The physical and psychological sequences which occur during the yawning process have been widely studied. Nevertheless, the reasons for yawning are disputed and unclear - as is identifying an evolutionary reason to explain the need for it.

“There are a number of theories that attempt to explain why animals yawn. It is likely that there are a number of triggers for the behavior. However, there are a few select theories that attempt to explain the primary evolutionary reason for the yawn. None of them has been empirically substantiated.”

Source : Wikipedia

Yawns are common not only in most (if not all) species of mammals, but also occur in reptiles and fish - suggesting a very deep-rooted and ancient,, but as yet unknown, origin.

Also see: Contagious Yawningplugin-autotooltip__plain plugin-autotooltip_bigContagious Yawning

Most people have seen examples of Contagious Yawning in humans. If one person in a group yawns (or convincingly feigns a yawn) it's often quite hard for others to resist. Current estimates are that 40-60% of human yawns are 'contagious'. and Stretchingplugin-autotooltip__plain plugin-autotooltip_bigStretching

"Pandiculation [stretching] is the involuntary stretching of the soft tissues, which occurs in most animal species and is associated with transitions between cyclic biological behaviors, especially the sleep-wake rhythm (Walusinski, 2006). Yawning is consid…

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