Temporal range: Early Paleocene – Present, 61.7–0Ma
|Left to right and top to bottom: Springhare (Anomaluromorpha), black rat (Myomorpha), red squirrel (Sciuromorpha), North American beaver (Castorimorpha), and capybara (Hystricomorpha)|
|Combined range of all rodent species|
About 40% of mammal species are rodents, and they are found in vast numbers on all continents other than Antarctica. Common rodents include mice, rats, squirrels, porcupines, beavers, guinea pigs, and hamsters. Rodents use their sharp incisors to gnaw wood, break into food, and bite predators. Most rodents eat seeds or plants, though some have more varied diets. Some species have historically been pests, eating seeds stored by people and spreading disease.
In terms of number of species—although not necessarily in terms of number of organisms (population) or biomass—rodents make up the largest order of mammals. With about 2,277 species of rodents, over 40% of mammalian species belong to the order. Their success is probably due to their small size, short breeding cycle, and ability to gnaw and eat a wide variety of foods.
Rodents are found in vast numbers on all continents except Antarctica, most islands, and in all habitats except oceans. They are the only placental order besides bats and pinnipeds to have reached Australia without human introduction. Members of non-rodent orders, such as Chiroptera (bats), Scandentia (treeshrews), Marsupials (opossums, possums, bandicoots), Soricomorpha (shrews and moles), Lagomorpha (hares, rabbits and pikas), and mustelid carnivores, such as weasels and mink, are sometimes confused with rodents.
Nearly all rodents share the characteristic of dentition highly specialized for gnawing. This specialization gives rodents their name from the Latin, rodere, to gnaw. All rodents have a single pair of upper and a single pair of lower incisors, followed by a gap (diastema), and then one or more molars or premolars; Paucidentomys vermidax is unique among the rodents in that it possesses no molars or premolars, and its incisors are so specialised, they are not for gnawing.
Typical rodent incisors grow continuously and must be kept worn down by gnawing. Their anterior and lateral surfaces are covered with enamel, but the posterior surface is exposed dentine. During gnawing, the incisors grind against each other, wearing away the softer dentine, leaving the enamel edge like the blade of a chisel. This ‘self-sharpening’ system is very effective and is one of the keys to the enormous success of rodents. Rodents lack canines, and have a diastema between their incisors and premolars. Their incisors are highly versatile and can be used for a range of functions, such as cutting wood, biting through the skin of fruit, prey capture, or defense, depending on the species. Nearly all rodents feed on plants, seeds in particular, but a number of species eat insects (grasshopper mouse, Onychomys leucogaster) or fish (fish-eating rats, Ichthyomyini). Some squirrels are known to eat passerine birds, such as cardinals and blue jays. One species, Paucidentomys vermidax, feeds primarily on worms and lacks the ability to gnaw or even chew, possessing bladelike, forked upper incisors and no molars.
Many rodents are small; the tiny African pygmy mouse, Mus minutoides, can be as small as 6 cm (2.4 in) in length and 7 g (0.25 oz) in weight at maturity, and the Baluchistan pygmy jerboa, Salpingotulus michaelis, is of roughly similar or slightly smaller dimensions. Conversely, the largest extant rodent, the capybara, Hydrochoerus hydrochaeris, usually weighs up to 65 kg (143 lb), with exceptional specimens weighing up to 91 kg (201 lb). Several enormous rodents are known from the fossil record, the largest known being Josephoartigasia monesi, which is estimated to have typically weighed about 1,000 kg (2,200 lb), and possibly up to 1,534 kg (3,382 lb) or 2,586 kg (5,701 lb) in large individuals.
Rodents are important in many ecosystems because they reproduce rapidly, and can function as food sources for predators, mechanisms for seed dispersal, and disease vectors. Humans use rodents as a source of fur, as pets, as model organisms in animal testing, for food, and even for detecting land mines. Due to the wide diversity of their characteristics, some of which are considered uncommon or unique amongst mammals, rodents are used widely in research. For example, the naked mole rat, Heterocephalus glaber, is the only known mammal that is poikilothermic and also does not produce the neurotransmitter substance P; it is therefore used in studies on thermoregulation and pain.
The fossil record of rodent-like mammals begins shortly after the extinction of the non-avian dinosaurs 66 million years ago, as early as the Paleocene. Some molecular clock data, however, suggest modern rodents (members of the order Rodentia) had appeared in the late Cretaceous, although other molecular divergence estimations are in agreement with the fossil record. By the end of the Eocene epoch, relatives of beavers, dormice, squirrels, and other groups appeared in the fossil record. They originated in Laurasia, the supercontinent composed of today's North America, Europe, and Asia. Some species colonized Africa, giving rise to the earliest hystricognaths. From Africa, hystricognaths rafted to South America, an isolated continent during the Oligocene and Miocene epochs. By the Miocene, Africa collided with Asia, allowing rodents, such as porcupines, to spread into Eurasia. During the Pliocene, rodent fossils appeared in Australia. Although marsupials are the most prominent mammals in Australia, rodents now make up almost 25% of the continent's mammal species. Meanwhile, the Americas became joined by the Isthmus of Panama, and some rodents participated in the resulting Great American Interchange; sigmodontines surged southward and caviomorphs headed north.
The rodents are part of the clades Glires (along with lagomorphs), Euarchontoglires (along with lagomorphs, primates, treeshrews, and colugos), and Boreoeutheria (along with most other placental mammals). The order Rodentia may be divided into suborders, infraorders, superfamilies and families.
Order Rodentia (from Latin, rodere, to gnaw)
The above taxonomy uses the shape of the lower jaw (sciurognath or hystricognath) as the primary character. This is the most commonly used approach for dividing the order into suborders. Many older references emphasize the zygomasseteric system (suborders Protrogomorpha, Sciuromorpha, Hystricomorpha, and Myomorpha).
Several molecular phylogenetic studies have used gene sequences to determine the relationships among rodents, but these studies have yet to produce a single, consistent and well-supported taxonomy. Some clades have been consistently produced, such as:
In 1991, a paper published by Nature proposed that caviomorphs should be reclassified as a separate order (similar to Lagomorpha), based on an analysis of the amino acid sequences of Guinea pig proteins. This hypothesis was refined in a 1992 paper, which asserted the possibility that caviomorphs may have diverged from myomorphs prior to later divergences of Myomorpha; this would mean caviomorphs, or possibly hystricomorphs, would be moved out of the rodent classification into a separate order. A minority scientific opinion exists that argues that guinea pigs, degus, and other caviomorphs are not rodents, while several papers were put forward in support of rodent monophyly. Subsequent studies published since 2002, using wider taxon and gene samples, have restored a majority opinion among mammalian biologists that the order Rodentia is monophyletic, although there is not a complete consensus.
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