Yawning is considered socially disrespectful not only among human beings but also some other animals. Among monkeys, only the dominant male monkey has the freedom to yawn whenever he feels like it. Females and subordinate males yawn only when the dominant male is not around. Yawning is a universal phenomenon among ﬁsh, reptiles, birds and mammals.
We still have neither a mechanistic nor a teleological answer to the question why we yawn. However we do have plenty of untested or refuted theories, claims and suggestions. It is often said that yawning is due to lack of oxygen or excess of carbon dioxide in the blood. But blood levels of oxygen and carbon dioxide show little correlation with yawning. The layman’s idea of a yawn is that it indicates boredom, fatigue and drowsiness. Possibly by stretching the jaw muscles, yawning helps overcome these. That is perhaps why an incomplete yawn, i.e. one concealed by clenching the teeth, is not fully satisfying. Yawns may sometimes have purely psychological reasons: watching somebody else yawn, or thinking or reading about yawns often makes one yawn. If you haven’t already had a yawn while reading this passage, you might have one now.
Source: Understanding of Medical Physiology 4th edition, RL Bijlani