It's true that one yawning is contagious and one leads to another within a group. This is an involuntary action that causes one to open your mouth wide and breathe in deeply. Yawning happens in the womb, it has been observed that an 11 month old fetus has.
There are three major theories of yawning. The first is the most commonly known one, that draw more oxygen in, which replaces the carbon dioxide that we exhale. The second theory.is to change the current activity. The third is bordom or tiredness.
However, in essence we have why we do yawn. But this is common in most if not all animals.
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