Tusks are elongated, continuously growing front teeth that protrude well beyond the mouth of certain mammalspecies. They are most commonly canine teeth, as with pigs, hippopotamuses and walruses, or, in the case of elephants, elongated incisors. Tusks share common features such as extra-oral position, growth pattern, composition and structure, and lack of contribution to ingestion. Tusks are thought to have adapted to the extra-oral environments, like dry or aquatic or arctic. In most tusked species both the males and the females have tusks although the males' are larger. Most mammals with tusks have a pair of them growing out from either side of the mouth. Tusks are generally curved and have a smooth, continuous surface. The male narwhal's straight single helical tusk, which usually grows out from the left of the mouth, is an exception to the typical features of tusks described above. Continuous growth of tusks is enabled by formative tissues in the apical openings of the roots of the teeth. Prior to over hunting and proliferation of the ivorytrade, elephant tusks weighing over 90 kg (200 lb) were not uncommon, though it is rare today to see any over 45 kg (100 lb).Other than mammals, dicynodonts are the only known vertebrates to have true tusks.
