Rodents (from Latin Rodere “to gnaw”) are mammals of the order Rodent, which are characterized by a single pair of continuously growing incisors in each of the Upper and lower jaws. About 40% of all mammal species are rodents (2,277 Species); they are found in vast numbers on all continents except Antarctica. They are the most diversified mammalian order and live in a variety of terrestrial habitats, including human-made environments.
Species can be arboreal, fossorial (burrowing), or semiaquatic. Well-known rodents include mice, rats, squirrels, prairie dogs, chipmunks, porcupines, beavers, guinea pigs, hamsters, gerbils, and capybaras. Other animals such as rabbits, hares, and pikas, whose incisors also grow continually, were once included with them, but are now considered to be in a separate order, the Lagomorph. Nonetheless, Rodent and Lagomorph are sister groups, sharing a most recent common ancestor and forming the clade of Glires. Most rodents are small animals with robust bodies, short limbs, and long tails. They use their sharp incisors to gnaw food, excavate burrows, and defend themselves. Most eat seeds or other plant material, but some have more varied diets.
They tend to be social animals and many species live in societies with complex ways of communicating with each other. Mating among rodents can vary from monogamy to polygyny, to promiscuity. Many have litters of underdeveloped, altricial young, while others are precocial (relatively well developed) at birth. The rodent fossil record dates back to the Paleocene on the supercontinent of Laurasia. Rodents greatly diversified in the Eocene, as they spread across continents, sometimes even crossing oceans. Rodents reached both South America and Madagascar from Africa and were the only terrestrial placental mammals to reach and colonize Australia.
Rodents have been used as food, for clothing, as pets, and as laboratory animals in research. Some species, in particular, the brown rat, the black rat, and the house mouse are serious pests, eating and spoiling food stored by humans, and spreading diseases.