For other animals, yawns can not only be invigorating but also symbolic - a way of communicating with others of the same species. A yawning hippopotamus is an awesome sight. It rears up out of the water and opens its huge mouth to reveal its tusk-like canine teeth. This is a message for other hippos around, but the meaning differs, depending on who is watching. If the spectators are male then the yawn is an aggressive, territorial gesture, intended to terrify any potential competitors with its immensity. If male hippos were to fight over territory or females they could inflict terrible injuries on one another. Disputes rarely end in violence, the hippos rely mainly on yawning shows of strength to quell brewing unrest.
If a hippo opens his mouth wide in the company of a female, however, the meaning is completely different. The yawn is part of courtship play-fighting - a gesture of 'affection' from a hippo.
Male baboons also yawn to intimidate would-be opponents, displaying their dangerous canine teeth. But yawning, stretching and scratching are also an essential part of the baboons' complicated ritual of grooming, which bonds their societies together.