Muroideamice, rats, gerbils, and relatives

Diversity

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)

Geographic Range

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

Habitat

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)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams

Physical Description

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)

  • Sexual Dimorphism
  • sexes alike
  • female larger
  • male larger

Reproduction

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. (Nowak, 1999)

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. (Carleton and Musser, 1984; Nowak, 1999)

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. (Gubernick and Teferi, 2000; Schradin and Pillay, 2003; Sommer, 2000)

  • 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

Lifespan/Longevity

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)

Behavior

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)

Communication and Perception

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)

Food Habits

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)

Predation

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)

Ecosystem Roles

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, et al., 2001; Manson, et al., 2001; Roberts and Janovy, Jr, 2000; Zhang, et al., 2003)

Species Used as Host
Commensal/Parasitic Species

Economic Importance for Humans: Positive

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, et al., 2003)

Economic Importance for Humans: Negative

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)

  • Negative Impacts
  • injures humans
    • carries human disease
  • crop pest
  • household pest

Conservation Status

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)

  • IUCN Red List [Link]
    Not Evaluated

Contributors

Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor, Allison Poor (author), University of Michigan-Ann Arbor, Tanya Dewey (editor), Animal Diversity Web.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

acoustic

uses sound to communicate

agricultural

living in landscapes dominated by human agriculture.

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

aposematic

having coloration that serves a protective function for the animal, usually used to refer to animals with colors that warn predators of their toxicity. For example: animals with bright red or yellow coloration are often toxic or distasteful.

arboreal

Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

bog

a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.

carnivore

an animal that mainly eats meat

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

crepuscular

active at dawn and dusk

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

delayed implantation

in mammals, a condition in which a fertilized egg reaches the uterus but delays its implantation in the uterine lining, sometimes for several months.

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

drug

a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

food

A substance that provides both nutrients and energy to a living thing.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

fossorial

Referring to a burrowing life-style or behavior, specialized for digging or burrowing.

frugivore

an animal that mainly eats fruit

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

holarctic

a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.

induced ovulation

ovulation is stimulated by the act of copulation (does not occur spontaneously)

insectivore

An animal that eats mainly insects or spiders.

internal fertilization

fertilization takes place within the female's body

introduced

referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.

island endemic

animals that live only on an island or set of islands.

iteroparous