If and when, a crocodile comes to the shore it wide opens its mouth; and a small bird enters its mouth and cleans up its teeth and gums, during which time the animal keeps its mouth shut; and when the bird finishes its job, it knocks the ceiling of the animal's mouth, and the crocodile opens its mouth, whereupon the bird comes outwards and flies away.
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This is based on mutual beneficial survival strategy.
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The deadly predator and the vulnerable bird are at ease with each other.
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