Rodents (from Latin ) are a group of mammals belonging to the order Rodentia ( or ) characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. Rodents make up about 40% of all mammal species. Rodents are native to all major landmasses except Antarctica and a handful of isolated oceanic islands — though humans have since introduced them to nearly all of these places too. 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 plant material, but some have more varied diets.
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. 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. Rodentia and Lagomorpha (rabbits, hares, and pikas) are sister groups, sharing a single common ancestor and forming the clade of Glires. Lagomorphs also have incisors that grow continuously, but are distinguished by an extra pair of incisors on the upper jaw.
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. Well-known rodents include mice, rats, squirrels, prairie dogs, porcupines, beavers, guinea pigs, and hamsters.
Characteristics The distinguishing feature of the rodents is their pairs of continuously growing, sharpened incisors. Enamel covers the front of the incisor, but the back is uncovered. Because they do not stop growing, the animal must continuously wear them down by gnawing. Otherwise, they can become so long that they pierce the skull. As the incisors grind against each other, the softer dentine on the rear of the teeth wears away more quickly, leaving the sharp enamel edge shaped like the blade of a chisel. Rodent species have 12–28 teeth total, usually fewer than 22, with no canines. A gap, or diastema, is present between the incisors and the cheek teeth. This allows rodents to suck in and block out inedible material as the incisors chip it off. Chinchillas and guinea pigs have a high-fiber diet; their molars lack roots and grow continuously like their incisors.
The complexly ridged surface of the rodent molar is well-equipped for grinding food into small particles. The jaw musculature is strong. The lower jaw is thrust forward while gnawing and shifts backward 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 muscles cause 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. However, 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 that 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 allows them to eat hard seeds and nuts, a niche that they presently dominate. This thumbnail is argued to be ancestral 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 (kangaroo rats and hopping mice), swimming, and even gliding. Flying squirrels can 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 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 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, and 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, inhabiting every continent except Antarctica. 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 (under snow) to hot deserts. Species range from arboreal (tree-dwelling) to fossorial (underground) and semiaquatic. Rodents have also thrived in human-created environments such as agricultural and urban areas.
Though some species are common pests for humans, rodents also play important ecological roles. 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 (which usually cannot thrive without them). As such, these rodents may help preserve healthy forests. 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 an important role in grassland habitats, contributing to soil aeration, which leads to more organic material and water absorption. Similarly 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 wetland habitats.
Behavior and life history
Feeding
Most rodents are primarily herbivorous, acting only opportunistically as predators; others maintain a more balanced omnivorous diet, while a few are primarily carnivorous and act as specialized predators. The field vole is a typical herbivorous rodent and feeds on grasses, herbs, root tubers, moss, and other vegetation, and gnaws on bark during the winter. It occasionally eats invertebrates such as insect larvae. The plains pocket gopher eats grasses, roots, and tubers in its cheek pouches and caches them in underground larder chambers. The Texas pocket gopher avoids emerging onto the surface to feed by pulling plants above them into their burrows by the roots. It also practices coprophagy. The African pouched rat forages on the surface, gathering anything that might be edible into its capacious cheek pouches until they fully stretch. It then returns to its burrow to sort through the material it has gathered and eats the nutritious items.
Agouti species are one of the few animal groups that can break open the large capsules of the Brazil nut fruit. Too many seeds are inside the fruit to be consumed in one meal, so the agouti carries some off and caches them. This helps dispersal of the seeds, as any that the agouti fails to retrieve are distant from the parent tree when they germinate. Desert-dwelling seed eaters like kangaroo rats must gather as many as they can since they are only available for a limited time. Some rodents eat as much as possible to store fat reserves for the long winter hibernation. Marmots do this, and may be 50% heavier in the autumn than in the spring. Beavers, which feed on tree bark and vegetation, store their winter food in "rafts", piles of wood immersed in water.