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With over 2000 living species placed in about 30 families, rodents are by far the largest order of mammals, at least in terms of number of taxa (well over 40% of mammalian species belong to the order Rodentia!). Rodents range in size from pygmy mice weighing 5 gms to capybaras, the largest of which weigh over 70 kg. They are found around the world except in Antarctica, New Zealand, and on some oceanic islands. Ecologically, they are incredibly diverse. Some species spend their entire lives above the ground in the canopy of rainforests; others seldom emerge from beneath the ground. Some species are highly aquatic, while others are equally specialized for life in deserts. Many are to some degree omnivorous; others are highly specialized, eating, for example, only a few species of invertebrates or fungi.
Despite their morphological and ecological diversity, all rodents share one characteristic: their dentition is highly specialized for gnawing. All rodents have a single pair of upper and a single pair of lower incisors, followed by a gap (diastema), followed by one or more molars or premolars. No rodent has more than one incisor in each quadrant, and no rodent has canines. Rodent incisors are rootless, growing continuously. Their anterior and lateral surfaces are covered with enamel, but their posterior surface is not. During gnawing, as the incisors grind against each other, they wear away the softer dentine, leaving the enamel edge as the blade of a chisel. This "self sharpening" system is very effective and is one of the keys to the enormous success of rodents. The condition of a dominant pair of incisors used for gnawing, followed by a long diastema, is not unique to rodents, and in fact rodents are relative latecomers to this condition (even though as a group, they have a very old fossil history, going back to Paleocene times). It is even seen in a group of therapsids (ancestors of mammals), the tritylodonts, which lived during the Jurassic. Multituberculates, a very large and successful but now extinct group of early mammals, had a similar pattern. So do wombats, hyraxes, aye-ayes, and lagomorphs, to give a few examples chosen from modern mammals. Rodents have specialized in gnawing to an extreme, however, seen in few or no other groups of vertebrates.
The main muscle used in chewing by rodents is the masseter, and the rodents can be divided into several groups based on exactly how they use these muscles. These groupings, and their use in classifying rodents, are discussed on an accompanying page. Other characteristics of rodents include a well-developed pterygoid region of the skull (the pterygoid muscles, which originate in this region, participate in the movement of the lower jaw), an elongate glenoid fossa with no postglenoid process (permitting the lower jaw to move forwards and backwards), complete zygomatic arch with the midportion spanned by the jugal, large paroccipital processes, alisphenoid canal present (but sometimes short and hard to see), clavicles usually present, feet primitively 5-toed (the pollex often very small), digits 2-5 with claws, pollex and hallux sometimes with a nail, reduced temporalis muscle, and many others.
Suborder Sciurognathi Family Aplodontidae Family Sciuridae Family Castoridae Family Geomyidae Family Heteromyidae Family Dipodidae Family Muridae Family Anomaluridae Family Pedetidae Family Ctenodactylidae Family Myoxidae
Suborder Hystricognathi Family Bathyergidae Family Hystricidae Family Petromuridae Family Thryonomyidae Family Erethizontidae Family Chinchillidae Family Dinomyidae Family Caviidae Family Hydrochaeridae Family Dasyproctidae Family Agoutidae Family Ctenomyidae Family Octodontidae Family Abrocomidae Family Echimyidae Family Capromyidae Family Heptaxodontidae Family Myocastoridae
Murids include most of the familiar rats and mice, but the family also encompasses an enormously diverse array of other rodents. Here, we follow recent authorities in treating murids as members of a single, very large family with a number of subfamilies. The systematic relationships of these groups among themselves and with other rodents has proved to be an extremely difficult problem, one that is by no means resolved. A number of characters link most murids. Not surprisingly in such an old and diverse group, even the most basic characters have been subject to continuing evolutionary change; most of the characters listed as diagnostic in the next paragraph do in fact show some variation within the group. All, however, are believed to have characterized primitive murids. Members of this family range from a few grams to a kilogram or more in weight. Most have relatively large eyes. External ears vary from almost absent in, for example, diggers like mole-rats (Spalax), to moderately large (e.g., deer mice, long-eared mice).
