To understand our irresistible urge to yawn, it might be a good idea to look at a thermometer, according to a recent experiment.

The theory puts forth that yawning might be a basic way to cool an overheated brain, especially when the surrounding air temperature is cooler than a person's body temperature.

Though all vertebrates yawn, the behavior may have social meaning in humans, some primates and dogs and seems to be contagious. Beyond the social reasons for yawning, scientists want to understand why it came about in the first place.

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For instance, many animals yawn without being around other members of their species. How can social factors cause such cases of yawning if the animals in question are alone?

Previous research from the same author shows that lowering temperatures can induce yawning in some birds and rats, suggesting a physiological — not social — function. Before now, the temperature hypothesis hasn't been tested in humans.

The team conducted an experiment during the summer and winter in Tucson, Ariz. Researchers randomly selected 160 participants walking outside. While outdoors, people in the experiment were asked to review 20 photos of yawning (since seeing others yawn promotes the behavior) and to report whether looking at the photos made them yawn.

They also provided the number of hours they'd slept the previous night and how long they had been outside that day. Meanwhile, the researchers kept tabs on the temperature and humidity.

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In the winter trial, 36 of 80 participants reported yawning during the survey, whereas 19 of 80 did in the summer. Considering people's sleeping patterns and time spent outside, temperature was the only factor that appeared to be linked to yawning frequency.

In addition, the longer a person was outside in the winter, the more likely he was to yawn. The exact opposite was observed in the summer, suggesting that yawning might not do much good in cooling the brain in warmer temperatures.

Exposing the roof of the mouth to cooler air is known to lower brain temperature, so the results suggest the same to be true for seasonal differences in yawning.

But the experiment isn't without drawbacks. Even though the researchers intentionally avoided interacting with the participants while they completed the survey, people may still have felt like they were being watched — a common hindrance to yawning. Also, being told the experiment involved yawning could have affected their comfort in yawning or refraining from the behavior.

It's likely more controlled studies are needed to rule out other unaccounted factors that might be at play.

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Although people living in colder climates generally have higher basal metabolic rates — a fancy term for the energy needed to keep the body functioning — it's not clear how temporary changes in temperature might affect BMR and the need to yawn.

As mentioned in a previous Discovery News article, the adaptation makes sense, according to one of the study's authors, because cooler brains equate to more alertness. Viewing yawning as a mechanism to make the brain more alert may account for why a person attributes yawning to tiredness or boredom.

First photo by sfllaw/ Second photo (painting) by Joseph Ducreux/Getty Museum/Wikimedia Commons.