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).
Members of the superfamily Muroidea can be found on all continents except Antarctica and on many oceanic islands.
Biogeographic Regions: nearctic (Introduced , Native ); palearctic (Native ); oriental (Native ); ethiopian (Native ); neotropical (Introduced , Native ); australian (Native ); oceanic islands (Introduced , Native )
Other Geographic Terms: holarctic ; cosmopolitan ; island endemic
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.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry ; polymorphic
Sexual Dimorphism: sexes alike; female larger; male larger
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.
Habitat Regions: temperate ; tropical ; polar ; terrestrial
Terrestrial Biomes: tundra ; taiga ; desert or dune ; savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest ; mountains
Aquatic Biomes: lakes and ponds; rivers and streams
Wetlands: marsh ; swamp ; bog
Other Habitat Features: urban ; suburban ; agricultural ; riparian
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.
Foraging Behavior: stores or caches food
Primary Diet: carnivore (Eats terrestrial vertebrates, Piscivore , Eats eggs, Insectivore , Eats non-insect arthropods, Molluscivore , Scavenger ); herbivore (Folivore , Frugivore , Granivore , Lignivore); omnivore ; mycophage
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.
Ecosystem Impact: disperses seeds; pollinates; creates habitat; soil aeration ; keystone species
Species Used as Host:
- humans (Homo sapiens)
- ticks and mites (Acari)
- fleas (Siphonaptera)
- lice (Anoplura)
- bot flies (Sarcophagidae)
- nematodes (Nematoda)
- tapeworms (Cestoda)
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.
- birds (Aves)
- reptiles (Reptilia)
- amphibians (Amphibia)
- fish (Actinopterygii)
- mammals (Mammalia)
Anti-predator Adaptations: mimic; aposematic ; cryptic
Life History and Behavior
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.
Communication Channels: visual ; tactile ; acoustic ; chemical
Other Communication Modes: pheromones ; scent marks ; vibrations
Perception Channels: visual ; tactile ; acoustic ; ultrasound ; vibrations ; chemical
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.
Muroids have monogamous, polygynous, and polygynandrous mating systems. Most commonly, they are polygynandrous, with males and females each having multiple mates over the course of a breeding period.
Mating System: monogamous ; polygynous ; polygynandrous (promiscuous)
Given the incredible diversity of this group, it is nearly impossible to generalize about the life-history characteristics of its members. The "typical" muroid species is characterized by a "fast" life: high reproductive output at an early age and a high mortality rate. The high reproductive output is made possible in many species by a postpartum estrus, which allows females to become pregnant again immediately after giving birth. Sometimes implantation of the embryos is delayed until after a female stops lactating, and in some species, the act of mating itself induces ovulation.
Key Reproductive Features: semelparous ; iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; induced ovulation ; fertilization (Internal ); viviparous ; delayed implantation ; post-partum estrous
Muroid mothers, like all female mammals, provide their young with milk until the young can eat solid food. Many muroid females build nests in which they raise and care for their young, which range from altricial to precocial. Male parental care is rare in this group, but it does occur in a few species. In most muroid species, the young disperse soon after they are weaned, but in a few, they stay with their parents for more than one breeding season.
Parental Investment: altricial ; precocial ; pre-fertilization (Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Male, Female); pre-independence (Provisioning: Female, Protecting: Male, Female); post-independence association with parents
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.
Relevance to Humans and Ecosystems
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).
Negative Impacts: injures humans (carries human disease); crop pest; household pest
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.
Positive Impacts: pet trade ; food ; body parts are source of valuable material; source of medicine or drug ; research and education; controls pest population
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