A rodent is any mammal that belongs to the order Rodentia, which includes most mammals equipped with continuously growing incisor teeth that are remarkably efficient for gnawing on tough plant matter. The name rodent comes from the Latin word rodere meaning "to gnaw." Rodents live in virtually every habitat, often in close association with humans. This close association between rodents and humans is frequently detrimental to human interests, since rodents (especially rats and mice) eat huge quantities of stored food and spread serious, often fatal, diseases. There are far more members in the order Rodentia than in any other order of mammals. Nearly 40% of all mammal species belong to this order.
Some rodents such as beavers have been economically important. Others, such as guinea pigs, hamsters, and gerbils, are fun pets. However, most of the about 1,600 species (the exact number changes frequently as various groups of rodents are studied closely) play little role in human lives. Instead, they carry on their own lives in virtually every environment, rarely noticed by the humans around them.
Rodents are distinguished from other mammals primarily by their 16 teeth. Lagomorphs (rabbits and hares) also have continuously growing incisors, and they were, for many years, included among the rodents. But they have an additional pair of tiny incisors that grows just behind the big front teeth, so they are now classified in a separate order.
The two pairs of rodent incisors work together, like scissors. They grow continuously from birth and must regularly be used for gnawing to keep them worn down and sharp. They have a heavy coating of enamel on the front surface but none on the back. Because the enamel wears away more slowly than the rest of the tooth, a sharp, chisel-like edge is maintained on the gnawing teeth. If a rodent breaks one of its incisors, the animal usually soon dies because it cannot eat properly.
Unlike many mammals, rodents have no canine teeth. Instead, there is an empty space between the incisors and flat-topped cheek-teeth, or molars, at the side of the mouth. This space lets rodents suck in their cheeks or lips to shield their mouths and throats from chips flying from whatever material they are gnawing. When using their cheek-teeth to grind up the plant matter they have gnawed, rodents have special jaw muscles that keep their incisors out of the way.
Rodents are divided into three groups according to the way their jaw muscles and associated skull structures are arranged. This is very important because these muscles control gnawing.
The squirrel-like rodents (Sciuromorpha) have a very simple jaw muscle that extends onto the snout in front of the eye. This group includes the squirrels as well as such unsquirrel-like animals as beavers and pocket gophers. They are mostly found in the northern hemisphere.
The mouse-like or rat-like rodents (Myomorpha) have jaw muscles that anchor on the side of the nose. Because their jaw muscles are the most efficient, this group contains the most species and is found all over the world. It includes the mice, rats, voles, lemmings, and even the riverbank-dwelling muskrat. Two-thirds of all rodents belong to only one family in this group, the mice.
The cavy-like rodents (Caviomorpha) have very large cheekbones and muscles that anchor to the side of the face. This group includes the porcupines, as well as primarily South American mammals such as the cavy. Some fossil mammals in this group were as large as bears. The Old World members of this group are sometimes placed in a separate group called the porcupine-like rodents (Hystricomorpha).
Most rodents are very small, averaging less than 5 oz (150 g). However, the capybara, a large South American rodent, may weigh as much as 145 lb (66 kg). Rodents usually breed easily and quickly, producing large litters. This fact played a major role in their worldwide distribution. Genetic changes can develop into new species quite rapidly when animals breed so quickly. Such changes allowed rodents to take over many habitats that might not otherwise have been suitable. Rodents swim, glide, burrow, climb, and survive different uncomfortable climates.
Rodents are known to carry disease-causing agents of at least 20 important human diseases including bubonic plague. About 500 years ago, at least 25 million people died in Europe from the "black death," as the plague was called. The plague-causing bacteria (Yersinia pestis) were carried by fleas that were spread from rodents to people.
Knight, Linsay. The Sierra Club Book of Small Mammals. San Francisco: Sierra Club Books, 1993.
The International Mouse Mutagenesis Consortium. "Functional Annotation of Mouse Genome Sequences." Science 291 (February 16, 2001): 1251-1255.
Jean F. Blashfield