Some species, especially fossorial murids, have relatively short and powerful legs; others are longer but no murids are really cursorial. The feet of some are relatively small; those of swimmers like muskrats are large and almost fully webbed; others (for example, many climbers) are broad and strongly grasping. Four clawed digits are found on each forefoot (the pollex or "thumb" is small and bears a nail); the hind foot in most has five clawed digits (but sometimes the hallux or first toe has a nail). Tails range from nearly absent in Siberian hamsters to much longer than the head and body in some tropical climbing rats. In many species the tail are nearly naked as in Norway rats; in others they are well furred; and in some, for example Philippine cloud rats, they are bushy. The pelage is also highly variable, in consistency (from densely wooly to spiny), pattern, and color. In the skull, the infraorbital foramen, which primitively transmits nerves to the rostral region of the skull, lies mostly above the zygomatic plate. It is enlarged above for the passage of a slip of muscle, the medial masseter (masseter medialis) that originates on the side of the rostrum and inserts on the lower jaw.
Below, the foramen is narrowed, but it nevertheless houses nerves and blood vessels en route to the rostrum. The foramen thus has a distinctive, "keyhole" shape in most forms (but the narrow ventral portion is lost in a few species). The zygomatic plate, formed by the anterior base of the zygomatic arch, is broad and a conspicuous feature of the cranium. It serves as attachment for another branch of the masseter, the lateral (masseter lateralis). This arrangement of the masseter, with one branch passing through the infraorbital foramen and and another originating on the zygomatic plate, is termed myomorphous. The jugal, one of the bones that participates in the zygomatic arch, is small and does not contact the lacrimal. The frontals are constricted above the orbits and there is no postorbital process or bar. Posteriorly, an interparietal bone is present and usually conspicuous. The lower jaw is sciurognathus. As in all rodents, one upper and one lower incisor are always found on each side of the jaw, and canines are always absent. Follow the incisor is a diastema. Premolars are never present, and no more than three cheekteeth occur on each side (but this number is sometimes reduced to two or even one). The nature of the molars (shape, size, surface structure, number of roots) varies greatly. Members of the family can be found on all continents except Antarctica and on many oceanic islands.
They occupy ecosystems ranging from dry desert to wet tropical forest, from tundra to savanna to temperate woodland. Some species are semiaquatic; others live underground; yet others spend their entire lives in the canopy of tropical forest. Their food habits range from true omnivory to specialization in earthworms, subterranean fungi, even aquatic invertebrates. Their importance to mankind cannot be overstated. Some species cause millions of damage to agricultural lands and stored foods. Others are the vectors or reservoirs of a number of diseases that have periodically devasted human populations (and continue to do so). On the other hand, many species are beneficial to man. Some are important biological controls of pestiferous insects. Others may be essential ("keystone") species in maintaining the health of our forests, through their role in dispersing seeds or spreading mycorrhizal fungi. Some even provide a source of meat. And a few species play an essential role as "domestic animals" used in medical research that has been enormously beneficial to mankind. Around 1325 living species of murid rodents have been described, making the Muridae by far the largest family of mammals -- but surely many more remain to be discovered. Murid species are currently placed in 281 genera, which are distributed among 17 subfamilies (for a brief description of each subfamily, click here). The fossil record of the family extends to the late Eocene.
Norway rat
Mass: 300 to 540 g On average, length from nose to tip of tail= 399 m while the tail is 187 mm. The Norway rat is a coarse-furred rat with prominent, naked ears and nearly naked scaly tail which is shorter than the head and body. Molars of the upper jaw have tubercles in three longitudinal rows, as in the house mouse. General color above is brown/dark gray with scattered black hairs, while underparts are pale gray or grayish brown. Females have 12 mammae.
Natural History Food Habits A true omnivore, the Norway rat shuns few items. It eats everything from soap to candy, milk, meat, vegetables, poultry, eggs and all grains, nuts and fruits. Its appetite is prodigious: it can eat a third of its weight in twenty-four hours. Norway rats are also capable of catching fish and small rodents, and they readily eat carrion. In cities, this rodent thrives on the uneaten and spoiled food constantly discarded by humans. Their main constraint is that they cannot go long without water unless their diet contains other liquids in adequate amounts.
Reproduction: Rats are among the most prolific of all mammals, and the Norway rat is no exception. If food is abundant and shelter adequate, rats will breed throughout the year, although fewer litters are produced in the winter. Each female becomes receptive for a period of about 20 hours every 4-6 days. Gestation is from 21-23 days, but it may be prolonged when a pregnant female is nursing an earlier litter. Young per litter numbers between 2 and 14, but the usual number is 7. Females experience a postpartum estrus, often mating within 18 hours of giving birth and thus easily produce 6-8 litters per year. The young are born blind, naked and helpless at birth; the eyes open in 14-17 days, and the young are weaned when 3 weeks old. Sexual maturity is reached at 3 months, although males must wait longer until they are able to challenge the dominant male. Rats continue to breed until they are 1.5-2 years old, and life span may reach 3 years.
