Vertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Infraclass Eutheria, or Placental Mammals
Order Rodentia

The order Rodentia comprises small to medium-sized mammals that feed primarily on plant material. Their external appearance varies considerably (Fig. 63), but the dental system in all species is adapted for gnawing and chewing hard plant food. Both the upper and lower jaws bear a single pair of large, chisel-shaped, rootless, and continuously growing incisors. The front surface of the incisors is covered with hard enamel, while the back lacks enamel and is soft. Due to uneven wear at the tip, the incisors remain constantly sharp. They are often exposed because the upper lip is frequently deeply cleft. The molars have a broad chewing surface set with cusps. Canines are absent, leaving a wide toothless gap—the diastema—between the incisors and the molars. The intestinal tract is long to accommodate a coarse plant diet. All species possess a caecum, which acts as a peculiar "Fermentation vat." Rodents are characterized by early sexual maturity and large litters.

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Fig. 63 Representatives of the orders Rodentia and Lagomorpha:

1 — squirrel; 2 — jerboa; 3 — nutria; 4 — common vole; 5 — wood mouse; 6 — European hare.

There are about 2,500 known species of rodents distributed across all PARTS OF THE world. Among them are terrestrial, subterranean, arboreal, and semiaquatic forms. About 130 species inhabit our country.

Squirrels (Sciuridae), apart from true squirrels, include chipmunks, ground squirrels, and marmots. The Eurasian red squirrel is found across almost the entire territory of Russia. It lives in nests built in trees or in tree hollows and does not hibernate in winter. It feeds primarily on conifer seeds, and less frequently on berries and mushrooms. It breeds 2 to 3 times a year, producing litters of 3 to 10 young. In years of food scarcity, squirrels undertake wide Migrations in search of food, frequently venturing to the outskirts of settlements, parks, and gardens. The squirrel is an important species in our fur trade.

Chipmunks are closely related to squirrels, featuring a striped coat pattern and a relatively less bushy tail. They lead a primarily terrestrial lifestyle and construct their nests in burrows. They hibernate in winter, but in autumn they cache food stores consisting of pine nuts, cereal grains, and legumes. They inhabit mainly the forests of Siberia and eastern Europe.

Numerous species of ground squirrels and marmots are terrestrial, inhabiting steppe and mountainous regions. They live in burrows, usually in colonies, and reproduce once a year. They feed on grass and seeds, predominantly of grasses. Most species are agricultural pests affecting cereal crops, and control measures are taken against them. Furthermore, ground squirrels and marmots act as vectors and reservoirs of dangerous diseases such as plague and tularemia. They hibernate in winter; for many species, hibernation begins In the second half of summer and is triggered by the drying up of forage vegetation. The longest hibernation is characteristic of the yellow suslik, an inhabitant of the Trans-Volga steppes, Kazakhstan, and the semi-deserts of Central Asia, which has an active period of only 4 months. The European part of our country is inhabited by the dwarf ground squirrel and the spotted ground squirrel.

Marmots are biologically close to ground squirrels but differ in their larger size. A typical species is the bobak marmot, widespread in the steppes of southeastern Europe, Kazakhstan, and western Siberia.

Among beavers, a single species—the Eurasian beaver—inhabits our country. In the past, it was numerous and found in many regions of Siberia and Europe; however, in the last century it was almost universally hunted to near extinction. Today, thanks to conservation efforts and reintroduction programs, the beaver population has recovered, and they are now found in many regions of European Russia. Beavers live in colonies along forest rivers, building lodges and burrows out of branches and tree trunks as dwellings. In their habitats, they construct dams that raise and stabilize the Water level in the river. In summer they feed on herbaceous vegetation, and in winter on tree bark and twigs. For the winter, they store branch fodder, placing it in the water near their dwellings. Beavers reproduce once a year, giving birth to 2–4 young, and reach sexual maturity in their third year of life.

The largest rodents are porcupines. In the mountains of Kazakhstan, Central Asia, and Azerbaijan, porcupines can reach a length of 70–90 cm and a weight of up to 30 kg. The front part of their body is covered with bristles, while the rear part bears long quills that are loosely attached and easily shed when the animal shakes. They are predominantly nocturnal, hiding in shelters during the day. They frequently dig long burrows (up to 10 m) with side chambers or take up residence in caves. They feed on bulbs, rhizomes, and roots, and locally cause damage by eating corn cobs, melons, and other crops. In summer, the female gives birth to 2–4 sighted young with soft quills. The offspring typically remain in the family burrow until the following year.

Murid rodents (Muridae) are the most numerous group of rodents, distributed across almost the entire globe. Most are small, burrowing animals that feed primarily on plant matter, although some species may occasionally consume insects. They are characterized by early sexual maturity and very high fecundity. The most common murids are listed below. Several species of mice inhabit almost everywhere; besides the well-known house mouse, the striped field mouse and the wood mouse are also common.

Voles differ from mice by having a shorter tail. Lemmings live in the Arctic, some of which turn white in winter. Gerbils inhabit deserts and are characterized by a relatively large size and a long, furred tail. Rats are represented primarily by the brown rat (or Norway rat). They live in human structures but may also be found far from human dwellings.

The muskrat—a large, valuable fur-bearing rodent native to North America—also belongs to the murids. It has been acclimatized almost everywhere in our country. This semiaquatic animal inhabits vegetation-choked water bodies, settling in lodges or burrows. It feeds on aquatic vegetation and reproduces very rapidly, producing 2–4 litters of 4–12 young per year. It reaches sexual maturity at less than a year of age. Today, the muskrat is one of the most important fur-bearing animals in Russia.

Other rodents found in our country, mainly in southern regions, include dormice, jerboas, and mole-rats. There are also domesticated forms, such as South American guinea pigs, which are widely used as laboratory animals. Valuable fur-bearing rodents such as the nutria and chinchilla are successfully bred in captivity.

Rodents are of immense economic importance. Some are vital objects of the fur trade; others are pests of agricultural crops and food supplies, as well as reservoirs and vectors of A number of dangerous diseases.