are ground dwellers and many rodent species dwell underground in burrows or tunnel networks of varying complexity and size. However, rodents also dwell in tree nests (squirrels) or lodges in ponds and streams (beavers), or simply run in herds (capybaras). Judging from fossil remains, rodents were widespread and plentiful fifty million years ago. It is believed that they evolved from small, insect-eating mammals, and did not develop into large species until a million years ago. The largest ancient rodents were giant, bear-sized beavers. Contemporary rodents are usually small. However, the largest modern rodents are herbivorous capybaras, which grow to approximately 100 pounds as adults.

Rodents also show remarkable diversity in their diets. These range from the vegetarian capybaras to the all-encompassing diet of omnivorous rats, which will eat meat. Rodents have many roles relative to humans. Hamsters and other small rodents are pets, capybaras are eaten as food, chinchillas are fur sources, and a few, such as rats and mice, are pests that compete with humans for their food crop supplies. The tremendous adaptability of rodents, especially rats, explains their wide geographical distribution in areas differing hugely in climate.

Physical Characteristics of Rodents

Among the two thousand known rodent species, size varies widely. Some small adult mouse species weigh about a fifteenth of a pound. At the other extreme, capybaras, largest of contemporary rodents, are the size of pigs.

Regardless of size, all rodents possess pairs of large, chisel-like front teeth in both the upper and lower jaws. The roots of these incisor teeth are located far back in rodent jawbones and grow continuously. Rodents lack the tearing teeth (cuspids) of carnivores as well as several premolars. Therefore, a large space exists between their incisors and molars. This allows the incisors to operate well in gnawing. The design of rodent dentition also allows the gnawed food to be transferred easily to the molars for efficient grinding. In addition, the muscles of the rodent lower jaw are arranged so as to enable its easy movement backward, forward, and laterally. This optimizes grinding of gnawed food.

Rodent incisors are different from those in other animals. Their continued growth from the

Rodents Living Underground
Groundhogs, like many rodents, live in underground dens where they hibernate through the winter. (Corbis)

root is valuable, especially because only the front surfaces of these teeth are protected by enamel, the hardest material in teeth. Thus, gnawing food causes the rear surfaces of the teeth to wear down faster than their front surfaces. This wear pattern is the basis for development of the chisel-like incisor edges. It continues as long as a rodent eats regularly, keeping the incisors sharp. Another interesting aspect of rodent mouths is that cheek fur grows inside the mouth and fills up the space between incisors and molars. This hair acts as padding and filters out food chunks too large to be swallowed comfortably.

Other than the special development of "gnawing machinery" of the mouth and teeth, rodents are anatomically unspecialized, with no other ubiquitous anatomic features. Where any special characteristic has developed in some rodents, it appears to be due to environmental need. For example, claws and front paws of burrowing rodents, such as woodchucks and moles, make them efficient diggers. In addition, gliding adaptations in some squirrels allow them to "fly" (or actually glide) from tree to tree. Furthermore, leaping ro dents such as the kangaroo rat use both hind feet together to enhance leaping capability. Yet another such adaptation is the webbed feet seen in beavers.

The Lives of Rodents

Rodents, like all other mammals, are warmblooded. They carry offspring to term in a uterus where each fetus is connected to the mother via the placenta, give birth to them, and nurse them. Depending on the rodent, the sequence of events between fertilization and the end of the nursing period takes between 5.5 weeks for a small mouse, to well over a year for large rodents. The process is easiest to describe for rats, although it is quite similar for mice and hamsters.

After fertilization, rat eggs make their way into a complex uterus which can hold eight to sixteen fetuses. There, each attaches to the uterine wall and develops, over three weeks, into a rat pup. The pups are born pink, hairless, blind, and incompletely developed. They are then nurtured by their mothers, who have the instinct of all mammals to care for their offspring. Rats breast-feed their pups for three weeks. At the end of this time, they are fully covered in hair, have full vision, and have begun to eat foods other than milk.

