Teeth (singular, tooth) are structures found in the jaws of many vertebrates. The primary function of teeth is to tear and chew food, and in some animals, particularly carnivores, for fighting and/or defence. The roots of the teeth are covered by gums. Adult teeth naturally darken with age as the pulp within the tooth shrinks and dentin is deposited in its place. Teeth are among the most distinctive features of mammal species and fossils. Paleontologists use them to identify fossil species and their relationships. The shape of the teeth is related to the animal's diet. For example, plant matter is hard to digest, so herbivores have many molars for chewing. Carnivores need canines to kill and tear meat. While humans develop two sets of teeth throughout life (diphyodont), some animals develop only one set (monophyodont) or develop many (polyphyodont). Sharks, for example, grow a new set of teeth every two weeks. Rodent teeth grow and wear away continually through the animal's gnawing, maintaining approximately constant length.