Why do we yawn?

An irrepressible urge to yawn sometimes happens when we are tired or when we see someone yawning. Perhaps you yourself have the urge to yawn after reading these few words... But what is the purpose of this behavior?

This behavior is universal because it is performed by all vertebrates, except the giraffe, but its utility is still very mysterious. We can then think that a behavior that is not useful would have been lost during the evolution, or at least modified. But this is not the case! Historically, the received ideas on yawning were that it would allow to regulate the internal temperature, or to oxygenate the brain.
However, recent studies have raised the possibility that yawning may serve as a signal that an individual is drowsy, thus signaling to other members of the group that they should be more vigilant in order to react in case of a predator attack. The contagious nature of this behavior would then be particularly interesting in this type of situation. Indeed, a human is six times more likely to yawn if he has seen another person do it. This phenomenon would be linked to particular neurons located in the premotor cortex, the mirror neurons, which allow us to imagine ourselves acting in the place of a person performing an action. Of course, in normal circumstances this imagined action is not reproduced in reality, but yawning seems to be the only exception.
In recent years, a theory has emerged concerning the contagious nature of yawning: it could be linked to empathy. Studies on this link in humans tend to confirm this claim, but research still has a lot to teach us on this subject, especially in animals...