Rodents are a group of mammals belonging to the order Rodentia characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. Rodents make up about 40% of all mammal species. They are native to all major landmasses except Antarctica and several oceanic islands, though they have subsequently been introduced to most of these landmasses by human activity. Most rodents are small animals with robust bodies, short limbs, and long tails. They use their sharp incisors to gnaw food, excavate burrows, and defend themselves. Most eat plant material, but some have more varied diets.