When you let forth a big, embarrassing yawn during a boring lecture or concert, you succumb to a reflex so universal among animals that Charles Darwin mentioned it in his field notes. “Seeing a dog & horse & man yawn, makes me feel how much all animals are built on one structure,” he wrote in 1838. Scientists, however, still don’t agree on why we yawn or where it came from. So in a new study, researchers watched YouTube videos of 29 different yawning mammals, including mice, kittens, foxes, hedgehogs, walruses, elephants, and humans. (Here is a particularly cute montage used in the study.) They discovered a pattern: Small-brained animals with fewer neurons in the wrinkly outer layer of the brain, called the cortex, had shorter yawns than large-brained animals with more cortical neurons, the scientists report today in Biology Letters. Primates tended to yawn longer than nonprimates, and humans, with about 12,000 million cortical neurons, had the longest average yawn, lasting a little more than 6 seconds. The yawns of tiny-brained mice, in contrast, were less than 1.5 seconds in duration. The study lends support to a long-held hypothesis that yawning has an important physiological effect, such as increasing blood flow to the brain and cooling it down, the scientists say.
*Correction, 6 October, 10:11 a.m.: A previous version of this story inaccurately suggested that a human brain is roughly the same weight as an elephant's. In fact, the African elephant brain is about three times heavier than the human brain.