Rodents (from Latin rodens 'gnawing') are a group of mammals belonging to the order Rodentia ( roh-DEN-shə or roh-DEN-chə) characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. Rodents make up about 40% of all mammal species. They are native to all major landmasses except Antarctica and several oceanic islands, though they have subsequently been introduced to most of these landmasses by human activity. 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 plant material, but some have more varied diets. Rodents are extremely diverse in their ecology and lifestyles and can be found in almost every terrestrial habitat, including human-made environments. Species can be arboreal, fossorial (burrowing), saltatorial/ricochetal (leaping on their hind legs), or semiaquatic. 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 of Asia. 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, until the arrival of Homo sapiens, were the only terrestrial placental mammals to reach and colonize Australia. Rodentia and Lagomorpha (rabbits, hares, and pikas) are sister groups, sharing a single common ancestor and forming the clade of Glires. Lagomorphs also have incisors that grow continuously, but are distinguished by an extra pair of incisors on the upper jaw. 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. Accidentally introduced species of rodents are often considered to be invasive. Well-known rodents include mice, rats, squirrels, prairie dogs, porcupines, beavers, guinea pigs, and hamsters.
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