Rodent

Rodent, largest order of mammals, including some 1,500 species of mice, rats, porcupines, and squirrels. Rodents have a single pair of incisors in the upper and lower jaws that continue to grow throughout life. Behind the incisors is a gap to allow recirculation of food in chewing. The cheek skin can be drawn across the gap, in front of the molars and premolars, leaving the incisors free for gnawing. Rodents are predominantly eaters of seeds, grain, and other vegetation.