Many theories have been proposed to explain why animals yawn, such as replenish oxygen supplies, cool the brain and even stretch the lungs.Now, a new study claims yawning evolved as a social cue to warn others that we are less alert, and so they need to be extra vigilant in the lookout for predators. Meanwhile, the phenomenon known as 'contagious yawning' – reflexively yawning after seeing or hearing another individual yawn – is thought to have further spread this cue among groups of social animals.
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