Rodents are mammals of the order Rodentia, which are characterized by a single pair of continuously growing incisors in both the upper and lower jaws. Roughly 40% of all mammal species are rodents. They are found in vast numbers on every continent except Antarctica. They are the most diversified mammalian order and live in a variety of habitats. Humans have allowed rodents to spread to many remote places in the world. Rodents have adapted to almost every habitat, from cold tundra to hot deserts. Some species, such as squirrels and porcupines, live in trees while others live almost completely underground. Gophers, prairie dogs, and mole rats build complex systems underground. Most rodents are small animals with robust bodies, short limbs, and long tails. They use their sharp incisors to gnaw food, excavate burrows, and even defend themselves. Most eat seeds or other plant material but some have more diverse diets. Rodents tend to be social animals and many species live in societies with complex ways of communicating with each other. Some rodent species are serious agricultural pests, eating large quantities of food stored by humans. Rodents are also significant vectors of bacteria and can spread disease.