Behavior: Norway rats are active year-round and are largely nocturnal, leaving the shelter of their nests as dusk approaches. They are wary creatures, often occurring in large numbers that go largely unsuspected, as they are adept at hiding or scurrying away upon human approach. When foraging, the Norway rat generally travels on the ground, but it is also a good swimmer, jumper and climber. They follow the same routes when they make their rounds foraging each night. Home ranges are generally only 50 m in diameter, although rats have been known to travel 2-3 miles a night to forage in farmers fields at harvest time. Rats are most frequently found living together in large groups in which a large male is dominant, maintaining preferential access to food, water, and resting sites. This male continuously guards a harem of females and aggressively prevents other males from mating. Females actively defend group resources against strangers and often nest together. Nests are built out of leaves, paper, rags, twigs, or anything useful the rat can find. If a suitable cavity or crevice is not available for nesting, rats make burrows in the ground and trails on the surface, particularly under the cover of old boards and the like. Dominance hierarchies within a group of rats are based on size/weight rather than age. Even though rats are nocturnal, some rats can often be found foraging during the daytime in areas of high population. These diurnal rats are generally socially low-ranked individuals who have been denied access to food by dominant rats during the night, when most of the activity takes place.
Habitat: Chiefly found in areas around human settlement, Norway rats occur wherever there is an abundance of food and shelter, from the subways and crowded tenements of metropolitan districts, to the corn and grain fields of farm country, to the salt marshes of the Atlantic Coast where edible flotsam is washed up on the beaches. Biomes: temperate forest & rainforest, temperate grassland, chaparral, desert, tropical rainforest, tropical deciduous forest, tropical scrub forest, tropical savanna & grasslands
Conservation/Biodiversity: no special status. Largely considered a menacing pest (see economic importance below) the Norway rat is the target of persistent control measures such as trapping, poisoning, and habitat elimination. It is estimated that there are between 150 and 175 million rats in the United States alone. Their numbers have been increased throughout the world as human numbers and habitation has increased.
Economic Benefits for Humans: Positive Selective breeding by humans in this species has produced the albino laboratory rat, widely used for medical and other research purposes. Negative: Many consider this rat to be the greatest mammal pest of all time. It has caused more deaths than all the wars in history. It harbors lice and fleas and has been the source of bubonic plague, typhus, trichina, infectious jaundice and many other serious diseases. These rats are usually a contributing factor of first importance in the spread of pandemics during war. They also cause considerable damage to property including crops, depletion and pollution of human food stores, and damage to buildings and their contents from destructive chewing of wiring, pipes, and walls. In the United States alone, economic losses due to rats were estimated in 1977 to be $500,000,000-$1,000,000,000,000 annually. There is also considerable evidence of rats attacking helpless human beings, especially infants.
Other Comments: The Norway rat was introduced to the Great Lakes region during historical times. It is not a native of Norway, as its name suggests. The species originated in Asia, reached Europe by the mid-1500's (when hoards of them reputedly swam the Volga River!) and arrived in North America about 1775 on ships from England.
House mouse
Geographic Range: Palearctic, Nearctic, Oriental, Ethiopian, Neotropical, Australian, Oceanic Islands: Mus musculus may originally be from the Mediterranean region to China, but has now been distributed throughout the world by humans and lives as a human commensal. ^ Physical Characteristics
Mass: 15 to 23 g Length of the head and body is 150-190 mm and the tail is 70-95 mm long. Fur coloration is generally light brown to black above, with white or buffy below. The long, tapered tail has obvious circular rows of scales (annulations) and is very sparsely furred. Mus musculus tend to have longer tails and darker coats when living commensally with humans as opposed to the wild forms. ^ Natural History Food Habits In the wild state, house mice eat many kinds of vegetable matter, such as seeds, fleshy roots, leaves and stems. Insects (beetle larvae, caterpillars, and cockroaches) and meat (carrion) may be taken when available. In human habitation, Mus musculus consumes any human food that is acessible as well as glue, soap, and other household materials. Many mice store their food or live within a human food storage facility.
