How wide a person yawns is an indication of how big a person’s brain is, scientists report. People who have larger brains can thank yawning for promoting brain growth and activity, while those with smaller brains can blame it on not yawning enough, Fox News reports.
Psychologist Andrew Gallup of the State University of New York at Oneonta studied 109 individuals from 19 different species and found that the average durations of yawns can predict brain weight and the number of cortical neurons in the species. These included humans, walruses, mice, rabbits, capuchin monkeys and African elephants.
In 2007, Gallup had proposed what is called the thermoregulatory theory of yawning. It posits that when a person opens their jaws wide to suck in air, it cools the brain – an idea that other studies have documented. Yawning is also likely to pull the brain out of its “default” mode, which is a resting state, into a more active state that pays attention by increasing the circulation of cerebrospinal fluid, according to a 2014 study.
Moreover, yawning can increase blood flow to the brain because of jaw stretching and the deep influx of air, which replaces warm blood in the brain with cool blood from the heart. This exchange also happens with the air outside, which is cooler than body temperature.
Gallup says,
Longer and/or [more] powerful yawns should provide greater physiological effects.
The theory prompted the idea that the duration of a yawn should parallel brain size and complexity because bigger brains with more neurons require more blood flow.
Gallup and a team of researchers timed yawns on YouTube videos, ranging in duration from 0.8 seconds in mice to 6.5 seconds in people. They found that the issue of jaw size was immaterial, as big animals like elephants and horses had shorter average yawns that people.
Critics have contended that no matter how wide a person’s jaw opens and how much air is sucked in, it isn’t enough to cool the brain. Gallup answered by saying, “Whether yawning functions specifically to cool the brain can still be debated, but there is no debate on whether yawning has thermoregulatory consequences.”
The researchers are currently testing the idea on human subjects.
