Rodentia

(Rodents)

Class Mammalia

Order Rodentia

Number of families 28

Number of genera, species 443 genera; 2,021 species

Evolution and systematics

Of the 4,629 species of mammals recognized by Wilson and Reeder, rodents represent 43% of species diversity within the class Mammalia. Several diagnostic characteristics associated with general morphology define rodents as a monophyletic group (e.g., group sharing a common ancestry). The primary characteristic is the pair of long incisors, resulting from the loss of canines and the creation of a diastema or gap between the incisors and cheek teeth, consisting of premolars and molars. Other characteristics of the skull and skeleton, dentition, and basic soft anatomy tend to support monophyly. Although the monophyly of Rodentia appears well supported on the basis of examinations of morphological traits of living forms as well as fossil lineages, several studies based primarily on amino acid sequence data from nuclear genes and some limited analysis of nucleotide sequence data have suggested that the order Rodentia is not monophyletic. According to these molecular studies, the guinea pig and its relatives group closer with other orders of mammals than they do to families of rodents, especially rats and mice of the family Muridae. However, as more extensive molecular studies have been initiated and more sophisticated analyses that take into account complex patterns of variation within rodents have been performed, molecular support for rodent monophyly has been forthcoming.

The order Rodentia presents some serious problems with respect to classification, especially the recognition of suborders. As specializations in rodents tend to focus on the masticatory apparatus (e.g., structure of the lower jaw, shape of the infraorbital foramen on the anterior end of the skull, and placement of musculature associated with jaw action), early classifications were based on the origin and insertion of jaw muscles (e.g., masseter muscles) and the arrangement of changes in portions of the skull associated with this musculature. Therefore, the more basic classifications focus on either changes in the infraorbital foramen and the placement of masseter muscles or structure of the lower jaw. Using these characteristics, the number of recognized rodent suborders has varied, depending upon the proposed classification scheme, from three (Sciuromorpha, squirrels; Myomorpha, rats and mice; and Hystricomorpha, guinea pigs and relatives) to as many as 16. Considering that the origin and insertion of jaw muscles and the structure of the skull relate directly to feeding, these features reveal high levels of parallel evolution (similar features independently derived). Molecular data, derived from nucleotide sequences, have confirmed the fallacy of using features that tend to converge in structure in response to similar functions. As a consequence of problems related to the origin and insertion of jaw muscles and modifications of the skull to accommodate such changes, the most consistent morphological feature is associated with the lower jaw. Based on this feature, two suborders of rodents can be identified, Hystrocognathi and Sciurognathi. The former suborder contains South American caviomorph rodents (guinea pigs and their relatives) and several Old World families including porcupines (Hystricidae), cane rats (Thryonomyidae), African mole-rats (Bathyergidae), and African dassie rats (Petromuridae). The remaining rodent families are placed in Sciurognathi.

In many cases, the phylogenetic relationships among families and genera of rodents are still controversial. Based on morphology and recent molecular data, families in the suborder Hystricognathi appear most closely related to the gundis of the family Ctenodactylidae. Several additional groups sharing a common ancestry appear to be well supported including: rats and mice of the superfamily Muroidea (families Muridae and Dipodidae); pocket gophers (family Geomyidae) and pocket mice and kangaroo rats (family Heteromyidae) of the superfamily Geomyoidea; squirrels (family

Sciuridae) and mountain beavers (family Aplodontidae). The placement of other families like the Castoridae (beavers), Pedetidae (springhares), Anomaluridae (scaly-tailed squirrels), and Myoxidae (dormice) are more tentative. The final outcome of ongoing studies of rodent relationships will alter our current knowledge of the classification and evolution of the order, especially with respect to interpretations of how morphology and life-history traits have changed throughout the rodent radiations.

Paleontologically, the order Rodentia dates to the Paleocene era at approximately 57 to 60 million years ago (mya). The earliest known family is the Paramyidae, containing sciurid-like rodents. In the Eocene (38 to 45 mya), rodents experienced a rapid rate of diversification, corresponding in time to the extinction of the Mesozoic order Multituberculata, a group containing rodent-like mammals. Many lineages appearing at this time still have living representatives. On several continents, including Africa, Australia, and South America, members of the family Muridae experienced rather recent adaptive radiations.

