Physical characteristics

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-crowned cheek

Coypu (Myocastor coypus) huddled for warmth on feeding platform. (Photo by Bill Goulet. Bruce Coleman, Inc. Reproduced by permission.)

teeth, whereas omnivorous species have low-crowned cheek teeth and well-defined arrangements of cusps. The arrangement of two primary jaw muscles, the masseters and ptery-goideus, allow for duel jaw action whereby chewing can be either transverse or front to back in motion. As noted earlier, the arrangement of these muscles relative to the infraorbital foramen has been used as a major diagnostic feature in many classifications of rodents, with the most primitive arrangement seen in the mountain beaver. Hamsters, pocket gophers, and pocket mice have cheek pouches that allow the animal to collect and transport food.

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-tucos (Ctenomyidae), coruros (Octodontidae), African mole-rats (Bathyergidae), bamboo rats (Muridae, Rhizomyinae), and blind mole-rats (Muridae, Spalacinae) show considerable convergent evolution for a subterranean lifestyle. Most species have a fusiform body with short limbs, small ears, and with eyes either reduced or absent. The tail is generally shorter than the head and body, and the feet are broad. The forefeet of many species have large clawed digits for digging, whereas some species dig with their incisors. Some species of rodents are modified for saltatorial (hopping) locomotion. Kangaroo rats, jerboas, springhares, and gerbils have long tails that serve as a counterbalance during hopping. In addition, the hind feet are generally large and the hind limbs are muscular, whereas the front limbs are shorter. Saltatorial locomotion has evolved several times in rodents and appears to be an adaptation to desert environments with patchily distributed resources. In addition to modifications of the postcranial skeleton, saltatorial rodents tend to have enlarged auditory bullae. Modifications for gliding locomotion can be seen in several species of squirrels (Sci-uridae) and scaly-tailed squirrels (Anomaluridae). 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-like with long hind legs, hoof-like claws, and a digitigrade foot posture (run on digits with heel off the ground).

A black rat (Rattus rattus) with baby rats on straw. (Photo by Hans Reinhard/OKAPIA/Photo Researchers, Inc. Reproduced by permission.)
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