Rodent

Rodents (from Latin rodere, 'to gnaw') are mammals of the order Rodentia (/roʊˈdɛnʃə/ roh-DEN-shə), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are native to all major land masses except for Antarctica, and several oceanic islands, though they have subsequently been introduced to most of these land masses by human activity.

Rodents are extremely diverse in their ecology and lifestyles and can be found in almost every terrestrial habitat, including human-made environments. Species can be arboreal, fossorial (burrowing), saltatorial/ricochetal (leaping on their hind legs), or semiaquatic. However, all rodents share several morphological features, including having only a single upper and lower pair of ever-growing incisors. Well-known rodents include mice, rats, squirrels, prairie dogs, porcupines, beavers, guinea pigs, and hamsters. Once included with rodents, rabbits, hares, and pikas, which also have incisors that grow continuously (but have two pairs of upper incisors instead of one), are now considered to be in a separate order, the Lagomorpha. Nonetheless, Rodentia and Lagomorpha are sister groups, sharing a single common ancestor and forming the clade of Glires.

Most rodents are small animals with robust bodies, short limbs, and long tails. They use their sharp incisors to gnaw food, excavate burrows, and defend themselves. Most eat seeds or other plant material, but some have more varied diets. They tend to be social animals and many species live in societies with complex ways of communicating with each other. Mating among rodents can vary from monogamy, to polygyny, to promiscuity. Many have litters of underdeveloped, altricial young, while others are precocial (relatively well developed) at birth.

The rodent fossil record dates back to the Paleocene of Asia. Rodents greatly diversified in the Eocene, as they spread across continents, sometimes even crossing oceans. Rodents reached both South America and Madagascar from Africa and, until the arrival of Homo sapiens, were the only terrestrial placental mammals to reach and colonize Australia.

Rodents have been used as food, for clothing, as pets, and as laboratory animals in research. Some species, in particular, the brown rat, the black rat, and the house mouse, are serious pests, eating and spoiling food stored by humans and spreading diseases. Accidentally introduced species of rodents are often considered to be invasive and have caused the extinction of numerous species, such as island birds, the dodo being an example, previously isolated from land-based predators.

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.^{[citation needed]} 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.