One biogeographic enigma pertains to the origin of the South American caviomorph families of suborder Hystricognathi. According to fossil evidence, the family is no older than late Eocene to early Oligocene (36 to 40 mya), yet the closest relatives of this group occur in Africa, a continent separated from South America by thousands of miles (kilometers) during the period at which caviomorphs first appear in South America. Although some paleontologists suggest that caviomorph rodents evolved the characteristics shared with African hystricognaths in parallel, all recent morphological and molecular data suggest that these two groups share a common ancestry. Based on shared ancestry, other paleontologists and biogeographers have suggested an over-water interchange between Africa and South America. Recent molecular data confirm a shared common ancestry for all families of South American caviomorph rodents, thus refuting earlier hypotheses of multiple origins from Africa. In addition, these molecular studies support African hystricognaths as being the closest relatives of caviomorphs. The timing of events associated with the exchange between Africa and South America is still controversial and is part of a broader debate over the origin of rodent lineages in genera. More recent studies based on amino acid and nucleotide sequences have employed a "molecular clock" to estimate divergence times for rodents. Rather than the 57 to 60 mya estimated from fossils for the origin of Rodentia, the molecules suggest a considerably older origin between 100 to 110 mya. Despite this contradiction, the origin of caviomorph lineages still appears younger than the dates at which Africa and South America were connected.

Physical characteristics

Rodents are gnawing mammals, and as such, all species share several features in common. The number of teeth in rodents seldom exceeds 22. The canine teeth are absent, and all rodents possess a single pair of upper and lower incisors characterized by enamel on the anterior surface and dentine on the posterior surface, allowing for differential wear of the teeth to maintain sharpness. A diastema or gap separates the incisors from premolars and molars, and this separation facilitates both gnawing with the incisors and grinding with the cheek teeth. Size and cusp patterns of cheek teeth in rodents relate to diet. Herbivorous rodents have high-crowned cheek

teeth, whereas omnivorous species have low-crowned cheek teeth and well-defined arrangements of cusps. The arrangement of two primary jaw muscles, the masseters and pterygoideus, allow for duel jaw action whereby chewing can be either transverse or front to back in motion. As noted earlier, the arrangement of these muscles relative to the infraorbital foramen has been used as a major diagnostic feature in many classifications of rodents, with the most primitive arrangement seen in the mountain beaver. Hamsters, pocket gophers, and pocket mice have cheek pouches that allow the animal to collect and transport food.

Rodents demonstrate considerable variation in size, ranging from a length of 4.7 in (12 cm) and weight of 0.1 oz (4g) for the pygmy mouse (Baiomys taylori) to 39.4 in (100 cm) and 10 lb (50 kg) for the capybara (Hydrochaeris hydrochaeris). Extinct lineages of the family Dinomyidae reached 882–1323 lb (400–600 kg) in size. In terms of their postcranial skeleton, unspecialized rodents have five digits, short limbs, a long tail, and a plantigrade foot posture (walk with the soles of feet on the ground). More specialized species tend to show considerable modification associated with their particular mode of locomotion and general lifestyle. Many unrelated fossorial species like pocket gophers, tuco-tucos (Ctenomyidae), coruros (Octodontidae), African mole-rats (Bathyergidae), bamboo rats (Muridae, Rhizomyinae), and blind mole-rats (Muridae, Spalacinae) show considerable convergent evolution for a subterranean lifestyle. Most species have a fusiform body with short limbs, small ears, and with eyes either reduced or absent. The tail is generally shorter than the head and body, and the feet are broad. The forefeet of many species have large clawed digits for digging, whereas some species dig with their incisors. Some species of rodents are modified for saltatorial (hopping) locomotion. Kangaroo rats, jerboas, springhares, and gerbils have long tails that serve as a counterbalance during hopping. In addition, the hind feet are generally large and the hind limbs are muscular, whereas the front limbs are shorter. Saltatorial locomotion has evolved several times in rodents and appears to be an adaptation to desert environments with patchily distributed resources. In addition to modifications of the postcranial skeleton, saltatorial rodents tend to have enlarged auditory bullae. Modifications for gliding locomotion can be seen in several species of squirrels (Sciuridae) and scaly-tailed squirrels (Anomaluridae). Gliders have modified membranes extending along the sides of the body and attached to the front and hind feet. The tail is generally well developed and is used as a rudder during the glide as well as a brace when landing. Several species of rodents reveal specializations for living in aquatic environments. These species have webbed feet and tails modified for swimming. The beaver's tail is flattened dorsoventrally, whereas the nutria and muskrat have tails that are laterally compressed. Tree squirrels show specializations for an arboreal lifestyle, revealing sharp claws on the digits and a modification of the bones in ankles that allows for the hind foot to be rotated as the animal descends from a tree head first. Some species of rodents are highly cursorial (fast running) and have highly modified limbs and feet. The mara, a fast running species that lives in steppe region of Patagonia and Argentina, appears rabbit-like with long hind legs, hoof-like claws, and a digitigrade foot posture (run on digits with heel off the ground).

