RODENTIA - the Rodent Family

The name Rodentia is derived from the Latin verb rodere (to gnaw), in allusion to the gnawing habits of the group. With 29 families, 426 genera, and 1, 814 species, the order Rodentia has more members than any other order of mammals. Most people are familiar with mice, rats, hamsters, and guinea pigs, which are commonly kept as pets. The Rodentia also includes beavers, muskrats, porcupines, woodchucks, chipmunks, squirrels, prairie dogs, marmots, chinchillas, voles, lemmings, and many others. Rodents range in size from pygmy mice weighing 5 gms to capybaras, the largest of which weigh over 70 kg. They are found around the world except in Antarctica, New Zealand, and on some oceanic islands. Ecologically, they are incredibly diverse. Some species spend their entire lives above the ground in the canopy of rainforests; others seldom emerge from beneath the ground. Some species are highly aquatic, while others are equally specialized for life in deserts. Many are to some degree omnivorous; others are highly specialized, eating, for example, only a few species of invertebrates or fungi.

Despite their great species diversity, all rodents share common features. Rodents have a single pair of incisors in each jaw, and the incisors grow continually throughout life. The incisors have thick enamel layers on the front but not on the back; this causes them to retain their chisel shape as they are worn down. Behind the incisors is a large gap in the tooth rows, or diastema; there are no canines, and typically only a few molars at the rear of the jaws. Rodents gnaw with their incisors by pushing the lower jaw forward, and chew with the molars by pulling the lower jaw backwards. In conjunction with these chewing patterns, rodents have large and complex jaw musculature, with modifications to the skull and jaws to accommodate it. Like some other mammal taxa, but unlike rabbits and other lagomorphs, male rodents have a baculum (penis bone). Some rodents have either internal or external cheek pouches that open near the angle of the mouth. The external pouches are fur-lined, and the animal can turn them wrong side out to clean them. The tongue, which is short and compressed, cannot be protruded beyond the incisors in many genera.

In addition to the dental characters, rodents have other anatomical features in common. The bones of the lower arm, the radius and ulna, are distinct, and the elbow joint permits free motion of the forearm. The hand usually has five fingers, though the thumb may be vestigial or absent. The toes number three to five. The stomach is variable, ranging from a simple sac to a complex, ruminantlike organ, as in the lemmings. The penis usually has a baculum, and the testes are inguinal or abdominal (in many cases they are inguinal only during the breeding season).

The tails of the members of some families break off readily when the animals are caught by the tail, enabling them to escape. A partial replacement of the lost portion of the tail then grows. The skin of the tail may also break readily and slip off beyond the break, leaving the flesh and bone exposed. The animal later amputates the exposed portion of the tail with its teeth, and the end heals.

In habits, members of this order are diverse. Most of them are nocturnal or crepuscular; ground squirrels and tree squirrels are strictly diurnal; others may be active either by day or by night. Considerable adaptive radiation occurs in the group. Some species (pocket gopher) are fossorial; others are aquatic (beaver), arboreal (tree squirrel), volant (flying squirrel), or terrestrial (cotton rat). Most rodents feed on vegetation, but a few species, notably the grasshopper mouse, feed extensively upon animal matter. Most rodents are active throughout the year, but others, notably ground squirrels, may hibernate for several months. Some are solitary such as porcupines and others highly social living in extensive colonies, such as prairie dogs and naked mole rats.

Rodents cost billions of dollars in lost crops each year, and some are carriers of human diseases such as bubonic plague, typhus, and Hanta fever. However, various rodent species are economically important as sources of food or fur in many parts of the world, and others are used extensively in biomedical research