Rodents

About 40 percent of all mammals are rodents, making Rodentia, with 2,024 species, the largest order of mammals. In adapting to so many different environments, rodents have evolved characteristics similar to those of mammals from other orders that fill similar types of habitats. One example of convergent development involves the semi-aquatic capybara, which is a forager of succulents in marshes and riverbeds, thus filling a similar ecological niche in South America as the hippopotamus does in Africa. Although the capybara, at about 145 pounds (66 kg), is the largest rodent, most members of the order, such as the European harvest mouse, weigh less than 5 ounces (150 g). This generally small size has helped rodent families radiate into a wide range of habitats and microhabitats.

All rodents lack canines; instead they have a pair of sharp incisors, which are constantly growing to compensate for being continually worn down by digging and by gnawing on coarse vegetation. Taxonomists divide Rodentia into two suborders based on the different ways the lateral and deep masseters, or jaw muscles, move and close the jaws: Sciurognathi, which includes squirrels, beavers and the mouselike murids that make up 66 percent of the entire order; and Hystricognathi, which includes porcupines, a group of highly specialized African mole-rats, and 14 widely radiated South American rodent families.

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