Distribution

The order Rodentia has a worldwide distribution, and species within this order can be found in every habitat ranging from Arctic tundra to desert. Nearly 70% of all rodent species are rats and mice of the family Muridae, and representatives of this family are found on all continents as well as Australia and oceanic islands. The house mouse (Mus musculus) and the black rat (Rattus rattus), two species that live in close association with humans, are cosmopolitan and have been transported to all areas frequented by humans. Squirrels of the family Sciuridae have a worldwide distribution except for Australia, the desert region of Africa, and the southernmost region of South America. The mountain beaver, a relative of squirrels, is more restricted in distribution, being confined to the Pacific Northwest of the United States. The two living species of beaver occur either in North America or parts of Europe and Asia. Pocket gophers are restricted to North and Central America, and members of the family Heteromyidae (kangaroo rats and pocket gophers) occur in North and Central America as well as the northern portion of South America. Scaly-tailed squirrels (Anomalurus), African mole-rats (family Bathyergidae), and springhares (Pedetes capensis) occur in parts of sub-Saharan Africa. Members of the family Dipodidae occur in portions of the Middle East and northern Africa, and jumping mice of the family Zapodidae occur in both North America and parts of Europe and Asia. Several families of caviomorph rodents originated in South America, and many are restricted to the southern half of South America. At least one family, Capromyidae, is restricted to islands in the Caribbean, and the North American porcupine (Erethizon dorsatum) is the only caviomorph with a range extending throughout most of North America. The nutria (family Myocastoridae) also occurs in North America, but it is the consequence of an introduction by humans.

Habitat

Most types of habitats are occupied by rodents. Some species are desert specialists. For instance, the red viscacha rat (Tympanoctomys barrerae) of the family Octodontidae occupies harsh desert habitats in parts of Argentina and demonstrates adaptations for feeding on plants with a high salt content. Kangaroo rats (Dipodomys) reveal several behavioral and physiological modifications for living in harsh desert environments, including a highly modified kidney that allows for concentration of urine and features of the nasal passage that aid in the extraction of water prior to exhaling through the nostrils. This species is active at night and occupies a closed burrow system during the day, and all the water necessary for survival is acquired through selective foraging of seeds high in water content. Other saltatorial rodents occupying similar desert habitats show strong physiological, behavioral, and morphological convergence, possessing many features analogous to the North American kangaroo rat. Squirrels are quite diverse in terms of habitat. Tree squirrels occur in most forested areas. Some species of squirrels (e.g., prairie dogs, ground squirrels, marmots) create elaborate burrow systems and can live in habitats ranging from grasslands to semidesert regions. The antelope ground squirrel (Ammospermophilus nelsoni) is a diurnal species that occupies desert areas in the southwestern United States. This particular species displays short activity bouts interrupted by periods in the shade where excess heat is released prior to the next foraging bout. The beaver, muskrat, and nutria prefer riparian habitats and wetlands. Some species of lemmings occur in tundra, and other microtines prefer either forest or grassland habitats. In many cases, rodents are essential components of the habitats that they occupy, and through their activities, plant communities can be modified as a result of foraging behavior, seed dispersal, and enhancing components of the soil. For instance, the plains viscacha (Lagostomus maximus) alters plant communities with forbs becoming more dominant than grasses in areas that are heavily grazed. Other studies on North American kangaroo rats have revealed significant changes in plant diversity in areas where a particular species has been removed.

