RODENT

House mouse (Mus musculus) any member of the order Rodentia, the most abundant order of mammals. At present, over a quarter of the families, 35 percent of the genera, and 50 percent of the species of living mammals are rodents. Probably an even higher percentage of individuals are rodents, for they tend to be small animals with dense populations. They are one of the few groups of animals that flourish in close association with men. Some, such as squirrels, live independently but fairly successfully near humans. Others, such as the house mouse (Mus musculus; see photograph) and black and Norway rats (Rattus rattus and R. norvegicus), have adapted themselves to human civilization, and live everywhere that man does. These two rats (and the Polynesian rat, Rattus exulans, of Australia and Oceania) have travelled in ships and boats of all sizes, and have populated the entire habitable world, especially near human habitations. a member of the mammalian order Rodentia, a diverse and abundant group of gnawing animals that includes such families as the squirrels and chipmunks (Sciuridae); Old World rats and mice (Muridae); voles, gerbils, hamsters, lemmings, and deer mice (Cricetidae); beavers (Castoridae); pocket gophers (Geomyidae); guinea pigs (Caviidae); pocket mice and kangaroo rats and mice (Heteromyidae); New World porcupines (Erethizontidae); and Old World porcupines (Hystricidae). Rodents account for half of the species of mammals living today; as individuals, rodents probably constitute more than 50 percent of living mammals. Although the largest living rodent, the South American capybara (Hydrochoerus hydrochoeris), may attain a length of 1.3 m (4 feet) and a weight of 50 kg (110 pounds), rodents are generally among the smallest animals, some mice and dormice reaching only 7.5 cm (3 inches) in length and 20 g (0.7 ounce) in weight. Although rodents appear in diverse forms and adaptations, they share two major characteristics, both relating to their specialization as gnawing animals. The first is an ability to shift the jaws into two positions: when the lower jaw is pulled back, the cheek teeth come into contact to allow the grinding of food; when the jaw is pulled forward and down, the large pairs of upper and lower incisors meet at their tips to allow gnawing. The second specialization has to do with the structure of these incisors. All rodents have only one pair each of upper and lower incisors, and these teeth grow continuously throughout the animal's life. This not only permits but requires the rodent to gnaw frequently on hard substances; if the teeth are not used enough they will grow past each other and eventually cause the animal's death, either by making it incapable of eating or by actually growing back into its skull. Rodents are specialized for many different habitats and lifestyles. Typical rodents scamper on four legs, but there are various modifications in limb structure found in the group. The hind limbs are enlarged for hopping in some groups, such as the kangaroo rats, while in specialized burrowing forms, such as pocket gophers, the forelimbs are short and massive with strong claws. The aquatic beavers have webbed hind feet and a dense coat. Many rodents have taken up an arboreal way of life, and various gliding species also occur. The fossil record shows identifiable rodents in North America in the Late Paleocene Epoch (63.5 to 57.8 million years ago). Diversification was wide and rapid, especially when new territory was colonized. Rodents were the only placental mammals, other than bats, to reach Australia before the arrival of humans. Human activities have helped a few species of rodents to continue to colonize new territories; the rat species have populated the entire habitable world by traveling on boats and ships. Most rodents are herbivorous, although many will eat animal food on occasion. Plant seeds are a preferred food of many species. These may be eaten at once or carried off to be eaten in the burrow or stored for later use. Some rodents prefer berries, grasses, leaves, acorns, buds, fruits, and other types of plant matter. The capybara and the agouti are grazers, as are the holarctic marmots and woodchuck. A few rodents are almost totally predatory. The majority of rodents live in nests dug in the ground, sometimes at the end of complex burrow systems such as those constructed by prairie dogs. Some, like the prairie dog, are communal; others, such as the muskrat, are solitary. Squirrels either nest in holes in trees or build leafy nests among the branches. Beaver lodges in ponds or streams are constructed of branches covered with mud; the entrance is underwater. Many rodents remain in their burrows during bad weather, and some, including the hamster and the woodchuck, undergo true hibernation. Large rodents such as capybaras and beavers breed only once a year, but the smaller species breed more often, and some voles may have up to 13 litters in one year. Some of the smaller species, such as harvest mice and voles, may breed within five to seven weeks of birth. Part of the reason for this high reproductive rate is the fact that small rodents form the staple diet of many mammalian, avian, and reptilian predators, so they must reproduce prolifically to compensate for their low survival rate. Rodents affect humans in several ways. Many species eat agricultural crops, destroying as much as one-third of stored grain during heavy infestations. They can also serve as reservoirs for various diseases that affect humans, such as the bubonic plague and tularemia. Beavers, nutria, and chinchillas are highly valued for their pelts, and much important medical research has been done with the laboratory strains of the house mouse and brown rat. Additional reading J.R. Ellerman, The Families and Genera of Living Rodents, 2 vol. in 1 (1940, reprinted 1966), a scientific catalog of existing rodents; C.S. Elton, Voles, Mice and Lemmings (1942, reprinted 1965), a popular, historical record of plagues of various rodents, followed by an analysis of the lemming population fluctuations in northern Labrador; P.P. Grasse and P.L Dekeyser, Ordre des Rongeurs, in Trait de zoologie, vol. 17, pt. 2, pp. 13211525 (1955), a section from a zoological reference work dealing with the anatomy, ecology, habits, and classification of rodents, with brief descriptions down to the generic level (in French); T.G. Hull, Diseases Transmitted from Animals to Man, 6th ed. (1975), descriptions of diseases, with discussion of causative factors and carriers, many of which are rodents; D. MacClintock, Squirrels of North America (1970), an account of the ecology, habits, and relationships of North American squirrels; H.G.Q. Rowett, The Rat as a Small Mammal, 3rd ed. (1974), a student laboratory manual with detailed notes on the dissection of the rat; M. Shorten, Squirrels (1954), a study of the red and gray squirrels in Great Britain for the general reader; G.D. Snell (ed.), Biology of the Laboratory Mouse, 2nd ed. (1966, reprinted 1975), a standard reference work on all aspects of mouse raising; E.P. Walker et al., Mammals of the World, 3 vol. (1964), descriptions at the generic level of all living mammals, including every recognized genus of rodent, in most cases with pictures; L. Wilsson, My Beaver Colony (1968; originally published in Swedish, 1964), a popular account of beaver behaviour; and S.E. Woods, The Squirrels of Canada (1980), a guide to 22 species. Albert E. Wood Evolution and paleontology Rodents are relatively poorly represented in collections of fossils, in spite of their great abundance at the present time. They have often been overlooked because of their small size, but modern intensive exploration for fossils usually results in a much more abundant representation of small mammals, especially rodents. Paleocene The earliest known rodents come from the late Paleocene (about 57,000,000 years ago) of North America, by which time they had already acquired all of the diagnostic features of the order. The ancestral family, the Paramyidae, was also present in Europe, where it first appeared in the earliest Eocene. There was very rapid diversification of the order during the Eocene, initially involving the Paramyidae, but with other families soon appearing. Most of the Eocene rodents were protrogomorphous, the masseter muscle restricted to its primitive origin on the cheekbone. By the middle Eocene of Europe, rodents with advanced types of masseters were present, and others occurred in the late Eocene of North America. The skeleton of the paramyids was basically that of a generalized scampering animal, approximating a rat in its method of locomotion. Before the end of the Eocene it is probable that both leaping and burrowing variations had arisen, although there is some uncertainty on this point.