听力原文： Today's discussion is about a common animal reaction — the yawning. The dictionary defines a yawn as "an involuntary reaction to fatigue or boredom". That's definitely true for human yawns but not necessarily for animal yawns. The same action, the yawn, can have quite different meanings in different species.
For instance, some animals yawn to intimidate intruders on their territory. Fish and lizards are examples of this. Hippos use yawns when they want to resolve a quarrel. Observers have seen two hippos yawn at each other for as long as two hours before they stop quarreling.
As for social animals like baboons or lions _ they yawn to set up the pecking order within social groups, and lions often yawn to calm social tensions. Sometimes these animals yawn for a strictly physiological reason __ that is to increase oxygen levels. And curiously enough, when they yawn for a physical reason like that, they do what men do __ they try to stifle the yawn by looking away or by covering their mouths.
What's the main topic of the monologue？

A. Different animals' yawn.
B. Human's yawn.
C. Fish's yawn.
D. Social animals' yawn.