Rodents make up the largest group of mammals, representing approximately 43 percent of all mammalian species. Families in the order Rodentia include rats, mice, porcupines, hamsters, beavers, squirrels, chipmunks, lemmings, muskrats, and guinea pigs (rabbits are not rodents). These families range in size from the pygmy mice, which are 4.7 inches long (12 centimeters) and weigh 0.1 ounces (4 grams), to the capybara, which is 39.4 inches long (100 centimeters) and can weigh 110 pounds (50 kilograms). Most rodents are relatively small animals, such as mice, rats, and squirrels.
While there is a broad range of characteristics among the families, the feature that sets rodents apart from other family members is their teeth. Rodents have one pair of upper incisors (the chisel-shaped teeth at the front of the mouth), and one pair of lower incisors. These teeth grow continually throughout their life. The outer surfaces of the incisors have a thick enamel (hard white substance) layer. Behind the incisors is a large gap in the tooth rows. There are no canines, spade-shaped teeth located next to the incisors. Typically there are only a few molars at the rear of the jaws. The number of teeth rarely is more than twenty-two.
The name Rodentia comes from the Latin verb rodere meaning to gnaw, a name suitable for a rodent that is constantly gnawing! Rodents' incisor teeth grow throughout their life and they grind their incisor teeth together to wear them down. If for some reason the rodent is unable to wear its incisors down, the tips may grow past each other and continue to grow outward into spiral. This may result in the upper teeth piercing the roof of the mouth, and the lower teeth growing upward in front of the nose, which could kill the animal.
In general, rodents have a compact body with short legs. They typically have four to five digits on each of the front feet and three to five digits on the rear feet. Generally, the sole of the foot is bare. Some rodents, such as hamsters and pocket gophers, have cheek pouches, which allow the animals to store and transport food. The tails of some rodent species break off when these animals are caught by the tail, which allows them to escape. The tail will partially grow back.
Other physical characteristics of rodents vary widely depending upon the species and where it lives. For example, rodents that live in the desert, such as American kangaroo rats, Australian hopping mice, and north African jerboas, have long, narrow hind legs and feet with a long tail used for hopping over the sandy desert floor. They all have well-developed hearing, small front limbs, and pale coloration. Animals that live in and around the water, such as the capybara and beaver, may have webbed or partially webbed feet and tails modified for swimming.