Subfamily Murinae
Old World rats and mice

Murinae, the Old World rats and mice, is the largest subfamily of muroid rodents. There are an astonishingly diverse 561 species in this subfamily, which are divided among 126 genera in 29 divisions. (Musser and Carleton, 2005)

Rats and mice are native to the Ethiopian, Palearctic, and Oriental regions, including Africa, the Arabian Peninsula, Europe, the Middle East, India, China, Taiwan, Korea, Japan, the Indo-Malayan region, the Philippines, New Guinea, Australia, and Tasmania. In addition, murines have been introduced around the world by humans, and now have a virtually cosmopolitan distribution. (Carleton and Musser, 1984)

Murines occupy a wide variety of boreal, temperate, subtropical, and tropical habitats, including: coniferous and deciduous forests, subtropical broadleaf forests, tropical rainforests, monsoon forests, savannahs, steppes, grasslands, scrub forests, alpine meadows, deserts, rocky outcrops, river valleys, marshes, swamps, lakes, rivers, streams, agricultural fields, cities, and towns. Murines span a greater elevational range than any other muroid subfamily; they have been found in high mountains at more than 4,000 meters, and in mine shafts more than 500 meters below the earth's surface. (Carleton and Musser, 1984; Nowak, 1999)

Over the years, the subfamily Murinae has been consistently placed within the family Muridae. The relationships within the subfamily have a much more complicated history. Previously, the genera Acomys, Uranomys, and Lophuromys were classified within the Murinae (Carleton and Musser 1984, Musser and Carleton 1993), but recent molecular studies have shown that these three genera, in addition to Deomys, form a monophyletic group, and that this group (Deomyinae) plus Gerbillinae is the sister group to Murinae (Jansa and Weksler 2004, Steppan et al. 2004, Michaux et al. 2001). Furthermore, the Deomyinae + Gerbillinae clade and the Murinae are estimated to have diverged about 21 million years ago, in the lower Miocene (Steppan et al. 2004).

As for those genera that remain in the subfamily Murinae, sorting through their relationships is a complicated, ongoing task due to the incredible amount of diversity in the group and the short time span in which the murine radiation occurred. Watts and Baverstock (1995) resolved the subfamily into nine clades: a Phloeomys clade; a Micromys + Vandeleuria clade; a Millardia clade; a New Guinea clade including Macruromys, Pogonomys, and Anisomys; an African clade including Rhabdomys, Grammomys, and Arvicanthis; a Mus clade; an Apodemus clade; an Australasian clade including Hydromys, Mesembriomys, and Conilurus; and a southeast Asian clade, including Bandicota, Maxomys, and Rattus. Based on their data, the Australasian clade and the southeast Asian clade underwent the most recent divergence, whereas Phloeomys is basal to the other groups. However, other studies have suggested evolutionary relationships inconsistent with those of Watts and Baverstock (Jansa and Weksler 2004; Steppan et al. 2004), and none has come close to including all 126 murine genera in an analysis. It is generally agreed that a group of "Philippine old endemics", including Phloeomys and Batomys forms a clade separate from all other murines. This divergence is estimated to have occurred 12 million years ago, and a rapid radiation of the non-"old endemic" groups ensued over the last three million years (Steppan et al. 2004). Furthermore, the non-"old endemic" radiation gave rise to at least seven murine clades, some of which may correspond with the groupings of Watts and Baverstock, but the relationships among these clades have yet to be satisfactorily resolved. (Carleton and Musser, 1984; Jansa and Weksler, 2004; Michaux, Reyes, and Catzeflis, 2001; Musser and Carleton, 1993; Steppan, 2004; Steppan, 2005; Watts and Baverstock, 1994)

There is an incredibly diverse range of body types in this subfamily. Murines can be shrew-like, gerbil-like, vole-like, gopher-like, squirrel-like, mouse-like, and rat-like, with many variations on each body plan. Some are small and gracile, like tiny African pygmy mice (Mus minutoides), which are less than 9 cm long and weigh in at under 5 grams, and some are large and robust, like southern Luzon giant cloud rats (Phloeomys cumingi), which grow to over 48 cm long and weigh over 2 kg. Murines usually have prominent ears, and their tails can be long or short. The fur is smooth and silky, woolly, short and velvety, coarse and thin, or spiny. The tail is naked to bushy, and is prehensile or semi-prehensile in some species. The ears can be either scantily-haired or furry, and the soles of the feet are hairless. The fur may be various shades of brown and gray on the dorsal surface, and is usually white, buff, or grayish on the ventral surface. Some species have dorsal stripes. The tail is usually monocolored but is sharply bicolored in some. Polymorphism is present in some species, with two or more color morphs living in sympatry. Male murines have large ventral sebaceous glands. There are no cheek pouches. The feet are cursorially adapted in most, and can be either short and wide or long and narrow. In some species, the feet are webbed. The front feet each have four digits that bear claws plus a stubby thumb bearing a nail. All five digits on each hind foot bear claws in most genera. Some arboreal species have semiopposable thumbs.

