Rodents

Text 2

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1. Give general definition of a mammal.

2. What is the smallest and the biggest animal in the group?

3. Enumerate unique mammalian features.

4. In what habitats can mammals live?

5. What can you tell about their feeding habits?

6. Why are mammals so important to human?

Rodents - any of more than 2,050 living species of mammals characterized by upper and lower pairs of ever-growing rootless incisor teeth. Rodents are the largest group of mammals, constituting almost half of the class Mammalia. They are indigenous to every land area except Antarctica, New Zealand, and a few Arctic and other oceanic islands, although some species have been introduced even to those places through their association with humans. This huge order of animals contains 27 separate families, including not only the “true” rats and mice (family Muridae) but also such diverse groups as porcupines, beavers, squirrels, marmots, pocket gophers, and chinchillas.

All rodents possess constantly growing rootless incisors that have a hard enamel layer on the front of each tooth and softer dentine behind. The differential wear from gnawing creates perpetually sharp chisel edges.

The range in body size between the mouse (18 grams, body 12 cm long) and the marmot (3,000 grams, body 50 cm long) spans the majority of living rodents.

Rodents have lived on the planet for at least 56 million years and modern humans for less than one million, but the consequences of their interactions during that short overlap of evolutionary time have been profound. For rodents, early humans were just another predator to avoid, but with Homo sapiens' transition from nomadic hunting and gathering to agricultural practices, humans became a reliable source of shelter and food for those species having the innate genetic and behavioral abilities to adapt to man-made habitats. The impact of these species upon human populations ranges from inconvenient to deadly.

Crops are damaged before harvest; stored food is contaminated by rodent waste; and objects are damaged by gnawing. Certain species are reservoirs for diseases such as plague, typhus, tularemia, etc. Only a few species are serious pests or vectors of disease, but it is these rodents that are most closely associated with people.

Various other rodents are beneficial, providing a source of food through hunting, apparel derived from their fur, test animals for biomedical and genetic research, pleasure as household pets, and insight on mammalian biology and evolutionary history.

Rodents may be diurnal, nocturnal, or sometimes active part of the day and night. Although some species are herbivorous, diets of most include vegetable and animal matter. Others are opportunistic generalists, and some are specialized predators. Food is either eaten where gathered or carried to burrows and stored. Species living in arid habitats and on oceanic islands are able to obtain their water requirements from their food. A wide variety of shelters are used or constructed; these range from tree holes, rock crevices, or simple burrows to hidden nests on the forest floor, leaf and stick structures in tree crowns, mounds of cut vegetation built in aquatic environments, or complex networks of tunnels and galleries. Rodents may be active all year or enter periods of dormancy or deep hibernation. Breeding time and frequency, length of gestation, and litter size vary widely, but two of the most prolific are both associated with humans. The brown rat (Rattus norvegicus) can give birth to litters of up to 22 offspring, and the house mouse (Mus musculus) can produce upto 14 litters annually. Population size may remain stable or fluctuate, and some species, most notably lemmings, migrate when populations become excessively large.

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