Charles Darwin was intrigued by the toucan’s monstrous beak. (The toucan’s bill accounts for about one-third of its body length.) He wrote that “toucans may owe the enormous size of their beaks to sexual selection, for the sake of displaying the diversified and vivid stripes of colour with which these organs are ornamented.”

In other words, Darwin suggested the big beaks attracted mates. Others have suggested the beaks are for peeling fruit, warning off territorial rivals, or are a visual warning to predators.

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