Thirty years ago, yawning was called “the least understood, common, human behavior,” and today it remains that way. Scientists still have no clear idea why people — or other mammals — yawn.
In 2007, Andrew Gallup, an evolutionary neuroscientist at the State University of New York at Oneonta, proposed the “thermoregulatory theory of yawning” — that opening one’s mouth and deeply inhaling pulls cool air into the brain, thus helping to keep the brain’s temperature balanced. Other research has suggested that yawning increases the circulation of cerebrospinal fluid, which then switches the brain from its “default mode” to a more alert state.
These theories remain controversial. But if true, Gallup thought, longer yawns would have greater physiological effects. And that meant larger brains would therefore require longer yawns.
He and his colleagues came up with a unique way of testing this hypothesis: timing and then averaging the duration of the yawns of various mammal species, collected mostly from YouTube videos (where else?).
It was not always easy to find good videos. “One video with multiple yawning clips from a litter of kittens was excluded owing to the inability to distinguish between individuals,” Gallup and his colleagues explain. “There were also nine cases where the start of the yawn was not clear (e.g., video began when the animal was already yawning, the animal was looking away during the execution of the yawn or there was no clear endpoint to identify).”
In the end, however, the researchers were able to find useable videos of 109 individual animals from 19 species, including African elephants, camels, cats, dogs, foxes, rats, gorillas and hedgehogs.
A strong correlation
After all the data was collected and analyzed, Gallup and his colleagues discovered something rather remarkable: The average yawn duration — which ranged from 0.8 seconds in mice to 6.5 seconds in humans — was a “robust” predictor of a species brain weight and size, as measured by its number of cortical neurons. (Cortical neurons are the brain cells located in the wrinkly outer layer of the cerebral cortex, which is “the seat of complex thought.”)
Small-brained animals with fewer neurons in the cerebral cortex had shorter yawns than large-brained animals with more cortical neurons.
In fact, yawn durations were a more reliable predictor of brain weight and cortical neuron counts than either the animal’s total body size or the size of its brain relative to its body.
African elephants, camels, horses and walruses, for example, had shorter yawns than humans — despite their massive body size.
And, yes, elephants have brains that weigh about three times more than those of humans, but their cerebral cortex contains only about one-third as many neurons.
Gorilla yawns were also shorter than those of humans. But gorillas, like all the primates in the study, had significantly longer yawns than the other mammals — even though many of the non-primates opened their mouths wider when yawning.
More questions to be answered
This study, which was published recently in the journal Biology Letters, doesn’t prove that the evolutionary purpose of yawning is to cool the brain, but it adds weight to that hypothesis.
“These combined effects represent a striking scaling relationship between brain and behaviour,” write Gallup and his colleagues. “Importantly, neither the size of the body nor the anatomical structures specific to yawning (cranium and mandible) are driving these effects.”
One more caveat: The study doesn’t say anything about the individual variations of yawn duration within species.
In other words, just because you yawn longer than, say, your spouse, you can’t claim bragging rights about having a bigger brain.
Gallup is, however, studying that issue right now. So, who knows? Stay tuned.
FYI: You can read Gallup’s study in full on the Biology Letters website. You’ll find detailed information about how long each mammal yawned (and links to the videos) in the study’s supplementary material. (Not to start a fight among pet owners, but the yawns of dogs averaged 2.4 seconds, while those of cats averaged only 1.97 seconds.)