The mystery of yawning: scientists suggest it’s proof of a bigger brain

Everybody yawns. Everybody. The reflexive deep, jaw-stretching inhale followed by a pause and a forced exhalation is pretty much ubiquitous in the animal kingdom, at least among creatures with the right anatomy for it.

It’s not clear why we yawn – or when our ancestors started the refreshing routine – but many scientists believe the action serves to cool down the brain. Brains use a lot of energy, and they run hot. Inhaling a rush of cool, ambient air chills the blood, and the widening of the jaw sends a nice blast of that breezy blood into the brain.

A new study in Biology Letters could add further support to this popular theory. If bigger yawns produce a greater cooling effect, the study authors hypothesised, then animals with bigger brains – and therefore more brain tissue to cool down – would produce more sustained yawns. Their data suggests that this is indeed the case. Forget the dream of having big brains and brawn; big brains and yawn is much cooler.

Led by Andrew Gallup of the State University of New York at Oneonta, who has long investigated this possible brain cooling mechanism, researchers studied 29 mammals whose brain weights had been documented. They tracked down videos of these animals yawning – mostly on YouTube – to calculate the average length of their yawns.

When the team crunched the numbers, they found that brain weight and “cortical neuron number” – the number of brain cells in the outer layer known as the cortex – were reliable predictors of yawn length, more reliable than total body size or relative brain size. Gorillas, camels, horses, lions, walruses and African elephants were all found to have shorter yawns than humans despite their massive sizes, which makes sense because their brains are smaller than ours.

In other words, the length of a yawn doesn’t seem to correlate to the size of your body. It seems to correlate to the size of your brain.