RODENTS: Rodentia

PHYSICAL CHARACTERISTICS

Rodents make up the largest group of mammals, representing approximately 43 percent of all mammalian species. Families in the order Rodentia include rats, mice, porcupines, hamsters, beavers, squirrels, chipmunks, lemmings, muskrats, and guinea pigs (rabbits are not rodents). These families range in size from the pygmy mice, which are 4.7 inches long (12 centimeters) and weigh 0.1 ounces (4 grams), to the capybara, which is 39.4 inches long (100 centimeters) and can weigh 110 pounds (50 kilograms). Most rodents are relatively small animals, such as mice, rats, and squirrels.

While there is a broad range of characteristics among the families, the feature that sets rodents apart from other family members is their teeth. Rodents have one pair of upper incisors (the chisel-shaped teeth at the front of the mouth), and one pair of lower incisors. These teeth grow continually throughout their life. The outer surfaces of the incisors have a thick enamel (hard white substance) layer. Behind the incisors is a large gap in the tooth rows. There are no canines, spade-shaped teeth located next to the incisors. Typically there are only a few molars at the rear of the jaws. The number of teeth rarely is more than twenty-two.

The name Rodentia comes from the Latin verb rodere meaning to gnaw, a name suitable for a rodent that is constantly gnawing! Rodents' incisor teeth grow throughout their life and they grind their incisor teeth together to wear them down. If for some reason the rodent is unable to wear its incisors down, the tips may grow past each other and continue to grow outward into spiral. This may result in the upper teeth piercing the roof of the mouth, and the lower teeth growing upward in front of the nose, which could kill the animal.

In general, rodents have a compact body with short legs. They typically have four to five digits on each of the front feet and three to five digits on the rear feet. Generally, the sole of the foot is bare. Some rodents, such as hamsters and pocket gophers, have cheek pouches, which allow the animals to store and transport food. The tails of some rodent species break off when these animals are caught by the tail, which allows them to escape. The tail will partially grow back.

Other physical characteristics of rodents vary widely depending upon the species and where it lives. For example, rodents that live in the desert, such as American kangaroo rats, Australian hopping mice, and north African jerboas, have long, narrow hind legs and feet with a long tail used for hopping over the sandy desert floor. They all have well-developed hearing, small front limbs, and pale coloration. Animals that live in and around the water, such as the capybara and beaver, may have webbed or partially webbed feet and tails modified for swimming.

GEOGRAPHIC RANGE

Rodents are found in all parts of the world, including the Arctic tundra, desert, and oceanic islands. About 70 percent of all rodents are rats and mice, and these animals are found on every continent.

HABITAT

The habitats of rodents are varied and numerous, from arid (extremely dry) deserts to the arctic tundra. There are rodents that live predominantly underground, others that live on land, and others are primarily arboreal (living in trees). Some species spend most of their life in the water, while others live in the desert. Some live close to humans in urban areas and even houses, while others make their home deep inside wetlands and rainforests. Rodents can be found in almost every habitat and on every continent except Antarctica.

DIET

All the families of rodents eat a wide range of foods. Most rodents are herbivorous, plant eaters, eating a wide range of plant materials, including seeds, stems, leaves, roots, and flowers. Many of these species eat primarily seeds. Some species, such as the grasshopper mouse, eat insects and spiders. Other species, such as the Australian water rat, are primarily carnivorous (meat-eating), preying (eating animals for food) on small fish, frogs, and mollusks. Many are to some degree omnivorous, eating both plants and animals. Still others have highly specialized meals, eating only a few species of invertebrates, animals without a backbone, or fungi.

BEHAVIOR AND REPRODUCTION

Rodents show a wide range of lifestyles and habits, depending upon the family and species. There are rodents that form burrows (holes or tunnels), such as gophers and moles; those that live in trees, such as the commonly called flying squirrels; rodents that spend most of their time in water, such as the capybara; and those that are specialized to life in the desert, such as kangaroo rats and jerboas.

