Before you finish reading this article, you’ll probably yawn.

That is, if you haven’t already done so, just from reading the headline.

As we all know, yawning is very contagious — so much so, that even thinking or reading about yawning can trigger the reflex.

What you may not know, however, is that yawning is also contagious among chimpanzees. Or that yawning can be contagious across species: Your yawning may cause your dog to do the same. (Skeptical? Here's a video.)

We humans tend to yawn (on our own) when we’re tired, bored or hungry. But, as biologist Steve Jones asks in a recent article in the British newspaper the Telegraph, “Why?”

That’s the question that continues to stump scientists.

Several theories
“Dogs do it, lions do it, even babies in the womb do it — but nobody really knows why,” writes Jones. “Theories abound. ... Some have suggested that a sudden drop in blood oxygen, or a surge of carbon dioxide pumped out by a tired body, sparks it off — but no, breathing air rich in that gas, or with extra oxygen, makes no difference.”

Other theories:

• Yawning helps cool the brain.True, notes Jones, we do tend to yawn more on hot days than on cold ones. But we yawn less often when we have a fever.

• Yawning signals an impending change of state — a “general preparation for some new mental experience,” says Jones. Parachutists, he points out, often yawn before they jump.

• Yawning is a form of erotic posturing and, thus, linked to sex. A paper presented at a recent scientific meeting on sexual medicine, Jones reports, described how women with depression who were given a particular (unnamed) mood-altering drug “immediately went into uncontrollable bouts of yawning, accompanied by repeated orgasms over many hours.”

Hmmm ...

A sign of empathy?
There's a growing scientific concensus around the idea that yawning’s contagiousness is a signal of empathy, the “ability to understand and to react to someone else’s state of mind,” Jones says. Research suggests that people with autism, who often have impaired empathy, are less likely to “catch” a yawn from someone else, he points out. And some research suggests that how quickly a person responds to someone else’s yawn may be an objective measurement of how empathic he or she is.

Although chimps “catch” yawns (even in reaction to a yawning computer avatar), they tend to do so as a “statement of dominance rather than sympathy (with a strong hint of sexual aggression built in),” says Jones.

The fact, then, that we humans tend to discreetly and politely cover up our yawns with our hands, may be “a deep insight into what it means to be human,” says Jones, “a sign of an ancient shift from a quarrelsome and sexually violent mental universe to a generally cooperative and agreeable one."