Reproduction: Mus musculus is characterized by tremendous reproductive potential. Breeding occurs throughout the year, although wild mice may have a reproductive season extending only from April to September. The estrous cycle is 4-6 days long, with estrus lasting less than a day. Females experience a postpartum estrus 12-18 hours after giving birth. Females generally have 5-10 litters per year if conditions are suitable, but as many as 14 have been reported. Gestation is 19-21 days but may be extended by several days if the female is lactating. Litters consist of 3-12 (generally 5 or 6) offspring, which are born naked and blind. They are fully furred after 10 days, open their eyes at 14 days, are weaned at 3 weeks, and reach sexual maturity at 5-7 weeks. Average life span is about 2 years in captivity, but individuals have lived for as long as 6 years. In the wild, most mice do not live beyond 12-18 months.
Behavior: In the wild state, house mice generally dwell in cracks in rocks or walls or make underground burrows consisting of a complex network of tunnels, several chambers for nesting and storage, and three or four exits. When living with humans, house mice nest behind rafters, in woodpiles, storage areas, or any hidden spot near a source of food. They construct nests from rags, paper, or other soft substances and line them with finer shredded material. House mice are generally nocturnal, although some are active during the day in human dwellings. House mice are quick runners (up to 8 miles per hour), good climbers, jumpers, and also swim well. Despite this, they rarely travel more than 50 feet from their established homes. Mus musculus is generally considered both territorial and colonial when living commensally with humans. Territoriality is not as pronounced in wild conditions, however. Dominant males set up a territory including a family group of several females and their young. Occasionally, subordinate males may occupy a territory or males may share territories. Females establish a loose hierarchy within the territories, but they are far less aggressive than males. Aggression within family groups is rare, but all the individuals in a territory will defend an area against outsiders. Young mice are generally made to disperse through adult aggression, although some (especially females) may remain in the vicinity of their parents.
Habitat: House mice generally live in close association with humans-- in houses, barns, granaries, etc. They also occupy cultivated fields, fencerows, and even wooded areas, but they seldom stray far from buildings. Some individuals spend the summer in fields and move into barns and houses with the onset of cool autumn weather. Because of their association with humans, house mice have been able inhabit inhospitable areas (such as tundra and desert) which they would not be able to occupy independently.
Biomes: tundra, taiga, temperate forest & rainforest, temperate grassland, chaparral, desert, tropical rainforest, tropical deciduous forest, tropical scrub forest, tropical savanna & grasslands ^ Conservation/Biodiversity
Status: no special status. Commensal populations of Mus musculus are generally stable and densities can be as high as 10 mice per square meter. In the wild, populations are less stable and densities may be less than 1 mouse per 100 square meters. Overall, populations are flourishing and are in fact aided by human construction of houses, barns, and other structures.
Economic Benefits for Humans: Positive: Domesticated forms and albinos have been developed which are commonly used as laboratory animals (especially in medicine and genetics), and as pets. Mus musculus also has a small role as an insect destroyer, but this is minimal. Negative: House mice do not cause such serious health and economic problems as Rattus norvegicus and R. rattus. Mice are agricultural pests in some areas, however, and they do consume and contaminate stored human food with their droppings. They also destroy woodwork, furniture, uphostery, and clothing. In addition, they contribute to the spread of diseases such as murine typhus, rickettsial pox, tularemia, food poisoning (Salmonella), and bubonic plague.
Other Comments: Mus musculus often refers to several fairly distinct kinds of mice. As many as seven separate species may be placed under Mus musculus. "Dancing" and "singing" mice are other names for the house mouse. The former refers to a genetic strain with inner ear defects, causing the mice to weave, turn in circles, and wobble when they walk. The latter refers to a pathological condition causing mice to twitter constantly with a "song" resembling that of a cricket.
Carleton, M. D. 1984. Introduction to rodents. Pp. 255-265 in Anderson, S. and J. K. Jones, Jr. (eds). Orders and Families of Recent Mammals of the World. John Wiley and Sons, N.Y. xii+686 pp.
Savage, R. J. G. and M. R. Long. 1986. Mammal Evolution, an Illustrated Guide. Facts of File Publications, New York. 259 pp.
Vaughan, T. A. 1986. Mammalogy. Third Edition. Saunders College Publishing, Fort Worth. vii+576 pp.
Wilson, D. E., and D. M. Reeder. 1993. Mammal Species of the World, A Taxonomic and Geographic Reference. 2nd edition. Smithsonian Institution Press, Washington. xviii+1206 pp.
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