In another month the pups are sexually mature and can breed. This makes it clear why omnivorous wild rats pose a threat to humans. Any pair of rats can produce up to eighty offspring per year. Furthermore, within six weeks after birth, any two offspring can, and do, reproduce.

Inbred laboratory rats live for two to three years, depending on the strain. Males are much larger than females (often twice their size) and may attain body weights up to two pounds. In the wild the life expectancy of rats varies greatly. However, reports of animals living for over five years occur. Some males have been reported to be as large as small cats or dogs. Wild rats live in complex tunnels as colonies of a hundred or more animals.

Other rodents live different versions of the life of rats. Litter size, gestation time, group organization all vary. For example, the larger rodents have only a few offspring per litter, and some rodents live in tree nests (squirrels) or lodges in ponds (beavers). Life expectancies may be ten years or more, assuming death by natural causes.

Destructive and Beneficial Rodents

Rats and mice interact extensively with humans in a destructive fashion. The problems involved are

Rodent Facts

Classification:

Kingdom: Animalia Subkingdom: Bilateria Phylum: Chordata Subphylum: Vertebrata Class: Mammalia Order: Rodentia (rodents)

Suborders: Sciurognathi (squirrel-like rodents), Myo-morpha (mouselike rodents), Caviomorpha (cavy-like rodents)

Families: Castoridae (beavers, 1 genus, 2 species); Aplodontidae (mountain beaver); Sciuridae (squirrels, 49 genera, 267 species); Geomyidae (pocket gophers, 5 genera, 34 species); Anomaluridae (scaly-tailed squirrels, 3 genera, 7 species); Het-eromyidae (pocket mice, 5 genera, 65 species); Pedetidae (springhares); Muridae (rats and mice, fifteen subfamilies, 241 genera, 1,082 species); Gliridae and Selevinidae (dormice, 8 genera, 11 species); Zapodidae (jumping mice and birchmice, 4 genera, 14 species); Dipodidae (jereboas, 11 genera, 31 species); Erethizontidae (northwestern porcupines, 4 genera, 10 species); Caviidae (cavies, 5 genera, 14 species); Hydrochoeridae (capybaras); Myocastoridae (coypus); Capromyidae (hutias, 4 genera, 13 species); Dinomyidae (pacaranas); Agoutidae (pacas, 1 genus, 2 species); Dasy-proctidae (agoutis and acouchis, 2 genera, 13 spe cies); Abrocomidae (chinchilla rats, 1 genus, 2 species); Echimyidae (spiny rats, 15 genera, 55 species); Chinchillidae (chinchillas and vicuñas, 3 genera, 6 species); Octodontidae (degus, 5 genera, 8 species); Ctenomyidae (tuco-tucos, 1 genus, 33 species); Thryonomyidae (cane rats, 1 genus, 2 species); Petromyidae (African rock rat species); Hystricidae (Old World porcupines, 4 genera, 11 species); Ctenodactylidae (gundis, 4 genera, 5 species); Bathyergidae (African mole rats, 5 genera, 9 species) Geographical location: Every continent except Antarctica

Habitat: Mostly on land or underground, in forests, plains, and deserts; some live in a partly freshwater environment, using ponds or streams Gestational period: Varies greatly, though generally two weeks in a mouse and about a month in a rat Life span: Most smaller species live for one to three years, while large rodents survive for over ten years

Special anatomy: All rodents possess incisor teeth designed for gnawing, which grow continuously from the roots and wear away at their tips, giving them chisel-like edges that can gnaw through very hard materials; however, they lack cuspids (tearing teeth) seen in carnivores

Several Kinds of Rodents

Beavers, the largest Northern Hemisphere rodents, may weigh over fifty pounds. They live in dammed streams and in ponds, in lodges made of logs, rocks, and mud. Prized for their fur, beavers are vegetarians that eat tree bark and the roots, stems, and twigs of aquatic plants.

