The crocodile and Egyptian plover
Crocodiles can open and close their jaws, but they cannot move them from side to side, nor can they use their tongues to propel food around their mouths. This means that a lot of food gets caught in the crocodile's teeth, attracting parasites and bacteria. Crocodiles 'invite' the plover over for a meal by opening their mouths wide and remaining very still. The plover will approach and enter the crocodile's mouth where it eats food scraps and parasites, cleaning the reptile's teeth in the process. An unlikely, but harmonious, win all round.