The order Rodentia, with ~2,020 living species placed in 28 families (approximately half of all mammalian species), is the largest order of mammals. They are found worldwide except in Antarctica and on some oceanic islands. Ecologically, they are remarkably diverse. Some species spend their entire lives above the ground in the canopy of rainforests; others rarely emerge from beneath the ground. Some species are aquatic, whereas others are equally specialized for life in deserts. Many rodents are to some degree omnivorous; others are highly specialized, eating, for example, only a few species of invertebrates or fungi.
Despite their morphologic and ecologic diversity, all rodents share one characteristic: a highly specialized dentition for gnawing. Rodents have a single pair of upper and a single pair of lower incisors. Between each incisor and the first cheek tooth is a toothless interval called the diastema. The incisors are rootless and grow continuously. Enamel is deposited on the anterior and lateral incisor surfaces; the posterior incisor surface is dentin. During gnawing, as the incisors chisel against each other, they wear away the softer dentin, leaving a sharp enamel edge. This "self-sharpening" system is very effective and is one of the keys to the enormous ecologic success of rodents.
Using the incisors together to chisel away at a surface requires muscle that forcefully brings the lower jaw forward. The masseter muscle does this in rodents. Rodents are traditionally divided into three groups based on how the masseter attachments evolved: sciuromorphs (eg, squirrels, beavers), hystricomorphs (eg, New and Old World porcupines, guinea pigs, jerboas), and myomorphs (eg, New and Old World rats and mice, hamsters, gerbils, voles).
Most modern rodents have adapted to eat seeds, which links them to the evolution of modern grasses. All rodents evolved from shrewlike carnivorous or insectivorous ancestors and have since diverged into various families and subfamilies.
Despite the large number of rodents, only a few species are owned as pets. The common pet rodents are chinchillas, gerbils, guinea pigs, hamsters, mice, and rats. Less common pet rodents are African giant pouched rats, degus, prairie dogs, spiny mice, and voles.
As prey animals, rodents do not show obvious signs of pain or disease until near death. Consequently, sick rodents are often presented late in disease progression when the prognosis is more guarded. Dermatologic conditions make up 25% of rodent cases presented for small animal consultation. Traumatic injuries are also frequent and seen in rodents of all ages. Although some diseases are caused by shortcomings in husbandry, often rodents subjected to “benign neglect” live long, healthy lives. The adverse effects of overfeeding pet rodents are seen in the early development of many spontaneous tumors and degenerative diseases.