They live most their lives underground in elaborate tunnels and emerge only at night mainly to breed. The order Rodentia consists of small to medium sized mammals which share the common characteristic of a single pair of upper and lower incisors that are specialized for gnawing. Rodentia is the largest order of living mammals, containing approximately 1500 species. The skulls of rodents vary in length from .25 inch to over 10 inches long. The tooth count does not exceed 22 except in the Mole Rat which has 28 teeth.
All rodents have incisors that grow continuously throughout their lives.
The molars of many rodents also grow continuously.