There are more species of rodent than of any other animal group both in the world and in North America. Nearly 40 percent of all mammal species are rodents, making them among the most successful animals on earth. They range in size from mice weighing less than ten g to the beaver that can weigh up to 49.5 km.
Rodents have many special features.
All rodents have one pair of upper and lower chisel-shaped 'incisor' teeth that are covered with hard enamel on the front and a softer substance like bone on the back. As the rodent gnaws, the backs of the chisel-shaped teeth wear away faster to maintain a sharp edge. The incisors grow all of the time or continuously. If the rodent does not keep gnawing, the front incisors will grow right out of its mouth and prevent it from eating. The incisors might cause death by growing inwards into the jaw or skull.