Rodents, usually not thought of one of humans favorite type of animals, are very opportunistic and adaptive. The evolutionary young order retains large stocks of genetic variablilty and large litter size exposes the variablilty to forces of the environment very rapidly. New genetic combinations quickly arise in sudden environmental changes. Rodents occur in many habitats, from tundra to deserts, living high in trees or underground burrows. Some live in water while others never touch water their whole lives.
Rodents show less variation in body plan than many other mamillian orders.
Most are squat, compact mammals with short limbs and a tail. There are a few large species, mostly in South America. The larger species have evolved long legs and a larger body fit for a niche in the grassy planes area where there is little competition by other large herbivores. One highly variable feture between rodents arises in the tail structure. All rodents have razor-sharp insisorts that can gnaw through anything.