Scientists aren't exactly sure why we yawn, but they know yawns are contagious.

yawnopar.jpg Credit: Mark Viveen

You might be sitting in a meeting or waiting in a supermarket checkout line, stuck in your own head. Then you see someone yawn. Soon, involuntarily, you inhale deeply. Your mouth opens wide, and you stretch your face, neck and respiratory muscles, filling your lungs until the flex reaches its acme. Then you emit a short, pleasurable exhalation. You weren’t bored. You weren’t tired. You couldn’t help it—yawning is contagious

Yawning is one of the animal kingdom’s oldest rites. Researchers have shown that nearly all vertebrates do it—reptiles, fish, birds and mammals alike. Merriam-Webster’s Dictionary says that humans yawn because of boredom or fatigue, but there is much more to this ancient behavior.

Scientists maintain that yawning has both social and physiological functions, and may even be useful clinically: Abnormal yawning can be symptomatic of pathology, such as tumors, hemorrhage or drug withdrawal. Researchers know that a system of several neurotransmitters and neuropeptides control yawning, but little is known about the exact mechanism underlying the action.

Until recently, it was thought that only humans and great apes were able to “catch” yawns. While humans yawn in the womb, they don’t fall prey to contagious yawning until about two years of age, which suggests a recent evolutionary origin.

Earlier this year, Annika Paukner, a psychologist at the University of Stirling, showed that higher primates weren’t the only ones who can’t control themselves when faced with a yawner. Showing stumptail macaques one video of monkeys yawning and another of them opening and closing mouths but not yawning, Paukner was surprised to find that the monkeys yawned and scratched significantly more often during and just after viewing the yawning video.

“Unlike chimpanzees, monkeys have never shown any strong signs of empathetic abilities,” she said. “[Yawning] might tell us not only about emotions and cognitive abilities in animals, but may also help us to trace the evolutionary origin of these abilities in humans.”

In examining why humans yawn, it’s helpful to look at testable common knowledge: In addition to yawning when others do, we yawn when we’re sleepy. We yawn most often in the morning. We yawn when we’re bored. We yawn when we’re hungry.

Robert Provine, a professor of psychology at the University of Maryland, Baltimore County who studies the neurobehavioral development and evolution of yawning, notes another characteristic of the yawn: It’s impossible to stop. He suggests simple experiments people can try that demonstrate how yawning requires both inhalation and a gaping mouth: When you start to yawn, clench your jaw, sucking in air through your teeth. Or, attempt to inhale through your nose.

“You’re sort of stuck in mid-yawn,” Provine says, about this unstable equilibrium state, “and that’s very unsatisfying.”

In 1987, Provine corrected the biggest misconception about yawning: that it is brought on by an oxygen shortage or an elevation of CO2 levels. In his study, he subjected participants to air with higher than normal levels of CO2 (3% to 5% versus the usual 0.03%). The experiment showed that neither elevated carbon dioxide nor depressed oxygen levels in the blood caused the frequency of yawning in subjects to change. In a second study, Provine found that having subjects exercise hard enough to double their breathing rate also had no effect on yawning. Together, the studies prove that yawning is not a primary respiratory function.

All of the accepted characteristics of yawning—hunger, boredom, lethargy—have something in common: They are associated with change. In fact, one function of yawning is its importance in changing behavioral state.

According to psychologist Olivier Walusinski, yawning is part of interoceptiveness, or sensitivity to events happening within the body. In a recent article in the journal Medical Hypotheses, Walusinski argues that yawning plays an important role in the link between REM sleep and arousal. He has shown that upon waking, yawning and stretching reverse the total muscle relaxation that characterizes REM sleep. Stretching corrects the loss of conscious imagery—the impaired sense of the position of various body parts in relation to neighboring parts—and yawning resets the mental self-image, thus increasing arousal and self-awareness.

We not only yawn to overcome drowsiness, but also before periods of high activity, says Provine. First-time paratroopers have been observed yawning before their first jump, he says, adding that dogs sometimes yawn on the threshold of attack and fish yawn before changing from one activity to another.

“Sometimes Olympic athletes, before their big event, will yawn—clearly, they’re not bored,” Provine says. “The yawn seems to stir things up, to facilitate these changes.”

If yawning is a mechanism for alertness, could that explain why it’s contagious? Though nobody knows for certain, researchers think yawning is an ancient form of social behavior, perhaps a primal form of empathy. An individual yawning could send yawns cascading through a group, synchronizing a clan for activity, such as hunting, going to battle or preparing to sleep.

By now, you’ve probably yawned a few times. Assuming that the subject matter hasn’t bored you, you’re experiencing just how contagious yawning is, according to Provine.

“Yawning is so contagious that virtually anything having to do with yawning will trigger it, even thinking about it or reading about it,” Provine says. “Someone reading your story will have yawns triggered, [just] by virtue of reading it.”

Originally published May 4, 2006

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