Rodents

Rodents are small to medium-size mammals (5 grams to 65 kilograms), with one pair of evergrowing, rootless upper and lower incisors. Most are herbivorous, though some are partly or exclusively insectivorous, or partly carnivorous. There are more than 2,100 living species (sorted into more than 460 genera in 27 families) in the Order Rodentia, and they constitute nearly half of the approximately 5,000 species of living mammals. Rodents are indigenous to every land area except Antarctica, New Zealand, and a few Arctic and oceanic islands, and some species have been introduced even into those places through their association with humans. Rodents are also the only terrestrial placental mammals occurring naturally in Australia and New Guinea.

Significant interactions occur between humans and species of rodents. Humans utilize rodents as a source of food through hunting and husbandry, for coats derived from their fur, as subjects used in biomedical and genetic research, and for pleasure in the form of household pets and zoo exhibits. Because living rodents form such a large group of species, and occur in so many kinds of habitats (from desert to tundra, from subterranean burrows to forest canopies) over so much of the planet's surface, study of their species-diversity, biology, geographic distributions, and evolutionary history are pragmatically and intellectually satisfying to researchers in a wide range of disciplines.

At some point in their evolutionary history, humans experienced a transition in resource use from nomadic hunting and gathering to sedentary agricultural practices. Through this transition humans inadvertently became, and remain, a reliable source of shelter and food for some rodents, primarily those species with the innate genetic and behavioral adaptive abilities to exploit resources available in anthropogenic habitats. The impact of these commensal species upon human populations is usually not benign. Some crops are damaged before harvest; stored grains are eaten by rodents, and what is not consumed is contaminated by their excrement. Water-impounding earthen dikes and dams leak and may even fail because of burrowing by some species, and objects (from water pipes to electrical wiring) are damaged by their gnawing. Certain species are natural reservoirs for disease, and these may be, or have been, transmitted to humans by arthropods, resulting in sickness or death.

As documented by fossils, the evolutionary history of rodents extends back 56 million years (to the Late Paleocene) in North America and Asia, and the actual origin of the group is probably even older. Approximately 26 extinct families have been described, containing more than 220 genera. The closest living relatives of rodents may be the lago-morphs (rabbits, hares, and pikas), and both groups may have evolved from a common ancestor. Some of the earliest lagomorphs represented by fossils are difficult to distinguish from primitive rodents.

Throughout their evolutionary history, rodents have been successful and significant members of terrestrial faunas. They are diverse in number of species and morphologies that represent different evolutionary lines of descent, and they provide stunning examples of parallel evolution (similar structures evolv ing in different evolutionary groups). Because of this history, the classification of rodents is a challenge to zoologists. Rodent specialists agree with definitions of most families but disagree about the grouping of families into suborders. Past classifications have either omitted suborders altogether and arranged the families into superfamilies, or into from two to sixteen suborders.

The most recent classification, and the one followed here, arranges families into five suborders. It is loosely based upon a combination of the classical arrangements of the jaw and associated musculature (protrogomor-phous, sciuromorphous, hystricomorphous, or myomorphous), histologic structure of incisor enamel layers, comparative anatomy of the head and postcranial skeletons and different organ systems, embryonic development of extra embryonic fetal membranes, and analyses of mitochondrial and nuclear DNA sequences. These five suborders represent a hypothesis of relationships among rodent families, and this classification may be modified by the discovery of new fossils, reanalyses of published data, and new analyses of data from new sources, most probably derived from rodent genomes. The five suborders, their contained families, and general native distributions are outlined below.

Suborder Sciuromorpha: Aplodontidae (mountain beaver; western North America), Sciuridae (tree squirrels, ground squirrels, flying squirrels, marmots, chipmunks, and prairie dogs; worldwide), Gliridae (dormice; Eurasia and Africa), and Castoridae (beavers; North America and Europe). Suborder Myomor-pha: Dipodidae (birch mice, jumping mice, and jerboas; Eurasia, North Africa, and North America), Muridae (rats, mice, hamsters, voles, lemmings, muskrats, gerbils, zokors, blind mole rats, bamboo rats, and African mole rats; worldwide), Geomyidae (pocket gophers;

