Everybody yawns. I mean, really, everybody: The reflexive deep, jaw-stretching inhale followed by a pause and a forced exhalation is pretty much ubiquitous in the animal kingdom, at least among creatures with the right anatomy for it.
It's not clear why we yawn (or when our ancestors started the refreshing routine) but many scientists believe the action serves to cool down the brain. Brains use a lot of energy, and they run hot. Inhaling a rush of cool, ambient air chills the blood, and the widening of the jaw sends a nice blast of that breezy blood into the brain.
A new study in Biology Letters could add further support to this popular theory. If bigger yawns produce a greater cooling effect, the study authors hypothesized, then animals with bigger brains — and therefore more brain tissue to cool down — would produce more sustained yawns. Their data suggests that this is indeed the case. Forget the dream of having big brains and brawn; big brains and yawn is much cooler.
Led by Andrew Gallup of the State University of New York at Oneonta, who has long investigated this possible brain cooling mechanism, researchers studied 29 mammals whose brain weights had been documented in a previous paper. They tracked down videos of these animals yawning — mostly on YouTube — to calculate the average length of their yawns.
When the team crunched the numbers, they found that brain weight and “cortical neuron number” — the number of brain cells in the outer layer known as the cortex — were reliable predictors of yawn length, more reliable than total body size or relative brain size. Gorillas, camels, horses, lions, walruses and African elephants were all found to have shorter yawns than humans despite their massive sizes, which makes sense because their brains are smaller than ours.
In other words, the length of a yawn doesn't seem to correlate to the size of your body. It seems to correlate to the size of your brain.
“Consistent with these results, we also found that primates tend to have longer and more variable yawn durations compared with other mammals,” the study authors write, even though some of the non-primates they studied opened their jaws wider.
“If yawning cools certain parts of the brain, it makes sense that larger brains with more cortical neurons require longer yawns and more influx of air,” Elainie Alenkær Madsen of Lund University, who was not involved in the new study, told the Atlantic. “It’s a very nice and intuitive finding.”
As you may recall from a recent installment of our weekly column Dear Science, not all scientists are convinced that yawns exist to cool the brain. Yawns have a tendency to be contagious, even among nonhuman animals — and there's some evidence that yawns are more catching among animals that experience empathy for patient zero of the yawn epidemic. Because of this strange social reflex, some researchers believe that yawns are actually some kind of communicative signal — something that evolved for psychological reasons, not physiological ones.
But the answer might be somewhere in the middle: Perhaps all animals developed yawns to keep ever-expanding brains cooled down, and only humans and their closest kin have developed a secondary emotional use for the reflex.
“Whether yawning functions specifically to cool the brain can still be debated,” Gallup told STAT, “but there is no debate on whether yawning has thermoregulatory consequences.”
One potential limit of the new study is kind of hard to believe: There weren't enough videos of these cute little guys yawning on the Internet for an analysis as statistically robust as the study authors would have liked. Further studies could endeavor to observe these yawns in person to get more data. But if you happen to know a capuchin monkey, keep a camera on him the next time he gets sleepy — it's for science.