Why do bigger-brained animals have longer yawns?
When I interviewed Andrew Gallup, I yawned five times in the space of half an hour. That wasn’t because he was boring. It was because he studies yawning—and not just in humans, either. Gallup, a psychologist based at State University of New York at Oneonta, is a connoisseur of animal yawns.
He recently gave his students a list of animals, including elephants, cats, gorillas, sheep, camels, and walruses. Go online, he said, and find as many videos as you can of these particular species yawning, and time the duration of their yawns.
How, incidentally, can you tell if an animal is yawning? It’s surprisingly easy. When you yawn, you typically open your mouth quickly, stretch your facial muscles, inhale deeply, close your eyes, and tilt your head back. You can see the same set of movements in other mammals, and even in birds. A parakeet’s yawn is unmistakably a yawn. And perhaps because these behaviors are so stereotyped and obvious, a lot of them turn up online. “It’s something that’s videotaped a lot because it’s so ubiquitous,” says Gallup.
The animals on his list were chosen for a reason. They had all appeared in a 2005 paper on brain evolution, which listed their brain weight, and the number of neurons in their cortex—the outer layer of the brain, which governs our most important mental skills. Using the videos they amassed, Gallup’s team showed that these two brain traits are strikingly correlated with the length of an animal’s yawn, irrespective of the creature’s body size or mouth size.
Put it this way: If you take a mammal and time its yawn, you can reasonably predict how heavy its brain is and how many cortical neurons they have. “We were just really blown out of the water,” says Gallup. “It’s such a strong predictor.”
Even locked-in patients who cannot voluntarily move their limbs can yawn. Conversely, you can’t stifle a yawn.
“I never would have thought to study this,” says Liz Cirulli Rogers, from Duke University School of Medicine. “This could be an important and interesting observation, though.” Why? Because this connection offers hints about why yawns exist at all.
“Yawns are not consciously controlled,” says Robert Provine, from University of Maryland, Baltimore County. Even locked-in patients who cannot voluntarily move their limbs can yawn. Conversely, you can’t stifle a yawn. Once it starts, Provine writes, it “progresses with the inevitability of a sneeze.”
In humans, yawning is famously contagious; if you see someone else yawn, chances are you’ll do it too. A covered mouth will still spread a yawn, as will an upside-down face. Contagious yawning isn’t just limited to humans; it’s also found in other species like apes, monkeys, and dogs—domestic dogs can even catch yawns from their human owners.
For this reason, some scientists have suggested that yawning is a way of involuntarily showing empathy. After all, the species that have contagious yawns are all highly social. But the empathy explanation has been disputed, and regardless, it can’t be the whole explanation since yawning is also found in many solitary birds and mammals. It’s even possible (although more contentious) that fish yawn too. So it’s likely that contagious yawning is a more advanced form of a more evolutionarily ancient behavior that has some deeper physiological significance. What might that be? “There’s no consensus,” says Gallup. “It remains hotly debated.”
The most popular answer is that yawning helps to cope with a build-up of carbon dioxide, by increasing the flow of oxygen to the brain. Nonsense, says Provine. Decades ago, he showed that air that’s loaded with a hundred times more carbon dioxide than usual doesn’t trigger yawning, and breathing in pure oxygen won’t suppress it.
Gallup’s favorite explanation is that yawning is like sweating—a way of keeping cool. Specifically, it’s a way of chilling the brain—a gas-guzzling organ that consumes a lot of calories and is prone to overheating. When you yawn, you constrict and relax your facial muscles, increasing the flow of warm blood around the skull, and allowing some of that heat to radiate into the surrounding air. Yawning also involves a deep inhalation, which brings cool air into nose and mouth, chilling our blood from within.
This hypothesis “has been more rigorously tested and supported than others,” Gallup says. For example, he implanted temperature probes into the brains of rats and showed that they yawn when their brains heat up, and that yawning does indeed cool their brains. He also showed that people who hold a cold pack to their foreheads are far less likely to catch a yawn. And he found that yawning is most common at a Goldilocks zone of temperature—when the air is neither too cold nor too hot. Again, this fits with his idea. If the world is too cold, the brain is less likely to heat up in the first place. If the world is too hot, outside air won’t help to cool the brain.
“Yawning may have the dubious distinction of being the least understood, common, human behavior.”
Why then do we associate yawns with being tired? Again, says Gallup, it boils down to heat. Our body temperatures rise and fall over a 24-hour cycle, and they’re highest in the evening hours when we’re most likely to yawn. They plummet when we fall asleep and rise sharply when we wake up—another time of day when we yawn spontaneously. (Incidentally, the link to sleepiness is also behind the cultural connection between yawning and boredom; we don’t actually yawn more when we’re bored.)
So, if yawning is a means of cooling the brain, it follows that larger brains might require longer yawns. That’s why Gallup looked for videos of yawning animals, and that’s why he found the correlations he did. “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,” says Elainie Alenkær Madsen, from Lund University. “It’s a very nice and intuitive finding.”
Gallup also found that bigger-brained animals were more variable in the length of their yawns. “Maybe more-intelligent animals have different types of yawns in response to different stimuli, and less-intelligent animals only have one type of yawn,” says Cirulli Rogers. But she also notes that the study only looked at a small number of individuals, especially for many of the smarter species like elephants and monkeys.
It’s a fair criticism, says Gallup, who notes that his team found whatever footage they could. He hopes that the results will spur bigger studies. If you look within a single species, does the length of a yawn relate to an individual’s brain traits or mental skills? Could it be an early sign of brain damage?
As Provine wrote in 1986, “Yawning may have the dubious distinction of being the least understood, common, human behavior.” Thirty years on, we’re not much further along. But at least, it seems clear that the study of yawning is far from yawn-worthy.