The order Rodentia is the largest mammalian order and is divided into three suborders: Sci-uromorpha, which contains the squirrellike rodents; Myomorpha, or mouselike rodents; and Hystricomorpha, or porcupinelike rodents. The word rodent means “gnawing animal.” These mammals were perhaps so named because their large incisors and their manner of eating are their most obvious characteristics.
Lagomorphs were once thought to constitute a suborder of Rodentia because of their long, large incisors, but they are now treated as a separate order, called Lagomorpha. The order contains two families: the Leporidae, the hares and rabbits, of which there are some 50 species, and the Ochotonidae, the pikas, or mousehares, with 14 species. Rodents and lagomorphs are similar in appearance and habits, but differ in certain aspects of anatomy. They are all relatively small animals and are found in most parts or the world.
General features of rodents
The two long pairs of chisel-shaped incisors in each jaw, characteristic of all rodents, project from the mouth and are used to gnaw on hard foods, such as nuts and wood. These teeth, which are segments of a circle, grow continuously and must constantly be worn down at the tips. Rodents have no canines. Because these animals feed on tough materials that are difficult to break down, they have a highly developed cecum -a branch of the gut at the junction of the small and large intestines that aids digestion. Living in the stomach are bacteria that break down cellulose, which then can be absorbed into the rodent’s body.
The limbs of rodents are constructed for plantigrade locomotion. There are usually five digits on the forelimbs and three to five on the hind limbs. The digits on the forelimbs are used for holding food. Hind limbs may be adapted for running, jumping, climbing, or swimming (in which case the three to five toes are webbed). The females are capable of bearing numerous young at one time. Some species can reproduce almost without interruption throughout the year. The uterus is usually divided, although in some species it is double. Most newborn rodents emerge both blind and naked and are entirely dependent on the mother for several days. But the guinea pigs (Caviidae); spiny mice; and nutria, or coypu, have precocious young that are active at birth and can soon take care of themselves.
Squirrels, marmots, gophers, and beavers make up the suborder Sciuromorpha, in which there are seven families. These animals are found everywhere except in Australia, Asia, Madagascar, and parts of South America. They have a variety of habitats. Some, such as the squirrels (Sciuridae), live in trees. Others, such as gophers, burrow underground. Still others, such as beavers (Castoridae), live a semiaquatic existence.
The squirrels are particularly noticeable for their bushy tails and large eyes and ears. They are agile and graceful, and they move equally well on the ground and in trees. The ground squirrels, such as prairie dogs, generally make complex underground tunnels and chambers, some of which extend for hundreds of feet. Ground squirrels sometimes use the nests or burrows for storing food, which they eat when food is scarce. The marmots and woodchucks are also ground-dwelling species of squirrels. Like the prairie dogs, they live in large social groups. They hibernate during the winter in their underground chambers and live off reserves of fat built up during summer feeding.
Beavers are the only members of the family Castoridae and are distributed throughout the Northern Hemisphere. Primarily waterdwelling rodents, they have a dense undercoat that is covered in coarse guard hairs. Beavers are excellent swimmers and they use their broad, flat tail as a rudder while they paddle with their webbed feet.
The springhaas, or Cape jumping hare, is the only species of the family Pedetidae. In most respects, it looks like a small version of the kangaroo, except that it has a bushy tail. It uses its long hind legs for jumping, but it also can move on all fours, like a rabbit Like the jumping hare, the kangaroo rats in the family Heteromyidae are nocturnal burrowing animals, and their hind limbs are also modified for jumping.
The brightly colored scaly-tailed squirrels (Anomaluridae) and the North American flying squirrels are the only airborne rodents. They glide from tree to tree by means of a membrane that joins their limbs, called a patagium.
The suborder Myomorpha contains more than 1,000 species, arranged in nine families. Their mouselike appearance and lack of premolars distinguish them from the sciuromorphs. The family Cricetidae contains about 570 species, including hamsters, lemmings, voles, and New World rats and mice. They live mainly above the ground, although some burrow and there are some semiaquatic species. Many have a thickset body and a short tail and legs.
The puna mouse is the only mammal to live at altitudes of about 16,400 feet (5,000 meters). It is found in the altiplana region of the Andes and is about 6 inches (15 centimeters) in length. The more familiar common hamster, which occurs across Europe into the Russian steppes, is a nocturnal species and may hibernate for a short period during winter. It stores food throughout the summer, some of which it carries to the winter burrow in its cheek pouches. This burrow is divided into separate compartments, for nesting, food storage, and body wastes. Each type of food, such as corn or potatoes, is stored in a different food compartment Hamsters may store up to 200 pounds (90 kilograms) of food to see them through the winter. In summer, they dig another burrow, which they use for nesting. The golden hamster exists today only as a pet or laboratory animal.
