Tusks, those ivory spears that protrude from the jaws of certain large mammals, are actually elongated teeth. Ivory, the stuff they're made of, is a type of dentine calcium-rich tooth tissue .

The tusks of elephants and narwhals are modified permanent upper incisors, whereas the tusks of walruses, warthogs and wild boars are outsized canines .

There's no one reason that tusks evolved from teeth in so many different species. Elephants use them to dig and bore. Male narwhals spar with them to impress narwhal-ladies. Walruses use them like picks, to grip ice. Boars use them as weapons.

The largest tusk ever recorded belonged to an African elephant . It weighed 214 pounds and was 11 feet six inches long.

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