Behavior

The social structure in rodents is highly variable. Some species, like pocket gophers, pocket mice, and kangaroo rats, are solitary. In the case of many fossorial species like pocket gophers (family Geomyidae) and Mediterranean blind mole-rats (Nannospalax), individuals tend to occupy burrow systems that do not overlap spatially, except for males and females during the breeding season. Multiple individuals in a burrow system are usually restricted to mother and offspring. In most cases, individuals tend to demonstrate high levels of aggression when confined in the same cage. Some fossorial species are considerably more social and maintain overlapping burrow systems. Many of these forms are highly colonial. For instance, African mole-rats of the family Bathyergidae have some solitary forms, yet several species including the naked mole-rat (Heterocephalus glaber), the Damaraland mole-rat (Cryptomys damarensis), and the common mole-rat (C. hottentotus) are highly colonial. In fact, the naked mole-rat and the Damaraland mole-rat have a social system analogous to eusocial insects (e.g., ants and termites). Multiple females of social tuco-tucos (Ctenomys sociabilis) share the same nest and burrow system along with their young. Group size in these social fossorial rodents varies greatly, to over 100 in some species. The degree of group living and the length of times that individuals remain in the group are influenced by multiple factors related to resource availability and cost of dispersal. In fact, individual naked mole-rats that disperse from their natal group appear to have a different phenotype characterized by large fat stores.

Other species of rodents tend to be gregarious. Prairie dogs, ground squirrels, and marmots live in colonies that have a well-defined structure consisting of related females that tend to stay within their natal group and males that disperse from neighboring groups. All of these species demonstrate sophisticated alarm calls in response to predators and other disturbances to alert other members of the colony, especially related individuals. In the case of Belding's ground squirrel (Spermophilus beldingi), the frequency of alarm calls, a behavior that can prove detrimental to the caller, tends to increase among related individuals. Under certain circumstances, both ground squirrels and prairie dogs display a form of infanticide known as marauding behavior, whereby adult individuals kill the young of a lactating female. In the case of ground squirrels this behavior appears to be directed towards non-kin.

Beavers live in colonies consisting of offspring and an adult male and female, and each colony occupies a defined territory. Individuals in the colony assist with construction of dams and lodges. Other rodents are also found to tend towards monogamy, e.g., South American acouchis and pacas, although they are less social than beavers. Additionally, some species of voles and deermice live in family groups like beavers. These species show strong incest avoidance.

Many species of South American caviomorph rodents are colonial, living in colonies with established male linear hierarchies maintained through dominance and aggression. Nearly all colonial species of caviomorph rodents are highly vocal and use a series of sounds to communicate warning, courtship, play, and aggression. Even some species of mice and rats are colonial and have dominance-based social systems similar to other unrelated social species of rodents.

Species of the order Rodentia communicate using visual, aural, olfactory, and vocal signaling. Vocalizations such as squeaks, grunts, and barks are used as alarm calls, for sexual and aggressive functions, and for seeking misplaced young. Tree squirrels of the family Sciuridae have been known to use visual communication during mating by waving and shivering their tails. Several Rodentia species employ footdrumming, a characteristic thumping or stamping pattern made with the feet, to differentiate between neighbors and outsiders—though in African mole-rats (Bathyergidae), footdrumming is also used during the breeding season to announce an individual's presence to potential mates. Olfactory messages are also vital in marking territorial boundaries and recognizing colony members. Scent-producing facial and anal glands, as well as urine and fecal matter, are used to produce these signals.

Feeding ecology and diet

Many species of rodents are herbivorous and feed on a variety of plant materials. These rodents tend to have a highly developed stomach and large intestines, and nutrients from plants are extracted through a fermentation system found in the enlarged cecum occurring in the large intestines. Some rodents, like the grasshopper mouse (Onychomys), are insectivorous, and others eat aquatic insects, mollusks, and fish. Like carnivores, dormice lack a cecum and appear to be more omnivorous in their diet, eating a variety of insects, worms, eggs, and fruits.

