Rodents (from Latin rodere, 'to gnaw') are mammals of the order Rodentia, 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. They are native to all major land masses except for New Zealand, Antarctica, and several oceanic islands, though they have subsequently been introduced to most of these land masses by human activity.
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/richochetal (leaping on their hind legs), or semiaquatic. However, all rodents share several morphological features, including having only a single upper and lower pair of ever-growing incisors. Well-known rodents include mice, rats, squirrels, prairie dogs, porcupines, beavers, guinea pigs, and hamsters. Rabbits, hares, and pikas, whose incisors also grow continually (but have two pairs of upper incisors instead of one), were once included with them, but are now considered to be in a separate order, the Lagomorpha. Nonetheless, Rodentia and Lagomorpha are sister groups, sharing a single 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, until the arrival of Homo sapiens, 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. Accidentally introduced species of rodents are often considered to be invasive and have caused the extinction of numerous species, such as island birds, the dodo being an example, previously isolated from land-based predators.
One of the most widespread groups of mammals, rodents can be found on every continent except Antarctica. They are the only terrestrial placental mammals to have colonized Australia and New Guinea without human intervention. Humans have also allowed the animals to spread to many remote oceanic islands (e.g., the Polynesian rat). Rodents have adapted to almost every terrestrial habitat, from cold tundra (where they can live under snow) to hot deserts.
Some species such as tree squirrels and New World porcupines are arboreal, while some, such as gophers, tuco-tucos, and mole rats, live almost completely underground, where they build complex burrow systems. Others dwell on the surface of the ground, but may have a burrow into which they can retreat. Beavers and muskrats are known for being semiaquatic, but the rodent best-adapted for aquatic life is probably the earless water rat from New Guinea. Rodents have also thrived in human-created environments such as agricultural and urban areas.
Though some species are common pests for humans, rodents also play important ecological roles. Some rodents are considered keystone species and ecosystem engineers in their respective habitats. In the Great Plains of North America, the burrowing activities of prairie dogs play important roles in soil aeration and nutrient redistribution, raising the organic content of the soil and increasing the absorption of water. They maintain these grassland habitats, and some large herbivores such as bison and pronghorn prefer to graze near prairie dog colonies due to the increased nutritional quality of forage.
Extirpation of prairie dogs can also contribute to regional and local biodiversity loss, increased seed depredation, and the establishment and spread of invasive shrubs. Burrowing rodents may eat the fruiting bodies of fungi and spread spores through their feces, thereby allowing the fungi to disperse and form symbiotic relationships with the roots of plants (which usually cannot thrive without them). As such, these rodents may play a role in maintaining healthy forests.
In many temperate regions, beavers play an essential hydrological role. When building their dams and lodges, beavers alter the paths of streams and rivers and allow for the creation of extensive wetland habitats. One study found that engineering by beavers leads to a 33 percent increase in the number of herbaceous plant species in riparian areas. Another study found that beavers increase wild salmon populations. Meanwhile, some rodents are seen as pests, due to their wide range.