Rodents (Rodentia) are a group of mammals that includes squirrels, dormice, mice, rats, gerbils, beavers, gophers, kangaroo rats, porcupines, pocket mice, springhares, and many others. There are more than 2000 species of rodents alive today, making them the most diverse of all mammal groups. Rodents are a widespread group of mammals, they occur in most terrestrial habitats and are only absent from Antarctica, New Zealand, and a handful of oceanic islands.
Rodents have teeth that are specialized for chewing and gnawing. They have one pair of incisors in each jaw (upper and lower) and a large gap (called a diastema) located between their incisors and molars. The incisors of rodents grow continuously and are maintained through constant use—grinding and gnawing wears away the tooth so that is always sharp and remains the correct length. Rodents also have one or multiple pairs of premolars or molars (these teeth, also called cheek teeth, are located towards the back of the animal's upper and lower jaws).
Rodents eat a variety of different foods including leaves, fruit, seeds, and small invertebrates. The cellulose rodents eat is processed in a structure called the caecum. The caecum is a pouch in the digestive tract that houses bacteria that are capable of breaking-down tough plant material into digestible form.
Rodents often play a key role in the communities in which they live because they serve as prey for other mammals and birds. In this way, they are similar to hares, rabbits, and pikas, a group of mammals whose members are also serve as prey for carnivorous birds and mammals. To counterbalance the intense predation pressures they suffer and to maintain healthy population levels, rodents must produce large litters of young every year.
The key characteristics of rodents include:
- one pair of incisors in each jaw (upper and lower)
- incisors grow continuously
- incisors lack enamel on the back of the tooth (and are worn down with use)
- large gap (diastema) behind incisors
- no canine teeth
- complex jaw musculature
- baculum (penis bone)
Rodents are classified within the following taxonomic hierarchy:
Rodents are divided into the following taxonomic groups:
- Hystricognath rodents (Hystricomorpha) - There are about 300 species of hystricognath rodents alive today. Members of this group include gundis, Old World porcupines, dassie rats, cane rats, New World porcupines, agoutis, acouchis, pacas, tucotucos, spiny rats, chinchilla rats, nutrias, cavies, capbaras, guinea pigs, and many others. Hystricognath rodents have a unique arrangement of their jaw muscles that differs from all other rodents.
- Mouse-like rodents (Myomorpha) - There are about 1,400 species of mouse-like rodents alive today. Members of this group include mice, rats, hamsters, voles, lemmings, dormice, harvest mice, muskrats, and gerbils. Most species of mouse-like rodents are nocturnal and feed on seeds and grains.
- Scaly-tailed squirrels and springhares (Anomaluromorpha) - There are nine species of scaly-tailed squirrels and springhares alive today. Members of this group include the Pel's flying squirrel, long-eared flying mouse, Cameroon scaly-tail, East African springhare, and the South African springhare. Some members of this group (notably the scaly-tailed squirrels) have membranes that stretch between their front and hind legs that enable them to glide.
- Squirrels-like rodents (Sciuromorpha) - There are about 273 species of squirrel-like rodents alive today. Members of this group include beavers, mountain beavers, squirrels, chipmunks, marmots, and flying squirrels. Squirrels-like rodents have a unique arrangement of their jaw muscles that differs from all other rodents.
Hickman C, Roberts L, Keen S, Larson A, l'Anson H, Eisenhour D. Integrated Principles of Zoology 14th ed. Boston MA: McGraw-Hill; 2006. 910 p.