Family Muridae
Old World mice and rats, gerbils, whistling rats, and relatives

Muridae, the rats and mice, is the largest, most diverse family of mammals. It includes 730 species in 150 genera in 5 subfamilies. These subfamilies include the Leimacomyinae (groove-toothed forest mouse), the Deomyinae (spiny mice, brush furred mice, and link rat), the Gerbillinae (gerbils, jirds, and sand rats), the Murinae (Old World rats and mice), and the Otomyinae (vlei rats, karoo rats, and whistling rats). (Musser and Carleton, 2005)

Murids are an Old World group. Their native range extends throughout Africa, Europe, Asia, and Australia. They have been introduced througout the world by humans and are now found on every continent (except for Antarctica) and on many oceanic islands. (Nowak, 1999)

Murids occupy just about every type of habitat imaginable. Murids live in dry sandy deserts, wet tropical forests, agricultural fields, and the world's largest cities. Murids inhabit alpine tundra at elevations exceeding 4000 m and can be found in mine shafts more that 500 m below the earth's surface. Other habitats include shrublands, grasslands, swamps, bogs, meadows, and every imaginable forest type. (Nowak, 1999)

The family Muridae, being the largest and most diverse family of rodents, also has the most complicated taxonomic history. Most systems of rodent classification have included a family called "Muridae", but ideas about what belongs in this family have varied widely. In his 1876 classification, Alston placed all muroid rodents (except for Spalax, Rhizomys and Lophiomys) and even some non-muroid rodents (the Dipodidae) within the family Muridae. Thomas (1896) followed a similar classification, only excluding the Rhizomyinae and the Spalacinae from the family Muridae. Tullberg's (1899) Muridae was less inclusive. It was divided into three tribes: Murini (which included Dendromus, Saccostomus, and Steatomys), Phloeomyini, and Otomyini. Similarly, Miller and Gidley (1918) included just four subfamilies in Muridae: Dendromyinae, Murinae, Phloeomyinae, and Hydromyinae, and Chaline et al. (1977) included just two: Murinae and Hydromyinae. Ellerman (1940, 1941) once again took an inclusive approach, and only separated the families Lophiomyidae, Spalacidae, and Rhizomyidae from Muridae. Simpson's Muridae (1945) contained two current murid subfamilies (Murinae and Otomyinae) and four other subfamilies that are either classified elsewhere now (Dendromurinae) or are no longer given subfamily status (Phloeomyinae, Rhynchomyinae, and Hydromyinae). Recent morphological taxonomies suggested that Muridae should be all-inclusive, essentially synonymous with the superfamily Muroidea, encompassing all muroid subfamilies (Musser and Carleton 1993). However, recent molecular evidence clearly demonstrates that muroid rodents fall into five distinct clades that only diverged as recently as the early Miocene (the sixth muroid family, Platacanthomyidae, has not yet been included in molecular analyses) (Jansa and Weksler 2004, Steppan et al. 2004, Michaux et al. 2001). Steppan et al. (2004) dated the divergence between the family Muridae and its sister taxon, the family Cricetidae, at about 24 million years ago. (Alston, 1876; Chaline, Mein, and Petter, 1977; Ellerman, 1940; Ellerman, 1941; Jansa and Weksler, 2004; Michaux, Reyes, and Catzeflis, 2001; Miller and Gidley, 1918; Musser and Carleton, 1993; Simpson, 1945; Steppan, Adkins, and Anderson, 2004; Thomas, 1896; Tullberg, 1899)

