Rodents must gnaw constantly or their teeth keep growing
About four in every ten mammal species are rodents, and all share one defining trait: a single pair of incisors top and bottom that never stops growing. Gnawing keeps them in check and sharpens them into chisels. Without it, the teeth can grow long enough to pierce the animal's own skull.
The name comes from the Latin rodens, gnawing. Only the front of each incisor is coated in hard enamel, so as upper and lower teeth grind together the softer dentine behind wears faster, leaving a self-sharpening blade. Rodents have no canines, and a gap behind the incisors lets them close off the mouth, sucking in their cheeks to keep out whatever inedible material they are chipping away. In chinchillas and guinea pigs, which eat tough, fibrous food, even the molars grow without end.
Their jaws are built for two jobs that cannot happen at once. The lower jaw shoves forward to gnaw with the incisors, then slides back so the ridged molars can grind. The masseter muscle does most of the work, making up 60 to 80 percent of the chewing muscle mass, and because it attaches behind the eyes, some rodents visibly jiggle their eyeballs up and down while gnawing. Squirrels, guinea pigs and rats each arrange these muscles differently, trading biting power against sideways chewing.
Most species weigh under 100 grams, though the capybara can reach 66 kilograms. A nail on the first digit, paired with deft incisors, lets them crack hard seeds and nuts, a niche they now dominate. They live almost everywhere, leaping, burrowing, swimming or climbing, and many are highly social with complex communication.
The fossil record starts in the Paleocene of Asia. Rodents spread widely in the Eocene, crossing from Africa to South America and Madagascar, and until modern humans arrived they were the only land-based placental mammals to colonise Australia. Their nearest relatives are rabbits, hares and pikas, which also have ever-growing incisors but carry an extra upper pair.
Source: Rodent
