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Suborder: Anomaluromorpha (scaly-tailed squirrels and springhares)
Suborder: Castorimorpha (beavers, gophers, kangaroo rats, pocket mice, and relatives)
Suborder: Hystricomorpha (hystricognath rodents)
--Infraorder: Ctenodactylomorphi (gundis)
--Infraorder: Hystricognathi (hystricognath rodents)
Suborder: Myomorpha (mice, rats, gerbils, jerboas, and relatives)
--Superfamily: Muroidea (mice, rats, gerbils, and relatives)
Suborder: Sciuromorpha (squirrels, dormice, and relatives)
Rodents are the largest order of mammals (40% of all mammals), with anywhere from 1500-2000 species in 30 different families. Rodents are adapted and specialized for their various environments, but all share one common characteristic: a pair of upper and lower incisors used for gnawing.  Rodents are common pets and are used in scientific studies. Unfortunately, rodents also carry diseases and cause the loss of billions of dollars in crops every year. 
All rodents are characterized by a single pair of upper and lower incisors especially adapted for gnawing. The jaws also contain a toothless gap known as a diastema, because rodents have no canines, although they have some premolars or molars. Rodent incisors are almost completely covered with enamel and have no roots, so they are constantly growing. While gnawing, a rodent's incisors wear away at the softer part of the tooth called dentine, leaving the enamel edge sharp.
Another prominent characteristic is the extensive use of the masseter, or main muscle used for chewing. Rodents can even be classified into various groups by their use of this muscle.  The skull and jaws are modified to enable various forms of gnawing. Male rodents have a bone called the baculum, or penis bone.  Rodent feet have five toes, having claws on four out of the five. Rodents range in size from 5 mg (pygmy mice) to up to 70 kg (capybara). Some rodents have internal or external mouth pouches for carrying food or seeds.  They are also known for having well-developed ears. 
Rodents reproduce by internal fertilization, and have a great variety of mating rituals. The gestation period differs from species to species. The guinea pig for example, has a gestation period of 68 days. Its young are born fully haired, have developed teeth, eyes open, and can eat solid food. The hamster however, has a gestation period of only 16 days, but the young are born with eyes and ears closed, partly haired, underdeveloped limbs, and only partially developed teeth. They also have to nurse from the mother until the teeth are fully grown.  Rodents have a high rate of reproduction , so their litters tend to be larger and more frequent than most mammals. Rodents are part of Eutheria, a subclass of Mammalia containing placental mammals. The placenta is an "internal structure that is formed when the embryo's tissues join with tissues from within the mother's body." The placenta gives nourishment from the mother to the embryo during gestation and allows a longer time for the young to fully develop and grow. (Miller, Levine Prentice Hall Biology pg 829)
Rodents are native to every continent except Antarctica, living in a number of different habitats. Capybaras and muskrats prefer an aquatic lifestyle, while others live in desert climates, such as jerboas and kangaroo rats. Gophers and mole rats burrow underground, but squirrels live in the treetops.  Flying squirrels are even semi-aerial, able to glide through the trees.  Some rodents are solitary and live in small numbers. Others however, like prairie dogs, live in large clans or groups.  Most rodents are herbivores, eating all sorts of plants, seeds, roots, flowers, leaves, and stems. There are also some omnivorous and carnivorous rodents that eat insects and other invertebrates. 
The majority of rodents are nocturnal, their activity peaking during the night rather than during the day.  During the winter, many rodents hibernate, especially if they live in extreme weather conditions. Rodents that live in hotter habitats can also estivate over the warmer summer months.  Hibernation can last for different lengths of time, some species hibernating for up to nine months. Rodents are prey for a number of carnivorous or omnivorous mammals, and are an important part of the food chain.  Because rodents are eaten in such large numbers, they have a high rate of reproduction and multiple ways to defend themselves. Rodents are typically very small and quick, fleeing to avoid capture.  Some rodents, such as the porcupine, have quills or use teeth to defend themselves against hungry predators. Rodents' keen sense of smell and hearing also enable them to flee at the first sign of danger. 
Impact on Humans
Rodents have both negative and positive impacts on humans. Many humans consider rodents as pests and vermin. They carry parasites and diseases that can harm both other animals and humans.  The Bubonic plague, Hanta fever, and typhus are all directly related to rodent carriers. Every year, billions of dollars are lost in crops due to rodents.  Stored seed or grain is also a problem because it attracts rodents in the winter. Prairie dog burrows have been known to hurt horses who fall into the holes, and beaver dams can cause flooding and river blockage. 
Despite all of the problems, rodents have also been very useful and helpful. Some rodents are sources of food in parts of the world.  Other rodents are valued for their fur, such as the beaver and muskrat. Beaver dams can prevent erosion, and albino rats and mice are used in biological research and experiments. In recent years, the Gambian and African giant pouched rat has been trained to sniff out land mines worldwide.  Some rodents are kept as pets, like gerbils, guinea pigs, and white mice.  Overall, rodents are both advantageous and detrimental to humans.
- Introduction to the Rodentia by UCMP (University of California and Museum of Paleontology)
- Order Rodentia Phil Myers (author), Museum of Zoology, University of Michigan
- Rodentia by americazoo.com
- Rodent by MSN Encarta
- The Rodents by geocities.com
- Comparison of Rodent Reproduction by MAABRE (Mid-Continent Association for Agriculture, Biomedical Research and Education)
- Gambian rodents risk death for bananas by New York Times, www.theage.com
- Rodent General Biology by Southern Illinois University
- Miller, Levine. Prentice Hall Biology pg 829.