Many rodents are social animals, living in large groups and interacting with one another frequently. Prairie dogs, naked mole-rats, and ground squirrels all live in these large colonies (groups). Other rodents live in smaller colonies. The beaver lives in a colony made up of the adult male and female, and their offspring. Each colony lives in a specific territory.

The prairie dog, for example, lives in a set area that can contain hundreds of these small animals (they look similar to squirrels, not dogs). These colonies or towns are broken up into certain neighborhoods. The prairie dogs post guards, they babysit and they help build one another's homes. There is a great deal of playing, mutual grooming, and vocal communication among the prairie dogs.

Some rodents are solitary, such as porcupines, pocket gophers, and pocket mice. Many desert species are solitary. Some of these species that burrow, dig, will construct and live in their own burrow system. However, during the mating season there may be more than one individual, or a mother and her offspring may live together.

Most rodents are active throughout the year. Some species, such as ground squirrels, may hibernate for several months. Species communicate with one another using sounds, smells, and sights. For example, squeaks, grunts, and calls can be used as alarm calls in mating and when a parent is searching for its young.

Many rodents have large numbers of offspring, which is one of the primary reasons they make up the largest group of mammals. Rodent reproduction can be divided into two forms. One group of families has a short gestation (pregnancy) period, produces multiple litters per year, and has large numbers of helpless offspring. Gestation periods can range from seventeen to forty-five days and the number of litters can be up to four. Rodents in this group include mice, rats, and pocket gophers. The other group of families has longer gestation periods (60 to 238 days), fewer litters per year (generally one to two), and have a relatively fewer number of offspring.

The mating system of rodents depends upon the species. A few species of rodents are monogamous (muh-NAH-guh-mus), such as the Patagonian mara, which forms male-female pairs that can last for multiple mating seasons. Other species have a harem-based (HARE-um based) mating system, one male with a set group of females for the mating season. Many rodents are promiscuous (prah-MISS-kyoo-us), meaning they mate randomly.

RODENTS AND PEOPLE

Rodents play a vital role in the ecosystem. They serve as the prey for many animals and some animals will use their burrows for shelter and protection.

People have caused the loss of population of many species of rodents by destroying their natural habitat, harming them directly, or introducing species that prey on rodents. Many species of rodents are considered pests and even dangerous to humans. Rodents cost billions of dollars in lost crops each year, eating the grain stored during the winter and the seeds of plants. Beavers can cause destruction by damming up creeks, causing water to back up into areas where its not wanted.

THE APPEARANCE OF MODERN RATS

Fossils show that the first rodents began scampering about an estimated fifty-four million years ago in Asia and North America. These original rodents were themselves descendants of rodent-like ancestors called anagalids, which also gave rise to the rabbits. It was not until about five million years ago that the modern Muridae family of rodents came on the scene. The murids (MYOO-rids) now make up more than half of all rodent species, including rats, mice, and hamsters. These mammals have flourished due to multiple, large litters and their ability to adapt quickly to environmental changes.

Rats carried the fleas that caused the plagues of Europe. Rats and mice help spread other deadly diseases as well, such as bubonic (byoo-BON-ik) plague and typhus (TIE-fus).

Rodents are important as sources of food for many people. Roasted, stuffed, or fried guinea pig, for example, is a popular dish in Ecuador, Peru, and other South American countries. In many parts of the world they have an economical importance for their fur, such as the chinchilla of South America, a rodent almost extinct in the wild but thriving in captivity.

Rodents such as mice and rats are also used extensively in medical research because their bodily processes are similar to humans' and they have a rapid reproduction rate. They are used to study many diseases and test medicines. People also use these and other rodents, such as guinea pigs, to test the safety of cosmetic and human food products. Many people also keep the small and "cute" rodents as pets.

