Superfamily Muroidea
mice, rats, gerbils, and relatives

The superfamily Muroidea includes most of the familiar rats and mice, but it also encompasses an enormously diverse array of other rodents. Currently there are 1517 recognized species and 310 genera of muroid rodents. These are divided among six families: Platacanthomyidae (Oriental dormice), Spalacidae (zokors, blind mole-rats, bamboo rats, root rats), Calomyscidae (mouse-like hamsters), Nesomyidae (climbing mice, African rock mice, Malagasy rats and mice, swamp mice, pouched rats, white-tailed rat), Cricetidae (hamsters, voles, lemmings, New World rats and mice), and Muridae (true mice and rats, gerbils). (Musser and Carleton, 2005)

Members of the superfamily Muroidea can be found on all continents except Antarctica and on many oceanic islands. (Nowak, 1999)

Muroid rodents 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. (Nowak, 1999)

The superfamily Muroidea is thought to be most closely related to the superfamily Dipodoidea, the jumping mice, birch mice, and jerboas. These two groups are estimated to have diverged about 50 million years ago, in the early Eocene. The relationships among the families within Muroidea have long been under debate; their taxonomic histories are described in detail in the accounts for those families. Some authors have placed all muroid rodents within a single family, Muridae (Musser and Carleton 1993). However, recent molecular evidence has justified classifying Muridae and five other families within the group Muroidea (Steppan et al., 2004). (Carleton and Musser, 1984; Carleton, 1984; Musser and Carleton, 1993; Steppan, Adkins, and Anderson, 2004)

A number of characters link most muroids. Not surprisingly, even the most basic characters are 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 muroids.

In the skull of muroids, 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 that inserts on the lower jaw, and narrowed in its lower region, through which pass 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. From it arise other parts of the same muscle (the masseter) that passes through the infraorbital foramen. 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. Following the incisor is a diastema. Canines and premolars are never present. No more than three molars 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.

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). Other external features (ears, eyes, tail, pelage, etc.) are extremely variable. To compound this variability, some populations of some species are polymorphic, and some exhibit sexual dimorphism in body size. (Carleton and Musser, 1984; Hubbard, 1972; Nowak, 1999)

Most muroids face a large array of predators and put all of their energy into a high reproductive output early in life, and therefore do not live more than a year or two in the wild. Captivity often extends the lifespan by several years. (Nowak, 1999)

Muroid rodents run the gamut of behaviors, from nocturnal to diurnal, from arboreal to fossorial, from solitary to colonial. There are muroid rodents specialized for climbing, burrowing, swimming, and hopping, and there are those that are generalists and are fairly good at doing all of the above. (Nowak, 1999)

To avoid the many predators that they face, and to find food and mates, muroid rodents have evolved acute visual, acoustic, tactile, and chemical senses, but the relative importance of these for each species varies widely. The means by which muroid rodents communicate also varies between species. A common theme in mammalian communication is the use of pheromones, which are used widely by muroid rodents to send and receive signals about an individual's status. In addition, some communicate using sounds (including ultrasounds) or vibrations. (Ehret, 2005; Johnston, 2003; Nowak, 1999; Smith, 1972; Thompson et al., 2004)

Muroid food habits range from true omnivores to generalist herbivores to specialists on insects, earthworms, subterranean fungi, and even aquatic invertebrates. Many species, especially herbivorous species, store their surplus food for later use. (Nowak, 1999)

Muroid rodents, as a group, have predators belonging to nearly every class of vertebrates, including birds and other reptiles, amphibians, fish, and other mammals. To avoid their numerous predators, muroid rodents have evolved strategies of hiding, running, swimming, hopping, climbing, and biting. There are even those that, when grabbed, lose their tails and buy themselves enough time to escape. One unique species, Lophiomys imhausi, is aposematic, exudes a musky odor, and may be a porcupine mimic through the use of stiff, erectile hairs. (Carleton and Musser, 1984; Cochran and Cochran, 1999; Nowak, 1999; Shargal et al., 1999)

Some muroid rodents may be essential ("keystone") species in maintaining the health of forests, through their role in spreading mycorrhizal fungi or dispersing seeds. Others affect the rate of forest succession by preying on tree seedlings. Some species are important pollinators. Others dig tunnels, and in doing so, create habitat for other species and aerate the soil. Many species are a vital food source for a wide range of predators, and muroids as a group support many different kinds of parasites, such as ticks and mites, fleas, lice, bot flies, nematodes, tapeworms, and trypanosomes. Finally, a few muroid species are commensal with humans, inhabiting cities and towns and relying on human-produced waste to survive. (Johnson, Pauw, and Midgley, 2001; Manson, Ostfeld, and Canham, 2001; Roberts, Janovy, and Jr, 2000; Zhang, Zhang, and Liu, 2003)

Some muroid species cause millions of dollars of damage to agricultural lands and stored foods. Several are pests that destroy household goods, cause structural damage, and even start fires by gnawing on electrical wires. Others are the vectors or reservoirs of a number of diseases that have periodically devasted human populations (and continue to do so). (Nowak, 1999)

Many muroid species are beneficial to man. Some are important biological controls of pest insects. Some are popular pets. Others are hunted for their meat, their skins, or their bones (which may be used in traditional medicine). And a few species play an essential role in medical research that has been enormously beneficial to human populations. (Nowak, 1999; Zhang, Zhang, and Liu, 2003)

Almost 26% of muroid species are on the IUCN Red List of Threatened Species. This includes 32 critically endangered species and 70 endangered species. Many of the threatened muroid species are endemic, and their restricted ranges render them especially vulnerable to habitat destruction and fragmentation, the two main threats to this and many other taxonomic groups. Few steps have been taken to save threatened muroid species; they are not particularly charismatic or popular with the public and in many cases there is simply not enough known about them to know where to begin. (IUCN, 2004)