Murids, as a group, are difficult to describe because few generalizations can be made. The typical murid body plan is what most people picture when they think of a rat or a mouse: a small brown or gray animal with prominent, rounded ears, long whiskers, and a long tail that scurries along on four small feet. While this description holds true for many murids, there are also many deviations from this basic body plan. Some species of murids are indeed small--tiny African pygmy mice (Mus minutoides) have bodies less than 9 cm long and weigh in at under 5 grams--but some are much larger, like southern Luzon giant cloud rats (Phloeomys cumingi), which grow to over 48 cm long and weigh over 2 kg. Populations of some species exhibit sexual dimorphism in body size. Murid fur may be long or short, and ranges from soft to spiny in texture. Coat colors vary from very pale to black, and include various shades of brown, gray, russet, white, and yellow. Many murids are rather uniformly colored, but others are mottled, striped, or blotchy. Most are paler on the feet and belly than on the dorsal surface. Some murid populations are polymorphic, with two or more color variants existing sympatrically. Murid ears may be rather long and rabbit-like, or very short and vole-like. Tails also vary from short to long and are completely naked in some and bushy and squirrel-like in others. The hind feet of some murids, most notably those in the subfamily Gerbillinae, are long and specialized for saltatorial movement, while other murids have prehensile tails that aid in climbing. (Hubbard, 1972; Nowak, 1999; Sinai et al., 2003)

The flip side of the high reproductive output of murids is that most live very short lives. Many do not live longer than a year, and those that do rarely make it to their third birthday. Captivity can drastically lengthen this time, however, and murids kept as pets may live for over a decade. (Nowak, 1999)

Behavioral characteristics, like everything else, vary widely in the family Muridae. Because they fill countless niches in a wide array of habitats, murids have also evolved a mind-boggling array of behaviors. There are terrestrial, arboreal, and fossorial murids, nocturnal, diurnal, and crepuscular murids, territorial, sedentary, migratory, and nomadic murids, social, colonial, and solitary murids, and murids with strict dominance hierarchies. There are murids that are specialized for hopping, climbing, swimming, and murids that are capable of all of these modes of locomotion. Many murids are active year-round, but some enter periods of torpor during cold periods. (Nowak, 1999)

Murids perceive the world using vision, hearing, touch, smell, and taste. The relative importance of these senses varies among species and relates to each species' lifestyle. For example, murids that forage under the cover of darkness might rely more on smell, touch, and hearing than on vision, while the opposite might be true for diurnal murids. The range of murid perception often surpasses that of humans; for example, some murids can hear ultrasounds, as young murids that have been separated from their mother often emit ultrasonic calls, to which mothers quickly respond (Ehret 2005). In general, murid communication involves a combination of chemical, tactile, visual, and auditory cues--the relative importance of which, again, varies among species. As is the case for many mammals, pheromones play a large role in intraspecific interactions in murids, allowing individuals to attract and locate mates, assess each other's status in the dominance hierarchy, or to synchronize their reproductive cycles (Thompson et al. 2004). (Ehret, 2005; Nowak, 1999; Thompson et al., 2004)

As a group, murids consume an astonishing array of food items, including (but not limited to) roots, grains, leaves, shoots, seeds, berries, nuts, fungi, fruits, insects and other invertebrates, small birds and eggs, turtles, carrion, and even household items such as glue, paste, and soap. Some species eat their own feces, a practice known as coprophagy. Individual murid species range from dietary generalists that will eat just about anything, such as commensal rats (Rattus norvegicus, for example) and mice (Mus musculus), to specialist herbivores (such as Hapalomys longicaudus, a bamboo specialist) and specialist carnivores (such as Rhynchomys isarogensis, an earthworm specialist). Many murid species cache their food in burrows or crevices for later use. (Nowak, 1999)

Murids are a food source for a myriad of predators belonging to almost every extant vertebrate class, including mammalian carnivores (such as foxes, cats, and weasels), birds of prey (such as hawks, eagles, and owls), non-bird reptiles (such as snakes and large lizards), amphibians (such as large frogs and toads), and even large fish (Cochran and Cochran 1999).

