Rodent

Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing. Forty percent of mammal species are rodents, and they are found in vast numbers on all continents other than Antarctica. Common rodents include mice, rats, squirrels, gerbils, porcupines, beavers, chipmunks, guinea pigs, and voles. Rodents have sharp incisors that they use to gnaw wood, break into food, and bite predators. Most eat seeds or plants, though some have more varied diets. Some species have historically been pests, eating seeds stored by people and spreading disease. The name comes from the Latin word rodens, "gnawing one" (from the verb rodere, "gnaw").

Size and range of order
In terms of number of species—although not necessarily in terms of number of organisms (population) or biomass—rodents make up the largest order of mammals. There are about 2,277 species of rodents (Wilson and Reeder, 2005), with over 40 percent of mammalian species belonging to the order. Their success is probably due to their small size, short breeding cycle, and ability to gnaw and eat a wide variety of foods. (Lambert, 2000)

Rodents are found in vast numbers on all continents except Antarctica, most islands, and in all habitats except oceans. They are the only non-volant, non-marine placental order—and in particular are the only placental order besides bats (Chiroptera) and Pinnipeds—to have reached Australia without human introduction.

Characteristics
Many rodents are small; the tiny African pygmy mouse can be as little as 6 cm (2.4 in) in length and 7 g (0.25 oz) in weight at maturity, and the Baluchistan Pygmy Jerboa is of roughly similar or slightly smaller dimensions. On the other hand, the capybara can weigh up to 80 kg (180 lb), and the largest known rodent, the extinct Josephoartigasia monesi, is estimated to have weighed about 1,000 kg (2,200 lb), and possibly up to 1,534 kg (3,380 lb) or 2,586 kg (5,700 lb).

Rodents have two incisors in the upper as well as in the lower jaw which grow continuously and must be kept worn down by gnawing; this is the origin of the name, from the Latin rodere, to gnaw. These teeth are used for cutting wood, biting through the skin of fruit, or for defense. The teeth have enamel on the outside and exposed dentine on the inside, so they self-sharpen during gnawing. Rodents lack canines, and have a space (diastema) between their incisors and premolars. Nearly all rodents feed on plants, seeds in particular, but there are a few exceptions which eat insects or fish. Some squirrels are known to eat passerine birds like cardinals and blue jays.