North and Central America, northern South America), and Heteromyidae (forest spiny mice, pocket mice, kangaroo rats and mice; North and Central America, northern South America). Suborder Anomaluromorpha: Pedetidae (spring hare; sub-Saharan Africa) and Anomaluridae (anomalures; sub-Saha-ran Africa). Suborder Sciuravida: Cten-odactylidae (gundis; North Africa). Suborder Hystricognatha: Hystricidae (African and Asian porcupines), Erethizontidae (North American, prehensile-tailed, stump-tailed, hairy dwarf, and thin-spined porcupines; North, Central, and South America), Petro-muridae (dassie rat; southern Africa), Thry-onomyidae (cane rats; sub-Saharan Africa), Bathyergidae (blesmols; sub-Saharan Africa), Dasyproctidae (agoutis and acouchys; Central and South America), Agoutidae (pacas; Central and South America), Dinomyidae (pacarana; South America), Caviidae (guinea pigs, cavies, and maras; South America), Hydrochoeridae (capybara; South America), Octodontidae (rock rats, degus, viscacha rat, plains viscacha rat, coruro, and tuco-tucos; South America), Echimyidae (American spiny rats; South America), Myocastoridae (nutria; South America), Capromyidae (hutias; West Indies), Chinchillidae (plains viscacha and chinchillas; South America), and Abrocomi-dae (chinchilla rats; South America).

All rodents have an upper and a lower pair of persistently growing, rootless incisors, with hard enamel layers on the front surfaces of each tooth and softer dentine behind. The differential wear from gnawing creates perpetually sharp, beveled, and chisel-like edges. Between incisors and cheek teeth is a long gap (diastema) devoid of other incisors and canines. The number of cheek teeth (premolars and molars) ranges from twenty-two (two premolars and three molars on each side of the upper jaws, and one premolar and three molars on each side of the lower jaws) to four (one molar in each quadrant of the jaw); they may be rooted or rootless and ever-growing, and low or high-crowned.

The configuration of the jaw articulation ensures that incisors do not meet when food is chewed, and that the cheek teeth do not occlude while the animal gnaws with its incisors. Powerful and complexly divided mas-seter muscles, attached to jaw and cranium in different arrangements, provide most of the power for chewing and gnawing. Incisors and cheek teeth perform different functions. The incisors cut, pry, slice, gouge, dig, stab, or delicately hold items like a pair of tweezers, and they can cut grass, open nuts, kill animal prey, excavate burrows, and fell large trees. The cheek teeth masticate (chew) the food obtained by the incisors. Chewing is crushing and grinding the food and involves the transverse and front-to-back movement of the cheek teeth in the lower jaw against the upper

An American groundhog (USDA Forest Service)

cheek teeth. This movement is possible because of a loose articulation between the lower jaw and cranium.

Except for saltatorial (jumping), gliding, and fossorial species, the postcranial skeleton of rodents is relatively unspecialized. The radius and ulna (bones of the lower arm) are unfused; the elbow joint permits free movement of the forearm; and front feet have four digits in most species, with a thumb that is vestigial or reduced in size. Tibia and fibula (bones of the lower leg) are usually fused near the ankle, and the hind feet have three to five digits. Some species have internal cheek pouches (chipmunks) or external, fur-lined cheek pouches (kangaroo rats and mice, pocket gophers) that open near the angle of the mouth. Rodents exhibit a wide range of stomach morphology, from a simple sac to a complex structure resembling that of ungulate ruminants. They have a relatively unspecial-ized brain. There is usually a baculum in the penis, and testes are either inguinal or abdominal in position. Ears range in size from slight ridges to large flaps; eyes are minute and covered with transparent tissue in some species but large in most. Tails may be rudimentary (mountain beaver and many fossorial species) or very long relative to the length of the head and body (most squirrels and arboreal species in other families); the tail is about as long as, or only slightly longer than, the head and body in most rodents.

