Reading that headline probably made you yawn.
And compared with other species, it was probably a pretty big yawn. That’s because as humans we have pretty big brains and the two are connected, at least according to a new study published this week in the journal Biology Letters.
It revealed a surprisingly tight correlation between the length of a yawn, the weight of an animal’s brain, and the number of number of neurons in the outer layer of the brain, the cortex.
To arrive at that conclusion, psychologist Andrew Gallup and his colleagues at State University of New York at Oneonta spent a lot of time watching clips of 24 different types of mammals yawning on YouTube. Camels, elephants, gibbons, rats, humans, lions, squirrels, several different kinds of monkeys, walruses, and so on. Sure enough, the species that yawned the longest had the weightiest brains and the most numerous neurons in the cortex.
It’s a curious connection, but one that makes sense if you subscribe to one of the (many) hypotheses that attempt to explain why yawns happen. Here’s Ed Yong, reporting for The Atlantic:
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.
The bigger the brain, the greater the need for cooling, the bigger the yawn.
Unfortunately, Gallup and his colleagues’ study still doesn’t answer why yawns are so contagious—or why you probably yawned a few times while reading this article. Psychologists have proffered a number of explanations over the years—empathy, intelligence, even time of day—but each of them has been shot down. So for now, we’re still waiting on a plausible answer to that question. Yawn.