Reproductive biology

Rodent reproduction can be subdivided into two forms. The families Muridae (mice and rats), Geomyidae (pocket gophers), and most other sciurognaths have short life expectancies, short gestation periods (17 to 45 days), produce multiple litters per year (one to four), and have large numbers of altricial (helpless) offspring per litter. Most hystricognath rodents such as the Caviidae (guinea pigs and relatives), Erethizontidae (porcupines), and other caviomorph families, have longer life spans, have long gestation periods (ranging from 60 to 283 days), produce few litters per year (generally one to two), and give birth to smaller numbers of precocial offspring per litter. Although body size is generally a good predictor of the form of reproduction employed, hystricognath rodents tend to have longer gestation periods than similar size sciurognath rodents. Hystricognath rodents also produce precocial young that are weaned and reach sexual maturity at an early age. Although species of rodents with large altricial litters tend to gamble in terms of reproduction, many species of rodents in more unpredictable environments tend to synchronize reproduction in an effort to produce offspring under optimal conditions (e.g., during periods of maximum plant productivity). Hibernating species in the families Sciuridae (squirrels) and Zapodidae (jumping mice) generally produce one litter per year, whereas non-hibernating species can be polyestrous, breeding more than one time per year. These hibernators have a narrow window where resources are

optimal for reproduction and raising offspring. For instance, the jumping mouse hibernates for nine months and has a three-month window for successful reproduction and weaning of offspring. Even within the same species of small rodent, such as the California meadow vole (Microtus californicus), the old-field mouse (Peromyscus polinotus), the Eastern woodrat (Neotoma floridana), timing of reproduction and litter size can vary with respect to environmental conditions such as rainfall and food abundance.

Chemical communication is important to the reproductive biology of rodents. Female house mice tend to experience increased ovulation in the presence of males. In rodents with short life spans, flexibility in the timing of ovulation to increase the chance of fertilization by a male optimizes reproductive success. The "strange male effect" (or Bruce effect, after the author who described it) in mice occurs when pregnancy is blocked in an inseminated female upon encountering an unknown male. After the new male inseminates the female, reintroduction of the previous male fails to block implantation. It has been proposed that the female presumably has "an olfactory memory" that prevents the female from blocking implantation upon encountering her first mate. Young prairie vole females do not come into heat until they are separated from their family and encounter an unfamiliar male (or his odor).

The mating system in rodents varies, depending upon the species. Many rodents are promiscuous, with offspring from

a single litter often being sired by more than one male. Recent genetic studies using DNA fingerprinting have confirmed a high incidence of multiple paternity in many promiscuous species. Some species of rodents have a monogamous mating system. For instance, male and female South American mara or Patagonian cavy (Dolichotis patagonum) establish pair bonds that can last for multiple mating seasons, and throughout this period the male and female stay in close proximity to each other as they forage. This same species has a tendency to form communal nurseries where several pairs house their young, visiting each day to provision their own young. Some species of rodents that demonstrate monogamy also show a correlation between male parental care and successful rearing of offspring. For instance, the California mouse (Peromyscus californicus) female is successful at raising a litter if the male is present. Male and female beavers also tend to form long-lasting pair bonds. Many species of caviomorph rodents have a harem based mating system defined by a linear hierarchy of males. Rock cavies (Kerodon rupestris) have a resource-based form of polygyny, whereby males defend rocky outcrops that are considered ideal resources, thus attracting females. Capybaras (Hydrochaeris hydrochaeris) are semi-aquatic rodents that live in social groups lasting multiple years. Breeding within these groups is harem-based with a dominant male and several females and subordinate males.

The naked mole-rat (Heterocephalus glaber), a species occupying regions of eastern Africa, has an unusual mating system, analogous to social insects, with a single reproductive female and a few reproductive males. Although mole-rat colonies can be quite large, breeding is suppressed in subordinate females, and the entire social system is based on reproductive and non-reproductive individuals. In this system, non-reproductive individuals perform duties related to excavation of the underground burrow system, foraging for food, and tending pups of the reproductive female. In captive populations, the same female can remain reproductive for multiple breeding seasons, and she uses a combination of aggression and possibly chemical communication to suppress reproduction by other adult females. This unusual social system has been considered analogous to the eusocial system seen in social insects that also have overlapping adult generations, recruitment into the natal group, and well defined reproductive and non-reproductive individuals. Apparently, several other species of African mole-rats have similar mating systems. These rodents have stimulated much debate regarding the prerequisites for such complex social behavior, including the possibility that unpredictable arid environments and monogamy may be early precursors that foster the eventual evolution of complex social systems.

