section_path: Characteristics
section_name: Characteristics
paragraph_text: Characteristics
  
  The distinguishing feature of the rodents is their pairs of continuously growing, razor-sharp, open-rooted incisors. These incisors have thick layers of enamel on the front and little enamel on the back. Because they do not stop growing, the animal must continue to wear them down so that they do not reach and pierce the skull. As the incisors grind against each other, the softer dentine on the rear of the teeth wears away, leaving the sharp enamel edge shaped like the blade of a chisel. Most species have up to 22 teeth with no canines or anterior premolars. A gap, or diastema, occurs between the incisors and the cheek teeth in most species. This allows rodents to suck in their cheeks or lips to shield their mouth and throat from wood shavings and other inedible material, discarding this waste from the sides of their mouths. Chinchillas and guinea pigs have a high-fiber diet; their molars have no roots and grow continuously like their incisors.
  
  In many species, the molars are relatively large, intricately structured, and highly cusped or ridged. Rodent molars are well equipped to grind food into small particles. The jaw musculature is strong. The lower jaw is thrust forward while gnawing and is pulled backwards during chewing. Gnawing uses incisors and chewing uses molars, however, due to the cranial anatomy of rodents these feeding methods cannot be used at the same time and are considered to be mutually exclusive. Among rodents, the masseter muscle plays a key role in chewing, making up 60% – 80% of the total muscle mass among masticatory muscles and reflects rodents' herbivorous diet. Rodent groups differ in the arrangement of the jaw muscles and associated skull structures, both from other mammals and amongst themselves.
  
  The Sciuromorpha, such as the eastern grey squirrel, have a large deep masseter, making them efficient at biting with the incisors. The Myomorpha, such as the brown rat, have enlarged temporalis and masseter muscles, making them able to chew powerfully with their molars. In rodents, masseter muscles insert behind the eyes and contribute to eye boggling that occurs during gnawing where the quick contraction and relaxation of the muscle causes the eyeballs to move up and down. The Hystricomorpha, such as the guinea pig, have larger superficial masseter muscles and smaller deep masseter muscles than rats or squirrels, possibly making them less efficient at biting with the incisors, but their enlarged internal pterygoid muscles may allow them to move the jaw further sideways when chewing. The cheek pouch is a specific morphological feature used for storing food and is evident in particular subgroups of rodents like kangaroo rats, hamsters, chipmunks and gophers which have two bags that may range from the mouth to the front of the shoulders. True mice and rats do not contain this structure but their cheeks are elastic due to a high degree of musculature and innervation in the region.
  
  While the largest species, the capybara, can weigh as much as 66 kg (146 lb), most rodents weigh less than 100 g (3.5 oz). Rodents have wide-ranging morphologies, but typically have squat bodies and short limbs. The fore limbs usually have five digits, including an opposable thumb, while the hind limbs have three to five digits. The elbow gives the forearms great flexibility. The majority of species are plantigrade, walking on both the palms and soles of their feet, and have claw-like nails. The nails of burrowing species tend to be long and strong, while arboreal rodents have shorter, sharper nails. Rodent species use a wide variety of methods of locomotion including quadrupedal walking, running, burrowing, climbing, bipedal hopping (kangaroo rats and hopping mice), swimming and even gliding. Scaly-tailed squirrels and flying squirrels, although not closely related, can both glide from tree to tree using parachute-like membranes that stretch from the fore to the hind limbs. The agouti is fleet-footed and antelope -like, being digitigrade and having hoof-like nails. The majority of rodents have tails, which can be of many shapes and sizes. Some tails are prehensile, as in the Eurasian harvest mouse, and the fur on the tails can vary from bushy to completely bald. The tail is sometimes used for communication, as when beavers slap their tails on the water surface or house mice rattle their tails to indicate alarm. Some species have vestigial tails or no tails at all. In some species, the tail is capable of regeneration if a part is broken off.
  
  Rodents generally have well-developed senses of smell, hearing, and vision. Nocturnal species often have enlarged eyes and some are sensitive to ultraviolet light. Many species have long, sensitive whiskers or vibrissae for touch or "whisking". Whisker action is mostly driven by the brain stem, which is itself provoked by the cortex. However Legg et al. 1989 find an alternate circuit between the cortex and whiskers through the cerebellar circuits, and Hemelt & Keller 2008 the superior colliculus. Some rodents have cheek pouches, which may be lined with fur. These can be turned inside out for cleaning. In many species, the tongue cannot reach past the incisors. Rodents have efficient digestive systems, absorbing nearly 80% of ingested energy. When eating cellulose, the food is softened in the stomach and passed to the cecum, where bacteria reduce it to its carbohydrate elements. The rodent then practices coprophagy, eating its own fecal pellets, so the nutrients can be absorbed by the gut. Rodents therefore often produce a hard and dry fecal pellet. Horn et al. 2013 makes the finding that rodents entirely lack the ability to vomit. In many species, the penis contains a bone, the baculum; the testes can be located either abdominally or at the groin.
  
  Sexual dimorphism occurs in many rodent species. In some rodents, males are larger than females, while in others the reverse is true. Male-bias sexual dimorphism is typical for ground squirrels, kangaroo rats, solitary mole rats and pocket gophers; it likely developed due to sexual selection and greater male–male combat. Female-bias sexual dimorphism exists among chipmunks and jumping mice. It is not understood why this pattern occurs, but in the case of yellow-pine chipmunks, males may have selected larger females due to their greater reproductive success. In some species, such as voles, sexual dimorphism can vary from population to population. In bank voles, females are typically larger than males, but male-bias sexual dimorphism occurs in alpine populations, possibly because of the lack of predators and greater competition between males.