Another member of this family, the true lemming, lives in the Northern Hemisphere, particularly in arctic regions. It is stocky, with very short ears and tail, and burrows in the soil during summer and under the snow in winter. When food supplies are plentiful, lemmings breed rapidly. But every four to five years, their population increases to the point that there are too many individuals for the available food. The lemmings then start to migrate in search of new food sources. In Norway, the migrating lemmings keep moving until they reach the sea. This water barrier does not stop them and they plunge into the sea, where most of them drown. In the following three to four years, the numbers gradually rise to a similar level and the cycle repeats. Lemmings compete for the same food as reindeer and caribou and, at the peak of their population numbers, they deprive the large ungulates of food and cause many of them to starve. Some voles are also prone to similar cyclical fluctuations in numbers. The peak of population is known as a “vole year,” during which there is a marked increase in the voles’ predators, such as birds of prey.
One of the most unique rodents is the naked mole-rat (family Spalacidae) of eastern Africa. The naked mole-rat is the only known mammal whose social behavior resembles that of such insects as ants, termites, and bees. These rodents live underground in organized communities. Groups of similar individuals perform different functions in the colony, such as maintaining the tunnels and defending the colony from predators. One female takes the role of “queen” and produces young.
In addition to bamboo rats (Rhizomyidae) and jumping mice (Zapodidae), there are also the Old World rats and mice of the family Muridae. Many of them, especially the rats and the house mouse, have been introduced accidentally to all parts of the world, although they originated in the Old World. Most have long snouts and a long, naked, scaly tail. The black rat probably originated in southeastern Asia, but like the house mouse, it is now found throughout the world. Like other rats and mice, the black rat lives in close association with humans, inhabiting buildings and living off human food and garbage. These close as sociations often mean that the rats transmit diseases to humans.
The suborder also contains the dormice (Gliridae) and spiny dormice (Platacanthomyi-dae). They are mainly tree-dwelling and squirrellike, with a bushy tail. Like other leaf-eating rodents, they consume large amounts of berries, grains, and nuts in autumn to accumu late as much fat as they can. This energy reserve enables them to hibernate throughout the winter, during which they may lose half their body weight.
Sixteen families comprise the suborder Hystri-comorpha, and include porcupines, guinea pigs, and coypus. The porcupines make up two families: the Hystricidae, or Old World porcupines (found in Africa, Italy, and southern Asia), and the Erethizontidae, the New World porcupines of North and South America. In addition to hair on the body and tail, they have long, sharp, black-and-white quills on their back and flanks. The spines are loosely attached to the skin, and when the rodent attacks, it lunges sideways, then immediately withdraws, so that some of the quills or spines may become lodged in the victim.
The New World porcupines have shorter spines and some of them are barbed. They are tree-dwelling, and their feet are adapted for climbing. The sole is widened, and the first toe on the hind foot is replaced by a flexible pad. This group includes the cavies and guinea pigs (family Caviidae), the capybaras (Hydro-choeridae), and the chinchillas and vizcachas (Chinchilladae).
The members of the order Lagomorpha differ from rodents in that they have an additional pair of sharp incisor teeth in the upper jaw, which are situated behind the normal pair. In addition, these animals have a curious method of making the best use of plant food. To extract the maximum nutrient from their food, they first digest it in the stomach with the aid of bacteria and excrete it in the form of pellets. These pellets are eaten and pass through the digestive system once more—a process known as refection.
The family Ochotonidae consists of the pikas, or mousehares, found in North America and Asia. Of the Asian species, one lives on Mount Everest at an altitude of 5,740 feet (1,750 meters). Others live in deserts, on grasslands, in rocky regions, and in forests. Most of them inhabit areas with harsh winters. Pikas store food for winter. During the summer, they dry vegetation in the sun. gathering it into small piles that resemble haystacks.
The rabbits and hares make up the family Leporidae. They are native to most countries except Australia and New Zealand, although some have been introduced there by humans. Rabbits and hares are very similar, but they differ in several ways. Both groups are distinguished by their very long ears; short, upturned tail; and their hind legs, which are usually longer than their forelimbs. Most are active at night or at twilight and feed on bark, root crops, and grasses. The common names of rabbits and hares also sometimes make it difficult to distinguish them. The Belgian hare is really a type of rabbit. The snowshoe rabbit of the Arctic, which turns completely white in the winter, and the jack rabbit of desert areas are, in fact, hares.
Hares are solitary except in the breeding season. They do not burrow, but make a depression in the grass, called a form, in which they rest during the day and give birth to their young, which are born with a set of teeth, a coat of hair, and their eyes open. Hares usually run swiftly to escape predators.
Rabbits escape predators by hiding. They build fur-lined nests, in which they give birth to naked, blind young. Unlike the solitary hare, rabbits live together in large groups of about 150 individuals. They inhabit lowlands and hills, generally where the soil is sandy.
The European wild rabbit has been introduced to most countries. It is extremely prolific-females are fertile at about six months of age and usually have five to seven litters a year, each with two to nine young. They are capable of becoming pregnant within 12 to 15 hours after giving birth. They are also voracious feeders, and these two facts make them serious agricultural pests when they are not controlled.