Conservation

The IUCN lists 669 species of rodents under varying degrees of threat and endangerment. Thirty-two species from 23 genera are considered Extinct, and 22 of these extinct species occurred on either Oceanic islands or islands in the Caribbean. Causes of extinction relate primarily to loss of habitat and the introduction of feral predators. Ninety-five species of rodents are listed as Endangered throughout the world, with three species in the squirrel family Sciuridae considered Endangered in North America. Vulnerable and Lower Risk species number 166 each.

Significance to humans

Many species of rodents are considered nuisance animals. Burrowing activities of nutria and muskrats can undermine water retention structures like earthen dams and levies. Woodchucks (or ground hogs), eastern chipmunks, prairie dogs, and ground squirrels also burrow, resulting in potential damage to vegetable gardens and structures. Tree squirrels are incredible pests in urban areas. They are true artisans at exploiting "squirrel–proof" bird feeders. Upon invading houses, tree squirrels can damage attics, especially insulation and wiring. Woodrats (genus Neotoma), sometimes known as packrats, frequent barns and other buildings associated with humans. These rodents are notorious for decorating their nests with objects taken from various parts of their territory, including human artifacts. North American pocket gophers of the family Geomyidae are both a pest and a benefit. These subterranean rodents are beneficial in terms of increasing soil fertility, aeration, and water infiltration. Their rotation of soil also reduces compaction. At the same time, pocket gophers cause serious damage to underground cables and irrigation lines by their gnawing activities. Underground telephone cables are vulnerable unless either protected by a gopher retardation device or buried below the soil's A zone. The common house mouse, Mus musculus, is a fixture in many homes, warehouses, and other human-made structures. In some circumstances, house mice can contaminate stored food, damage materials maintained in warehouses, and cause structural damage through their nest building and habit of gnawing.

Some rodent species cause serious damage to commercial forest operations. The mountain beaver, Aplodontia rufa, a species occurring from southern British Columbia to northern California, is a serious pest that feeds on conifers. Primary damage occurs as a result of girdling trees, damaging seedlings, and gnawing roots. It has been estimated that this rodent has extensively damaged over 300,000 acres (120,000 ha) of coniferous trees in parts of Washington state and Oregon. At a value of $10,000 per acre, this species has caused millions of dollars in damage. Voles also eat conifer seedlings at sites of reforestation and Christmas tree farms. These small rodents require high grass for cover. Therefore, prevention of damage to seedlings can be accomplished by minimizing the amount of grass cover. Although less damaging, nutrias are known to girdle trees, and in forest plantations and fruit orchards, the North American porcupine can cause damage. Like the pocket gopher, the porcupine also is beneficial by creating diverse habitat for many forms of wildlife, especially birds. Cotton rats (Sigmodon) occur throughout regions of the southern and western United States. These rodents are herbivorous and prolific breeders. When cotton rat populations are high, these rodents can cause damage to alfalfa and other crops. Although prairie dogs generally prefer overgrazed or disturbed habitat, they can impact rangelands by reducing the amount of forage available to livestock. At the same time, the burrow of prairie dogs supports many species of vertebrates and invertebrates, and the prairie dog is a major prey item of both the endangered black-footed ferret and birds of prey.

Rodents cause considerable damage to cash crops in several countries. Studies have shown that rodents consume from 1 to 20% of crops in industrialized nations and as much as 50% in less developed countries. In parts of Southeast Asia, where rodents destroy large portions of the rice crop, reduction of rodent populations resulted in a 200% increase in rice crops. The reproductive ability of many species of the family Muridae make these rodents especially harmful to crops in many parts of the world. For instance, rodent outbreaks can dramatically increase population densities of rodents, with numbers being in the thousands per 400 acres (1 ha), thus posing a serious threat to crops. For instance, a population of the European hamster (Cricetus cricetus) in Hungary resulted in nealy 988,422 acres (400,000 ha) being impacted. Several genera in the murid subfamily Gerbillinae (e.g., Meriones, Tatera, and Rhombomys) are considered major pests of cultivated crops from northern Africa to regions in India. These gerbils can severely damage cereal as well as vegetables, olive saplings, and other economically important crops.

