Of the 5,000 species of mammals, over 2,000 are rodents. The main features that characterize the rodents are their teeth and their jaw muscles. These animals evolved to gnaw things. Rodents have four incisor teeth for cutting, two in the upper jaw and two in the lower. The four main groups of rodents are the relatives of mice and rats, beavers, squirrels and porcupines. The shapes of their skulls and the arrangements of the muscles that operate their jaws distinguish these various groups.

The relatives of mice and rats comprise the largest group and represent over 1,500 species. This group includes gerbils, hamsters, voles, lemmings, and jerboas. Muskrats are also an example of this group.

The group of beaver relatives includes pocket gophers, kangaroo rats, kangaroo mice, and pocket mice. That encompasses about 100 species. Out of numerous ancestors, only two species of the namesake beavers exist today: the North American beaver and the Eurasian beaver.

The squirrel relatives are familiar to most people. This group of 300 species includes tree squirrels, like the red and gray squirrels in North America, and various ground squirrels. Chipmunks are examples of the ground squirrels, as are their giant cousins, the marmots.

Lastly, the group of porcupine relatives includes not only the porcupines, but spiny rats, mole rats, cavies, agoutis, nutrias, pacas, chinchillas and hutias. This group also includes about 300 species. The capybara and the mara shown here belong to the family of cavies. The yellow-crowned brush-tailed tree rat is one of the spiny rats, which have very stiff bristles on their tails.