All rodents, except Paucidentomys vermidax, have a single pair of upper and a single pair of lower incisors, followed by a gap (diastema), and then one or more molars or premolars; Paucidentomys vermidax is unique among the rodents in that it possesses no molars or premolars, and its incisors are so specialised, they are not for gnawing.
Typical rodent incisors grow continuously and must be kept worn down by gnawing. Their anterior and lateral surfaces are covered with enamel, but the posterior surface is exposed dentine. During gnawing, the incisors grind against each other, wearing away the softer dentine, leaving the enamel edge as the blade of a chisel.
This ‘self-sharpening’ system is very effective and is one of the keys to the enormous success of rodents.
Rodents lack canines, and have a diastema between their incisors and premolars. They use their teeth for cutting wood, biting through the skin of fruit, or for defense. Nearly all rodents feed on plants, seeds in particular, but a few exceptions eat insects (grasshopper mouse, Onychomys leucogaster) or fish. Some squirrels are known to eat passerine birds, such as cardinals and blue jays. One species, Paucidentomys vermidax, feeds primarily on worms and lacks the ability to gnaw or even chew, possessing bladelike, forked upper incisors and no molars.