All rodents possess constantly growing rootless incisors that have a hardenamel layer on the front of each tooth and softer dentine behind. The differential wear from gnawing creates perpetually sharp chisel edges. Rodents’ absence of other incisors and canine teeth results in a gap, or diastema, between incisors and cheek teeth, which number from 22 (5 on each side of the upper and lower jaws to 4, may be rooted or rootless and ever-growing, and may be low- or high-crowned. The nature of thejaw articulation ensures that incisors do not meet when food is chewed and that upper and lower cheek teeth, premolars and molars, do not make contact while the animal gnaws. Powerful and intricately dividedmasseter muscles, attached to jaw and skull in different arrangements, provide most of the power for chewing and gnawing.

The range in body size between the mouse,18 grams 0.64-ounce, body 12 cm ,4.7 inches long, and the marmot, 3,000 grams, body 50 cm long, spans the majority of living rodents, but the extremes are remarkable. One of the smallest is Delany’s swamp mouse associated with bamboo in the marshes and mountain forests in Africa. It weighs 5 to 7 grams, and the body is 5 to 6 cm long. The largest is the capybara of Central and South America, which weighs 35 to 66 kg ,77 to 146 pounds and stands 50 to 60 cm at the shoulder, with a body 100 to 135 cm long. Some extinct species were even larger, attaining the size of a black bear or small rhinoceros. The largest rodent ever recorded, lived some two to four million years ago, during the Pleistocene and Pliocene epochs; by some estimates it grew to a length of about 3 metres about 10 feet and weighed nearly 1000 kgs.

Rodents have lived on the planet for at least 56 million years and modern humans for less than one million, but the consequences of their interactions during that short overlap of evolutionary time have been profound. For rodents, early humans were just another predator to avoid, but with Homo Sapiens transition from nomadic hunting and gathering to sedentary agricultural practices, humans became a reliable source of shelter and food for those species having the innate genetic and behavioural abilities to adapt to man-made habitats. The impact of these species upon human populations ranges from inconvenient to deadly. Crops are damaged before harvest; stored food is contaminated by rodent waste; water-impounding structures leak from burrowing; and objects are damaged by gnawing. Certain species are reservoirs for diseases such as plague, murine, typhus, scrub typhus, tularaemia, rat bite fever, rocky mountain spotted fever and Lassa fever, among others. Only a few species are serious pests or vectors of disease, but it is these rodents that are most closely associated with people.

Rodents may be diurnal, nocturnal, or sometimes active part of the day and night. Although some species are herbivorous, diets of most include vegetable and animal matter. Others are opportunistic generalists, and some are specialized predators, not only of arthropods but sometimes of vertebrates. Food is either eaten where gathered or carried to burrows and stored. Species living in arid habitats and on oceanic islands are able to obtain their water requirements from their food. A wide variety of shelters are used or constructed; these range from tree holes, rock crevices, or simple burrows to hidden nests on the forest floor, leaf and stick structures in tree crowns, mounds of cut vegetation built in aquatic environments or complex networks of tunnels and galleries. Rodents may be active all year or enter periods of dormancy or deep hibernation. Breeding time and frequency, length of gestation, and litter size vary widely, but two of the mostprolific are both associated with humans. The brown rat can give birth to litters of up to 22 offspring, and thehouse mouse can produce up to 14 litters annually. Population size may remain stable or fluctuate, and some species, most notably lemmings, migrate when populations become excessively large.

The body form of tree squirrels may be the model for the earliest, and presumably generalized, rodents. With their ability to adhere to bark with their claws, squirrels adeptly scamper up tree trunks, run along branches, and leap to adjacenttrees; but they are equally agile on the ground, and some are capable swimmers. Burrowers are also represented in the form of long-tailed ground squirrels.

The specialized body forms of other kinds of rodents tie them closer to particular locomotor patterns and ecologies. Some strictly arboreal species have a prehensile tail; others glide from tree to tree supported by fur-covered membranes between appendages. Highly specialized fossorial rodents, including blind mole rats, blesmols, and ground squirrels, are cylindrical and furry with protruding, strong incisors, small eyes and ears, and large forefeet bearing powerful digging claws. Semiaquatic rodents such as beavers, muskrats, nutrias, and water rats, possess specialized traits allowing them to forage in aquatic habitats yet den in ground burrows. Terrestrial leaping species, such as kangaroos, rats, jumping mice, gerbils, and jerboas, have short forelimbs, long and powerful hind limbs and feet, and a long tail used for balance. Body forms of some rodents converge on those in nonrodent orders, resembling pigs or deer. There is also convergence between distantly related groups of rodents in particular body forms and associated natural histories.Regardless of body form, all rodents share the same basic tools.