The range in body size between the house mouse (15 grams, body 10 cm long) and the woodchuck (6,000 grams, body 50 cm long) brackets most species of living rodents. At one extreme is the Old World harvest mouse (Micromys minutus), one of the smallest rodents, weighing 5 to 8 grams, with a body up to 8 cm long (the length of a person's little finger). The largest living rodent is the capybara (Hydrochaeris hydrochaeris) of Central and

South America, which weighs 35 to 65 kg and has a body 100 to 135 cm long, with a shoulder height up to 60 cm. Some extinct species were even larger, attaining the size of a black bear (the giant beaver, Castoroides, from Pliocene-Pleistocene sediments of North America) or small rhinoceros (Telicomys, related to the living South American pacarana, Dinomys, and found in Late Miocene sediments of Argentina).

Rodents may be active during the day (diurnal), only at night (nocturnal), or sometimes part of the day and night. The popular conception is that rodents are strictly herbivorous, and that is true for some species, but diets of most include vegetative and reproductive parts of plants, fungi, invertebrates, and vertebrates. Some species are opportunistic generalists; others are specialized predators of arthropods and vertebrates. Food is either eaten where gathered or carried to burrows and stored. Species living in arid habitats and on oceanic islands are able to obtain their water requirements from their food. Shelters may consist of a simple nest on the forest floor beneath cover, in tree holes, in leaf and stick structures in tree crowns, in rock crevices, or in mounds of cut vegetation built in aquatic environments or burrows. Rodents may be active all year, or enter periods of dormancy or deep hibernation. Breeding time and frequency, length of gestation, and litter size vary widely. Population size may remain stable or fluctuate, and some species migrate when populations become excessively large.

The body form of tree squirrels (Sciuridae) may be similar to that of the earliest (Paleocene and Early Eocene) and presumably generalized, rodents (species of extinct North American Paramys, for example). By adhering to bark with their claws, tree squirrels can adeptly scamper up tree trunks, run along branches, and leap to adjacent trees; they nest in tree holes or in stick nests constructed in the crowns (characteristics of an arboreal manner of life). Other arboreal squirrels nest and forage in trees but glide from one tree to the next (flying squirrels). Many species are equally agile on the ground, and shelter in burrow systems that they excavate (terrestrial and semifossorial ground squirrels, prairie dogs, marmots), and some are capable swimmers (occasionally amphibious). A few species utilize burrows for nesting but forage in trees (long-tailed ground squirrels).

The specialized body forms of other rodent species signal particular locomotor patterns and ecologies. Some strictly arboreal species have a prehensile tail (South American porcupines, for example); others glide from tree to tree, supported by fur-covered membranes between their extended appendages (flying squirrels and anomalures). Highly specialized fossorial rodents are basically furry cylinders with protruding, strong incisors, small eyes (sometimes not discernible externally) and ears (represented by only a low ridge in some species), and expansive front feet bearing powerful digging claws (zokors, blind mole rats, African mole rats, pocket gophers, bamboo rats, blesmols). Amphibious rodents possess specialized traits (such as thick and waterproof fur, webbing between digits, side-to-side flattened tail), allowing them to forage in aquatic habitats but den in ground burrows or stick houses (muskrat and beaver are the most familiar). Terrestrial, leaping species have short forelimbs, long and powerful hind limbs and feet, and a long tail used for balance (jerboas, kangaroo rats and mice). The body forms of some rodents converge on those in nonrodent orders, resembling shrews (Indo-Australian species of shrew mice), hares (viscachas), pikas (gundis), small pigs (pacas), miniature hippopotamus (capy-

baras), and small-bodied forest deer (acouchys and agoutis).

—Mary Ellen Holden

See also: Biogeography; Deserts and Semiarid Scrublands; Evolutionary Biodiversity; Herbivory; Lago-morpha; Mammalia; Tropical Rain Forests

Bibliography:

Carleton, Michael D. 1984. "Introduction to Rodents." In Orders and Families of Recent Mammals of the World, edited by Sydney Anderson and J. Knox Jones, Jr., pp. 255-265. New York: John Wiley and Sons; Carroll, Robert. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman; McKenna, Malcolm C., and Susan K. Bell. 1997. Classification of Mammals above the Species Level. New York: Columbia University Press; Nowak, Ronald M. 1999. Walker's Mammals of the World, 6th ed. Vol. 2. Baltimore: Johns Hopkins University Press; Vaughan, Terry A., James M. Ryan, and Nicholas J. Czaplewski. 2000. Mammalogy, 4th ed. Orlando: Harcourt.

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