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 considered a special case of pandiculation that affects the musculature of the mouth, respiratory system and upper spine (Baenninger, 1997). When, as often happens, yawning occurs simultaneously with pandiculation in other body regions …