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 hypothesized, 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.