North America's largest rodent, the beaver (Castor canadensis), has some positive and negative attributes. Historically, the beaver has been prized for its pelt, and beaver trapping had a severely negative impact on natural populations throughout portions of its range in North America. As "nature's engineers," beavers are capable of modifying habitat that favors wetlands. Through their dam building activities, beavers create habitat suitable for waterfowl and other wetland species. However, alteration of habitat and harvesting trees for food and support materials also threatens both agricultural and forest interests. Dams can initiate flooding in some areas resulting in the loss of trees that are intolerant of high water levels. In addition, beavers can pose a threat to human health. They are known to carry Giarda, an intestinal parasite, transmitted to humans through drinking water, and exposure to beaver lodges can result in humans contracting Gilchrist's disease, which causes pneumonia-like symptoms.

Many species of rodent endanger human health. Bubonic plague or the "Black Death" is a dreaded disease that created havoc during the sixth and fourteenth centuries in Europe by killing almost one third of its population. The total number of deaths from bubonic plague was approximately 137 million people. Thousands died as a result of this rodent-borne bacterial disease. Plague is transmitted by the bite of an infected flea as well as through inhalation of the bacteria or direct contact with body fluids, and during the European epidemic, the primary rodent host was the black rat, Rattus rattus, of the family Muridae. Even today, several species of rodents carry bubonic plague, even though the frequency of human infections has declined. Squirrels and chipmunks are the major rodent hosts of plague in California, and humans most at risk live in the more rural areas of the state. Prairie dogs are also bubonic plague carriers, and although they rarely transmit the

disease to humans, whole prairie towns have ceased to exist as a result of plague epidemics.

Lyme disease is another bacterial infection transmitted indirectly from rodent reservoirs to humans through tick bites. This disease occurs worldwide and has become endemic throughout the United States. The symptoms are flu-like and can become chronic. Rodents, especially the white-footed mouse (Peromyscus leucopus), act as reservoirs by supporting the larval and nymphal stages of tick species known to transmit the disease. In the northeastern United States, the cycle of Lyme disease involves a rodent intermediate host followed by the adult stage of the tick infesting white-tailed deer. Humans catch the disease through incidental bites from infected ticks.

Hantavirus causes pulmonary distress in humans and is contracted through exposure primarily to the urine and droppings of several species of mice. This exposure generally occurs as a result of breathing the virus during contact with dust in areas of heavy rodent infestation. In North America, there are several rodent hosts of the subfamily Sigmodontinae (family Muridae) including the deer mouse (Peromyscus maniculatus), the white-footed mouse (Peromyscus leucopus), the cotton rat (Sigmodon hispidus), and the rice rat (Oryzomys palustris). The most recent and famous hantavirus outbreak in North America occurred in the "Four Corners" of the southwestern United States. In South America, outbreaks of viruses related to hantavirus have occurred in Chile. Again, the primary rodent hosts are sigmodontines, such as the genus Oligoryzomys. Even in parts of Europe and Asia, several species of rodents harbor similar viruses that cause pulmonary disorders and hemorrhagic fever in humans. In many cases virus outbreaks are cyclical and are more virulent during population increases in response to increased rainfall and plant production.

Rodents provide both indirect and direct benefits to humans. Many species serve as the primary prey of many vertebrate predators, and through their burrowing, dam building, seed hoarding, and other activities, rodents can have a positive impact on wildlife habitat. In terms of human health, rodents provide excellent animal models for studying human disease. In fact, approximately 95% of animals used in research are either rats or mice. The house mouse, in particular, provides a very useful animal model for biomedical research. Inbred strains, first developed by pet fanciers, of rats and mice are widely used to study human diseases. Transgenic and knock-out mice represent genetically manipulated strains of the house mouse that are used in research on a variety of human diseases (e.g., Parkinson's diseases, cancer, heart disease, etc.). Similar rodents are being used to study Alzheimer's disease, aging, and cystic fibrosis, and they are also useful for testing potentially beneficial cancer treatments and other drugs used to combat human disease. Research supported by the National Aeronautics and Space Administration uses rodent models to study bone loss in response to being in an environment without gravity.