PLAGUE OF RATS

In 1347 a ship from Caffa, on the Black Sea, came ashore at Messina, Sicily. Along with its goods, the ship was also carrying flea-infested rats. Most people on board were already dead, and the ships were ordered out of the harbor, but it was too late—the plague, or Black Death, had reached Europe. The disease, which may have begun in Asia or Egypt, killed within days of infection and the European population was quickly decimated. (The nursery rhyme "Ring Around the Rosy" is traced to the plague's rose-colored wounds). By 1349, the Black Death had swept through nearly every town and village in Britain. It is estimated that the plague killed a quarter of the European population, about twenty-five million people.

At that time, people blamed the plague on many causes, including fumes, God's wrath, and the unlucky alignment of planets. It is now known that the plague was caused by a bacterium that lived in the stomach of fleas. These fleas mainly infect rodents, particularly black rats. When the flea bites, either a rat or a human, it spits some bacteria out into the bite wound. The bacteria were passed from one rat to another by these fleas, causing the bacterium to spread. Large numbers of infected rats died and the fleas began biting humans.

CONSERVATION STATUS

The IUCN lists 669 species of rodents under varying degrees of threat and endangerment (facing varying risks of extinction in the wild) as of 2004. There are 32 species that are listed as Extinct (died out). Loss of habitats and the introduction of species are the two main reasons for the loss of populations.

FOR MORE INFORMATION

Books:

Clutton-Brock, Juliet, and Don E. Wilson, eds. Smithsonian Handbooks: Mammals. New York: Dorling Kindersley Publishing, 2002.

Jordan, E. L. Animal Atlas of the World. Maplewood, NJ: Hammond Incorporated, 1969.

Macdonald, David, ed. The Encyclopedia of Mammals. New York: Facts on File Publications, 1984.

Nowak, Ronald M. Walker's Mammals of the World, 5th ed. Baltimore and London: The Johns Hopkins University Press, 1991.

Nowak, Ronald M. "Order Rodentia." Walker's Mammals of the World 5.1 Online. Baltimore: Johns Hopkins University Press: 1997. http://www.press.jhu.edu/books/walkers_mammals_of_the_world/rodentia/rodentia.html (accessed on July 8, 2004).

Periodicals:

Adams, Jonathan. "A Nose for Trouble (trained rats)." Newsweek International (January 12, 2004): 42.

Dickson-Ramos, Holly. "The World's Largest Rodent." Faces: People, Places, and Culture (January 2004): 36.

Marchand, Peter J. "A Squirrel's Glide to a Long Life." Natural History 110, no. 8 (October 2001): 18.

Ostfeld, Richard S. "Little Loggers Make a Big Difference: The Tastes of Two Small Rodents—the Meadow Vole and the White-Footed Mouse— Can Determine What Trees Grow in a Forest." Natural History (May 2002): 64.

Travis, John. "Mining the Mouse: A Rodent's DNA Sheds Light on the Human Genome." Science News (February 22, 2003): 122.

"World's Largest Rodent Risks Extinction." United Press International (September 20, 2002): 1008263 .

Web sites:

Myers, P. "Order Rodentia (Rodents)." Animal Diversity Web. http://animaldiversity.ummz.umich.edu/site/accounts/information/Rodentia.html (accessed on July 8, 2004).

Davis, William B., and David J. Schmidly. "Order: Rodentia." The Mammals of Texas—Online Edition.http://www.nsrl.ttu.edu/tmot1/ordroden.htm (accessed on July 8, 2004).

Lockwood, Burleigh. "Order: Rodentia." The Chaffee Zoo. http://www.chaffeezoo.org/animals/rodentia.html (accessed on July 8, 2004).

"Introduction to the Rodentia." University of California Berkeley, Museum of Paleontology. http://www.ucmp.berkeley.edu/mammal/rodentia/rodentia.html (accessed on July 8, 2004).

"Rat and Mouse FAQs." The Rat & Mice Club of America. http://www.rmca.org/Resources/faqs.htm (accessed on July 8, 2004).