The dental formula is 1/1, 0/0, 0/0, 3/3 = 16 in most murine genera. The incisors can be opisthodont, orthodont, or proodont. Most have ungrooved incisors. The molars are rooted and are not evergrowing. The molars range from brachydont to hypsodont, and the third molars are always smaller than the first and second molars. Most murines have three lingual cusps on the upper molars, giving a triserial cusp arrangement; there is always at least an anterolingual cusp on the second upper molars. In addition, the lower molars usually have labial cusplets. Murines vary widely in skull characteristics, and the diversity is so great that no synapomorphies of the skull can be identified, except of the lack of a sphenofrontal foramen or squamosoalisphenoid groove. A skeletal characteristic that all murine genera share is the presence of a prominent neural spine on the second thoracic vertebra. Diploid chromosome numbers for murines range from 25 to 68. (Carleton and Musser, 1984; Hubbard, 1972; Nowak, 1999)

Murines usually do not live more than a few months in the wild, and those that do rarely live to be three years old. In captivity, however, some murines may live nearly a decade. (Nowak, 1999)

Behavioral characteristics, like other traits, vary widely in the subfamily Murinae. Because they fill countless niches in a wide array of habitats, murines have also evolved a mind-boggling array of behaviors. There are murines that are terrestrial, arboreal, and aquatic, and those that are nocturnal, diurnal, and crepuscular. Some are territorial and solitary, others are social or colonial. Some have strict dominance hierarchies. Some are sedentary, others are migratory. Most murines have a regular spot where they seek shelter, which may be a nest in a tree or shrub, a burrow, a hollow tree, a crevice between two rocks, a crack in the wall of a house, or hundreds of other possibilities. The majority are cursorial, but there are murines that are specialized for hopping, climbing, swimming, and locomoting in all of these ways. (Nowak, 1999)

Murines 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, murines 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 murines. The range of murine perception often surpasses that of humans; for example, some murines can hear ultrasounds, as youngsters that have been separated from their mothers often emit ultrasonic calls, to which mothers quickly respond (Ehret 2005). In general, murine 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 murines, 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). Males of many territorial species demarcate their boundaries by scent-marking with their large ventral sebaceous glands. (Carleton and Musser, 1984; Ehret, 2005; Nowak, 1999; Thompson et al., 2004)

As a group, murines consume an astonishing array of food items, including (but not limited to) roots, grains, leaves, shoots, seeds, berries, nuts, fungi, fruits, insects, earthworms, arachnids, fish, small birds and eggs, turtles, lizards, frogs, mussels, carrion, and even household items such as glue, paste, and soap. Individual murine species range from dietary generalists that will eat just about anything to specialist herbivores and specialist carnivores. Many murine species cache their food in burrows or crevices for later use. (Nowak, 1999)

Murines 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 a large array of predators, murines 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. Murines 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. Murines tend to have neutral-colored coats that blend in with the natural backgrounds of their habitats, affording them some degree of camouflage. Finally, like most wild mammals, murines often bite viciously when attacked and may inflict enough surprise or damage that predators release them. (Cochran and Cochran, 1999; Nowak, 1999)

Murines 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 murine 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 murine 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 murines as hosts, including ticks and mites, fleas, lice, bot flies, nematodes, tapeworms, and trypanosomes. (Johnson, Pauw, and Midgely, 2001; Nowak, 1999; Roberts and Janovy Jr., 2000)

Although most murines have no direct impact whatsover on humans, those that do cause enough damage and suffering to give the entire group a bad name. Every year, 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 and Janovy Jr., 2000)

Murines have an immense positive economic impact on human populations. First, some murine species are kept as pets, and some are sold by pet stores as food for other types of pets, such as snakes and lizards. Also, murines 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 probably contributes greatly to the spread of disease), and some murine species are prized as food or for their pelts and hunted regularly. (Nowak, 1999)

The subfamily Murinae contains some of the most common species on Earth--the house mouse (Mus musculus) and Norway rat (Rattus norvegicus) come to mind--but it also contains a large number of species with small populations and restricted ranges. In fact, 41% of the species in this subfamily are on the IUCN's Red List of threatened species. This includes 20 critically endangered species, 41 endangered species, 66 vulnerable species, 13 near threatened species, 53 lower risk species, and 25 species that cannot be classified due to lack of information. Another 13 species are presumed to have gone extinct in recent years. The largest threat to most of these species is also the largest threat to the Earth's biodiversity overall: human-induced habitat loss and degradation. 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)