The members belonging to the order Rodentia account for half of the mammalian species living today. Rodents are widely distributed throughout the globe except Antarctica and some islands. They inhabit in almost every habitat. The order Rodentia is divided into three suborders: Sciuromorpha, Myomorpha and Caviomorpha. The classification is based primarily on the arrangement of the jaw muscles especially the deep masseter muscles. The members belonging to the suborder Sciuromorpha (squirrel-like rodents) include squirrels, beavers, spring hares, pocket mice, kangaroo rats, scaly-tailed squirrels, pocket gophers and mountain beavers. Rodents belonging to the suborder Myomorpha (mouse-like rodents) are rats, mice, dormice, jumping dormice, jerboas, birch and desert dormice. Some notable members in the suborder Caviomorpha (cavy-like rodents) are cavies, porcupines, capybaras, coypus, pacas, chinchillas, chinchilla rats and agoutis. Most members of this order have short limbs. The forelimbs are slightly smaller than the hindlimbs. All species possess long and sensitive whiskers. Some are nocturnal (e.g. porcupines), while others are active during day light hours (e.g. squirrels). Rodents may be semi aquatic (e.g. capybara), aquatic or semi aquatic (e.g. beavers) and arboreal (e.g. New World porcupines). Some are burrowing animals (e.g. voles), while others are grazing rodents (e.g. capybara).