Capybaras, the largest rodents, weigh about one hundred pounds. Found in Central and South America, they are land animals, travel in herds, and eat grass.

Chinchillas, originally South American burrow-ers, are the source of valuable, silky fur. In the wild, they are colonial and venture above ground only at night to forage for vegetable foods.

Chipmunks are small ground squirrels about ten inches in length. They are most often brown, with characteristic black and white stripes along their backs, and short tails.

Gerbils are desert rodents which dig extensive bur-

rows. These gray, brown, or black rodents are related to rats but are more streamlined, and have prominent ears and furred tails.

Muskrats are semiaquatic rodents, named for musk glands in their hindquarters. Their luxuriant fur is used in fur garments.

Porcupines are arboreal or ground-living rodents with fur modified into defensive quills. When endangered, they backpedal at high speeds to impale predators with sharp quills that inflict painful wounds.

Tree squirrels are arboreal climbers and leapers. They are from four inches to several feet long and feed mostly on nuts.

Rats and mice belong to the largest rodent family, which also includes hamsters. "Rat" denotes the large species, while smaller species are "mice." Often, they are crop-eating pests. Rats and mice are often used as lab animals.

competition for food, and disease transmission from rodents to humans. Rats and mice, viewed as pests, are known to eat 10 to 25 percent of grain crops grown, harvested, and stored worldwide. This percentage varies depending upon the extent of use of rodenticides, such as warfarin, in various nations and the extent of agricultural technology. Very careful use of rodenticides is important because they are quite toxic to humans.

Rodents are disease vectors, historically causing outbreaks of serious epidemics of the bubonic plague and tularemia. This was especially serious during the Middle Ages, when rats were responsible for the transmission of the Black Death. Currently, most sporadic outbreaks of rodent-derived infectious disease are handled by use of rodenti-cides to kill carriers and antibiotics to destroy rodent-borne microorganisms that infect humans. Most often it is not the rodents themselves that cause disease outbreaks. Rather, infection occurs as contaminated fleas and ticks move from rodents to humans. Rats are seen as the main disease vectors because they abound near and in human habitations. However, mice and any other infected rodents can be disease vectors.

Concerning beneficial use of rodents, one can point to the myriad rats, mice, hamsters, and guinea pigs utilized as laboratory animals in testing and developing pharmaceuticals, the identification of toxic cosmetic, paint, and food components, isolation of disease cures, and so on. This aspect of research is likely to become less common because a large segment of the population deems it morally inappropriate to submit animals to these testing procedures.

Another benefit of rodents that is becoming morally unacceptable is harvesting rodent fur. Beaver fur was once hugely important to the world fur trade. Presently, as beaver are nearly extinct, the use of rodents to provide fur for human use has shifted to muskrats, nutria, and chinchillas, which are valued for their attractive, luxuriant coats.

—Sanford S. Singer See also: Beavers; Diseases; Gophers; Home building; Mice and rats; Porcupines; Squirrels.

Bibliography

Alderton, David. Rodents of the World. New York: Facts on File, 1996. Lots of information and numerous fine illustrations of rodents of the world, as well as useful bibliographical references.

Lacey, Eileen A., James P. Patton, and Guy N. Cameron, eds. Life Underground: The Biology of Subterranean Rodents. Chicago: University of Chicago Press, 2000. Offers considerable information on rodents and their life cycles underground. Many useful illustrations and a solid bibliography.

Stoddart, D. Michael, ed. Ecology of Mammals. New York: John Wiley & Sons, 1979. This classic text describes and compares many ecological aspects of mammal life, including rodents. It is well-designed, nicely illustrated, with a good bibliography.

Webster, Douglas, and Molly Webster. Comparative Vertebrate Morphology. New York: Academic Press, 1974. Excellent description and comparison of vertebrate anatomy, morphology, and histology, done in a clear and comprehensive fashion.

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