Yawning doesn't mean you're bored, it just means you've got one big, hot brain in your head, according to a study published Tuesday in Biology Letters.
The Atlantic reports that researcher Andrew Gallup had his students go online to collect videos of animals yawning (not a hard task considering yawning animals are the dang cutest). He then found he was able to predict the brain weight and the number of brain cells in the cortexes of various animals based on their yawn length. Perhaps surprisingly, body and mouth size had nothing to do with yawn length.
"We were just really blown out of the water," Gallup tells the Atlantic. "It’s such a strong predictor."
The explanation for brain size correlating to yawn length comes from a theory first proposed by Gallup in 2007, Stat reports. He argues that animals yawn to cool their brains, according to the Washington Post. If that's true, it stands to reason that larger brains would require a larger yawn to cool. And it explains why humans' yawns, at 6.5 seconds, are the longest yawns in the animal kingdom, beating out those of gorillas, lions, elephants, and more.
However, not all scientists are convinced yawning serves a cooling function, and some point to its contagiousness as evidence of a sociological function. Gallup isn't swayed. "Whether yawning functions specifically to cool the brain can still be debated, but there is no debate on whether yawning has thermoregulatory consequences," he says.
(In related news, men are less likely to catch a yawn.)