Because they are up against such an astonishing array of predators, murids have evolved numerous strategies for avoiding being eaten. Many are only active after dark, when diurnal predators (like snakes and hawks) may have a difficult time hunting them. Murids often seek refuge in burrows or crevices that are too small for predators to enter. In addition, many rely on their versatility to escape predators, and can run, leap, climb or swim in a pinch, even if they do not normally do so. Murids tend to have neutral-colored coats that blend in with the natural backgrounds of their habitats, affording them some degree of camouflage. Some murids, such as common spiny mice (Acomys cahirinus), have evolved tails that break off easily when grabbed, which may allow the then tailless mice time to escape predation (Shargal et al. 1999). Finally, like most wild mammals, murids often bite viciously when attacked and may inflict enough surprise or damage that predators release them. (Cochran and Cochran, 1999; Nowak, 1999; Shargal et al., 1999)

Murid rodents are essential components of many ecosystems. They have roles as seed dispersers, pollinators (Johnson et al. 2001), predators, and/or prey. Not all ecosystem roles are positive, however. Some murid species have been introduced to areas where they were previously absent, and they have devastated ecosystems by outcompeting or feeding on native wildlife. A few murid species have developed a commensal relationship with humans, and, especially in urban areas, rely on human-produced waste to survive. In turn, various parasites use murids as hosts, including species of ticks and mites, fleas, lice, bot flies, nematodes, tapeworms, and trypanosomes. (Johnson, Pauw, and Midgley, 2001; Nowak, 1999; Roberts, Janovy, and Jr., 2000)

Although most murids have no direct impact whatsoever on humans, those that do cause enough damage and suffering to give the entire group a bad name. Every year, commensal rats and mice cause billions of dollars worth of property damage worldwide by gnawing on structures and on electrical wires, damaging buildings, and starting fires. They are common household pests, raiding kitchens and granaries and causing much crop damage when they are abundant. In addition, they are carriers of numerous human diseases, from mild cases of food poisoning, to murine typhus and the highly deadly plague, which has had an enormous impact on human history, wiping out a quarter of Europe's population in a single 14th century epidemic. (Nowak, 1999; Roberts, Janovy, and Jr., 2000)

Murids have an immense positive economic impact on humankind. First, many murid species are kept as pets, and some are sold as food for other types of pets, such as snakes and lizards. Even more importantly, murids have been used as model organisms in laboratories for years, and their contribution to scientific and medical research cannot be overstated. Throughout history, humans have resorted to eating rats during times of famine to avoid starvation (although this practice may contribute to the spread of disease), and some murid species are prized as food or for their pelts and hunted regularly. (Nowak, 1999)

The family Muridae contains some of the most common species on Earth--such as house mice (Mus musculus) and Norway rats (Rattus norvegicus)--but it also contains a large number of species with small populations and restricted ranges. In fact, 36% of the species in this family are on the IUCN's Red List of threatened species. This includes 22 critically endangered species (such as Rattus enganus), 45 endangered species (such as Bunomys coelestis), 71 vulnerable species (such as Mastomys awashensis), 18 near threatened species (such as Grammomys aridulus), 54 lower risk species (such as Mus spicilegus), and 54 species that cannot be classified due to lack of information. Another 13 species are presumed to have gone extinct in recent years: lesser Australian stick-nest rats, Leporillus apicalis, Flores cave rats, Spelaeomys florensis, white-footed rabbit rats, Conilurus albipes, 4 species of Australian hopping mice: Notomys amplus, N. longicaudatus, N. macrotis, and N. mordax, Verhoeven's giant tree rats, Papagomys theodorverhoeveni, Maclear's rats, Rattus macleari, bulldog rats, R. nativitatis, emperor rats, Uromys imperator, Guadalcanal rats, U. porculus, and Gould's mice, Pseudomys gouldii. The largest threat to most murine species is also the largest threat to Earth's biodiversity overall: human-induced habitat loss. Specific conservation measures have not been enacted for many species, but for some, research is underway to better understand their ecology and, for a few, protected areas have been established to offset the effects of habitat loss. (IUCN, 2004)