Tusks are elongated, continuously growing front teeth, usually but not always in pairs, that protrude well beyond the mouth of certain mammal species. They are most commonly canines, as with warthogs, pig, and walruses, or, in the case of elephants, elongated incisors. In most tusked species both the males and the females have tusks although the males' are larger. Tusks are generally curved, though the narwhal's sole tusk is straight and has a helical structure. In the elephant, the tusks were originally second incisors. Continuous growth is enabled by formative tissues in the apical openings of the roots of the teeth.