MuridaeOld World mice and rats, gerbils, whistling rats, and relatives

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Murids include most of the familiar rats and mice, but the family also encompasses an enormously diverse array of other rodents. Here, we follow recent authorities in treating murids as members of a single, very large family with a number of subfamilies. The systematic relationships of these groups among themselves and with other rodents has proved to be an extremely difficult problem, one that is by no means resolved.

A number of characters link most murids. 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 murids.

In the skull of murids, 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. Follow 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.

Members of the family can be found on all continents except Antarctica and on many oceanic islands. They 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. Their food habits range from true omnivores to specialists on earthworms, subterranean fungi, even aquatic invertebrates. Their importance to mankind cannot be overstated. Some species cause millions of damage to agricultural lands and stored foods. Others are the vectors or reservoirs of a number of diseases that have periodically devasted human populations (and continue to do so). On the other hand, many species are beneficial to man. Some are important biological controls of pestiferous insects. Others may be essential ("keystone") species in maintaining the health of our forests, through their role in spreading mycorrhizal fungi or dispersing seeds. Some even provide a source of meat. And a few species play an essential role as "domestic animals" used in medical research that has been enormously beneficial to mankind.

Around 1150 living species of murid rodents have been described, but surely many more remain to be discovered. These are placed in around 260 genera, which are distributed among 17 subfamilies.

Reference: Anderson, S. and J. K. Jones, Jr., 1984. Orders and Families of Recent Mammals of the World. John Wiley and Sons, New York. 686pp.

Contributors

Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.

Glossary

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.

chemical

uses smells or other chemicals to communicate

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.

motile

having the capacity to move from one place to another.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate