Rodents are found in all terrestrial situations, and some spend a major portion of their time in fresh water. They do not live in salt water, and they do not fly. Several species are very well adapted to cohabitation with man, and they have spread with him to all continents and subpolar islands. Some people disparage rats for leaving a sinking ship, as though that wasn't a smart thing to do. This complexity of evolutionary relationship has been enhanced by its broad distribution.
Most species are rather small and have much the same biology.
As a result there has been considerable convergence of body form, physiology, and behavior among otherwise distantly related and distantly distributed groups. The families are listed alphabetically within each morphological pattern. Their common name is derived from the scaled underside of the tail, which is bushy on its dorsal side. There are 12 species in four genera. The head of these animals has a superficial resemblance to that of beavers, but the body is much smaller, and the tail is slender.