I just found this site and surfed through the archives. A very old column about Why yawning is contagious brought a theory to my mind, that I read some time ago. Unfortunately I don’t have any references to the source anymore.
However, the theory is as follows:
Back in time, when we were still monkey-like neanderthals, living in caves with other members of the tribe, the yawning was a sign to the others “I’m tired and want to go to sleep.” But when you go to sleep you’re defenseless against hostile tribe members.
So if others are also yawning, you can be (quite) sure, that the other tribe members are also tired, will go to sleep soon, too, and are not able to club you to death.
The first problem is that I know of no social animal anywhere in the world that will challenge for leadership when the rival is asleep.
I can’t even imagine how such a trait could evolve since killing a sleeping rival is not a sign of fitness and as such the females would simply stop responding to social status. Remember sexually attractive traits have to have some relationship to genetic fitness or very rapidly females who don’t prefer that trait will have more offspring and so forth and thus the trait will die out.
The next problem is that social animals have evolved means of determining reciprocity. Any individuals that killed another group member while they slept would be immediately shunned and mistrusted. that is not survival trait.
Finally, and most importantly, we have the problem that the trait as you describe it is an evolutionary disadvantage. Any organism would be best served by paying no heed at all to yawning if yawning was purely social mimicry and gave no indication of being tired. And if everyone ignores yawning because we know it;s just mimicry then why would mimicry serve any purpose?
It seems that this would remove contagious yawning. Your entire tribe starts yawning and goes to sleep so you get overrun by another tribe which doesn’t suffer from this.