Everyone knows yawning is the pinkeye of social cues: powerfully contagious and not that attractive. Yet scientists aren't sure what the point of it is. Is yawning a form of communication that evolved to send some message to our companions? Or is the basis of yawning physiological, and its social contagiousness unrelated? A new paper suggests that yawning--even when triggered by seeing another person yawn--is meant to cool down overheated brains.

We're not the only species that feels compelled to yawn when we see others doing it. Other primates, and possibly dogs, have been observed catching a case of the yawns. But Princeton researcher Andrew Gallup thinks the root cause of yawning is in the body, not the mind. After all, we yawn when we're alone, not just when we're with other people.

Previously, Gallup worked on a study that involved sticking tiny thermometers into the brains of rats and waiting for them to yawn. The researchers observed that yawning and stretching came after a rapid temperature rise in the frontal cortex. After the yawn and the stretch, rats' brain temperatures dropped back to normal. The authors speculated that yawning cools the blood off (by taking in a large amount of air from outside the body) and increases blood flow, thereby bringing cooler blood to the brain.

If yawning's function is to cool the brain, Gallup reasoned, then people should yawn less often when they're in a hot environment. If the air outside you is the same temperature as your body, it won't make you less hot.

To test that theory, researchers went out into the field--namely, the sidewalks of Tuscon, Arizona--in both the winter and the summer. They recruited subjects walking down the street (80 people in each season) and asked them to look at pictures of people yawning. Then the subjects answered questions about whether they yawned while looking at the pictures, how much sleep they'd gotten the night before, and how long they'd been outside.

The researchers found that the main variable affecting whether people yawned was the season. It's worth noting that "winter" in Tuscon was a balmy 22 degrees Celsius (71 degrees Fahrenheit), while summer was right around body temperature. In the summer, 24% of subjects reported yawning while they looked at the pictures. In the winter, that number went up to 45%.

Additionally, the longer people had been outside in the summer heat, the less likely they were to yawn. But in the winter, the opposite was true: People were more likely to yawn after spending more time outside. Gallup speculates that because the testing took place in direct sunlight, subjects' bodies were heating up, even though the air around them remained cooler. So a yawn became more refreshing to the brain the longer subjects stood outside in the winter, but only got less refreshing as they sweltered in the summer.

The study used contagious yawning rather than spontaneous yawning, presumably because it's easier to hand subjects pictures of yawning people than to aggressively bore them. Gallup notes that contagious and spontaneous yawning are physically identical ("a stretching of the jaw and a deep inhalation of air," if you were wondering), so one can stand in for the other. Still, it would be informative to study people in a more controlled setting--in a lab rather than on the street, and preferably not aware that they're part of a yawning study.

A lab experiment would also allow researchers to directly observe whether their subjects yawned, rather than just asking them. In the field, researchers walked away while subjects were looking at the pictures, since people who know they're being watched are less likely to yawn. But self-reported results might not be accurate. The paper points out that "four participants in the winter condition did not report yawning during the experiment but yawned while handing in the survey to the experimenter."

Still, it seems there's real connection between brain temperature and yawning. It will take more research (and more helplessly yawning subjects) to elucidate exactly what the connection is. Even if brain temperatures always rise right before a yawn and fall afterward, cooling the brain might not be the point of the yawn--another factor could be causing the impulse to yawn, and the temperature changes could be a side effect. Studying subjects in a truly cold environment, and showing that they are once again less likely to yawn (because outside air would cool their brains too much), would provide another piece of evidence that temperature triggers the yawn in the first place.

None of this tells us why yawning is so catching, though. Personally, I think I yawned at least a thousand times while reading and writing about this paper. Maybe I should have taken some advice from an older study by Andrew Gallup, which found that you can inhibit yawning by breathing through your nose or putting something chilly on your forehead.

Photo: Wikipedia/National Media Museum

Andrew C. Gallup, & Omar Tonsi Eldakar (2011). Contagious yawning and seasonal climate variation. Frontiers in Evolutionary Neuroscience