In addition to serving as research models, many rodents are popular pets, including rats, mice, gerbils, hamsters, and guinea pigs. Rodents are also eaten in many parts of the world. For example, roasted, fried, or stewed cuy (guinea pig) is popular in Ecuador, Peru, and other South American countries.

Resources

Books

Bronson, F. H. Mammalian Reproductive Biology. Chicago: University of Chicago Press, 1989.

Eisenberg, J. F. The Mammalian Radiations: An Analysis of Trends in Evolution, Adaptation, and Behavior. Chicago: University of Chicago Press, 1981.

Lacey, E. A. "Spatial and Social Systems of Subterranean Rodents." In Life Underground: The Biology of Subterranean Rodents, edited by Eileen A. Lacey, James L. Patton, and Guy N. Cameron. Chicago: University of Chicago Press, 2000.

Macdonald, D. The New Encyclopedia of Mammals. Oxford: Oxford University Press, 2001.

Myllymaki, A. "Importance of Small Mammals as Pests in Agriculture and Stored Products." In Ecology of Small Mammals, edited by D. Michael Stoddart. London: Chapman and Hall, 1979.

Nowak, R. M. Walker's Mammals of the World. Vol. 2. Baltimore: Johns Hopkins University Press, 1991.

Stein, B. R. "Morphology of Subterranean Rodents." In Life Underground: The Biology of Subterranean Rodents, edited by Eileen A. Lacey, James L. Patton, and Guy N. Cameron. Chicago: University of Chicago Press, 2000.

Vaughn, T. A. Mammalogy, 3rd ed. New York: CBS College Publishing, 1986.

Wilson, D. E., and D. M. Reeder. Mammal Species of The World. Washington, DC: Smithsonian Institution Press, 1993.

Wood, B. J. "Rodents in Agriculture and Forestry." In Rodent Pests and Their Control, edited by A. P. Buckle and R. H. Smith. Wallingford, UK: CAB International, 1994.

Periodicals

Adkins, R. M., A. H. Walton, and R. L. Honeycutt. "Higher-Level Systematics of Rodents and Divergence Time Estimates Based on Two Congruent Nuclear Genes." Molecular Phylogenetics and Evolution 26 (2003): 409–420.

Burda, H., R. L. Honeycutt, S. Begall, O. Locker-Grutjen, and A. Scharff. "Are Naked and Common Mole-rats Eusocial and if so, Why?" Behavioural Ecology and Sociobiology 47 (2000): 293–303.

Cantoni, D., and R. Brown. "Paternal investment and reproductive success in the California mouse, Peromyscus californicus." Animal Behaviour 54 (1997): 377–386.

Honeycutt, R. L. "Naked Mole-rats." American Scientist 80 (1992): 43–53.

Huchon, D., F. M. Catzeflis, and E. J. P. Douzery. "Variance of Molecular Datings, Evolution of Rodents and the Phylogenetic Affinities between Ctenodactylidae and Hystricognathi." Proceedings of the Royal Society of London 267 (2000): 393–402.

Jarvis, J. U. M. "Eusociality in a Mammal: Cooperative Breeding in Naked Mole-rat Colonies." Science 212 (1981): 571–573.

Nedbal, M. A., R. L. Honeycutt, and D. A. Schlitter. "Higher-Level Systematics of Rodents (Mammalia, Rodentia): Evidence from the Mitochondrial 12S rRNA Gene." Journal of Mammalian Evolution 3 (1996): 201–237.

Sherman, P. W. "Nepotism and the Evolution of Alarm Calls." Science 197 (1977): 1246–1253.

Sullivan, J., and D. L. Swofford. "Are Guinea Pigs Rodents? The Importance of Adequate Models in Molecular Phylogenetics." Journal of Mammalian Evolution 4 (1997): 77–86.

Organizations

Foundation for Biomedical Research. 818 Connecticut Avenue, NW Suite 200,, Washington, DC 20006 USA. Phone: (202) 457 0654. Fax: (202) 457 0659. Web site: <http://www.fbresearch.org/education/species-sheet-rodents.htm>

National Center for Infectious Diseases, Division of Bacterial and Mycotic Diseases. 1600 Clifton Road, Atlanta, Georgia 30333 USA. Web site: <http://www.cdc.gov/ncidod/dbmd/mspb.htm>

Rodney L. Honeycutt, PhD