Rodent
Rodents are mammals characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species belong to the order Rodentia. 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, saltatorial/ricochetal, 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, are now considered to be in a separate order, the Lagomorpha, distinguished by an extra pair of incisors. Both 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 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.
Characteristics
The distinguishing feature of the rodents is their pairs of continuously growing, sharpened incisors. Enamel covers the front of the incisor back the back is uncovered. 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, they sharpen the enamel like the blade of a chisel. Rodent species have 12–28 teeth total, usually less than 22, with no canines. A gap, or diastema, is present between the incisors the cheek teeth. This allows rodents to suck in and block out inedible material as the incisors chip them off. Chinchillas and guinea pigs have a high-fiber diet; their molars have no roots and grow continuously like their incisors.The complexly ridged surface of the molars are well equipped for grinding food into small particles. The jaw musculature is strong. The lower jaw is thrust forward while gnawing and shifts backwards during chewing. The incisors do the gnawing while molars chew, however, due to the cranial anatomy of rodents these feeding methods cannot be used at the same time. Among rodents, the masseter muscle plays a key role in chewing, making up 60% – 80% of the total muscle mass among masticatory muscles.
In rodents, masseter muscles are attached 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. Variations in the zygomasseteric system are associated with different specializations in the chewing apparatus. Sciuromorphous rodents, such as the eastern grey squirrel, have a large deep masseter, making them efficient at biting with the incisors. Hystricomorphous rodents, 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. Myomorphous rodents, such as the brown rat, have enlarged temporalis and masseter muscles, making them efficient with both gnawing and chewing.
While the largest species, the capybara, can reach, most species weigh less than. Rodents have wide-ranging morphologies, but typically have stout bodies and short limbs. The forefeet usually have five digits, including an opposable thumb, while the hind feet have three to five digits. The elbow gives the forearms great flexibility. The majority of species are plantigrade, walking on the whole foot and have claw-like nails which vary in size. Rodents have nails on their first digit which they use in food handling. Such a nail combined with dexterous feeding movement with incisors which allow them to eat hard seeds and nuts, a niche that they presently dominate. This thumbnail is argued to be ancestorial with exceptions being linked to its replacement by claws in burrowing and for oral-only feeding.
Rodent species use a wide variety of methods of locomotion including quadrupedal walking, running, burrowing, climbing, bipedal hopping, 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 with 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 mostly bald. Some species have vestigial or non-visible tails. In some species, the tail is capable of regeneration if a part is broken off.
Rodents generally have well-developed senses of smell and hearing and while the eyes are enlarged in species that are active at night. 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, though alternative routes have been found. Many species have cheek pouches for storing food. In squirrels and species of Muroidea, these structures are extensions of the oral cavity while in gophers and the Heteromyidae, they are separate. Both types reach to the shoulders. The digestive system is efficient enough to absorb nearly 80% of the energy in their food. When eating cellulose, the food is softened in the stomach and directed 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. The final fecal pellet is hard and dry. They may 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. The former is typical for ground squirrels, kangaroo rats, solitary mole rats and pocket gophers; it likely developed due to sexual selection with larger males being more reproductively successful. Female-bias sexual dimorphism exists among chipmunks and jumping mice. Its function is not understood, but in the case of yellow-pine chipmunks, males may choose larger females due to their greater fitness. In some species, such as voles, sexual dimorphism can vary from population to population. In bank voles, females are usually the larger sex, but males may be larger in certain alpine populations, possibly because of the lack of predators and greater competition between them.
Distribution and habitat
Rodents have a near global distribution. They are the only terrestrial placental mammals to have colonized Australia and New Guinea without human intervention. Humans have also allowed the animals to spread to many remote oceanic islands. Rodents have adapted to almost every terrestrial habitat, from cold tundra to hot deserts. Species range from arboreal, to fossorial and semiaquatic. Rodents have also thrived in human-created environments such as agricultural and urban areas.File:American Beaver with dam.JPG|thumb|right|Some rodents, like this North American beaver with its dam of gnawed tree trunks and the lake it has created, are considered ecosystem engineers.
Though some species are common pests for humans, rodents also play important ecological roles. Some rodents are considered keystone species and ecosystem engineers in their respective habitats. In the Great Plains of North America, the burrowing activities of prairie dogs play important roles in soil aeration and nutrient redistribution, raising the organic content of the soil and increasing the absorption of water. They maintain these grassland habitats, and some large herbivores such as bison and pronghorn prefer to graze near prairie dog colonies due to the increased nutritional quality of forage.
Extirpation of prairie dogs can also contribute to regional and local biodiversity loss, increased seed depredation, and the establishment and spread of invasive shrubs. Burrowing rodents may eat the fruiting bodies of fungi and spread spores through their feces, thereby allowing the fungi to disperse and form symbiotic relationships with the roots of plants. As such, these rodents may play a role in maintaining healthy forests.
In many temperate regions, beavers play an essential hydrological role. When building their dams and lodges, beavers alter the paths of streams and rivers and allow for the creation of extensive wetland habitats. One study found that engineering by beavers leads to a 33 percent increase in the number of herbaceous plant species in riparian areas. Another study found that beavers increase wild salmon populations. Meanwhile, some rodents are seen as pests, due to their wide range.