Rodentia is an order of mammals also known as rodents, characterised by two continuously-growing incisors in the upper and lower jaws which must be kept short by gnawing. The order Rodentia may be divided into suborders, infraorders, superfamilies and families. Forty-percent of mammal species are rodents, and they are found in vast numbers on all continents other than Antarctica. Common rodents include mice, rats, squirrels, chipmunks, gophers, porcupines, beavers, hamsters, gerbils, and guinea pigs.Rodents have sharp incisors that they use to gnaw wood, break into food, and bite predators. Most eat seeds or plants, though some have more varied diets. Some species of rodents have historically been pests, eating human seed stores and spreading disease. Some of these same rodents also are popular choices for pets. In terms of number of species, rodents make up the largest order of mammals. There are about 2,277 species of rodents .Their success is probably due to their small size, short breeding cycle, and ability to gnaw and eat a wide variety of foods. All rodents have a single pair of upper and a single pair of lower incisors, followed by a gap ( diastema), followed by one or more molars or premolars. No rodent has more than one incisor in each quadrant, and no rodent has canines. Rodent incisors are rootless, growing continuously. Their anterior and lateral surfaces are covered with enamel, but their posterior surface is not. During gnawing, as the incisors grind against each other, they wear away the softer dentine, leaving the enamel edge as the blade of a chisel. This "self sharpening" system is very effective and is one of the keys to the enormous success of rodents. |
