Rodents are gnawing mammals, and as such, all species share several features in common. The number of teeth in rodents seldom exceeds 22. The canine teeth are absent, and all rodents possess a single pair of upper and lower incisors characterized by enamel on the anterior surface and dentine on the posterior surface, allowing for differential wear of the teeth to maintain sharpness.
A diastema or gap separates the incisors from premolars and molars, and this separation facilitates both gnawing with the incisors and grinding with the cheek teeth. Size and cusp patterns of cheek teeth in rodents relate to diet. Herbivorous rodents have high-
Rodents demonstrate considerable variation in size, ranging from a length of 4.7 in (12 cm) and weight of 0.1 oz (4 g) for the pygmy mouse (Baiomys taylori) to 39.4 in (100 cm) and 10 lb (50 kg) for the capybara (Hydrochaeris hydrochaeris). Extinct lineages of the family Dinomyidae reached 882–1323 lb (400–600 kg) in size. In terms of their postcranial skeleton, unspecialized rodents have five digits, short limbs, a long tail, and a plantigrade foot posture (walk with the soles of feet on the ground). More specialized species tend to show considerable modification associated with their particular mode of locomotion and general lifestyle.
Many unrelated fossorial species like pocket gophers, tuco-
Gliders have modified membranes extending along the sides of the body and attached to the front and hind feet. The tail is generally well developed and is used as a rudder during the glide as well as a brace when landing. Several species of rodents reveal specializations for living in aquatic environments. These species have webbed feet and tails modified for swimming. The beaver’s tail is flattened dorsoventrally, whereas the nutria and muskrat have tails that are laterally compressed. Tree squirrels show specializations for an arboreal lifestyle, revealing sharp claws on the digits and a modification of the bones in ankles that allows for the hind foot to be rotated as the animal descends from a tree head first. Some species of rodents are highly cursorial (fast running) and have highly modified limbs and feet. The mara, a fast running species that lives in steppe region of Patagonia and Argentina, appears rabbit-