Why a drawn-out yawn is the sign of a bigger brain

IT’S often considered a sign of tiredness, boredom – not to mention rudeness.

And now researchers have found that a big yawn is indicative of a fourth thing – a larger brain.

A study of animals from mice to elephants has equated bigger brains with lengthier yawns.

The find could explain why humans have the longest yawns in the animal kingdom.

To make the connection, the US researchers scoured YouTube for videos of yawning animals. Yawn length was then compared against data on the average brain size of the various species.

It wasn’t just the case that larger animals with gaping jaws had the biggest yawns because animals such as gorillas, hippos and elephants have shorter yawns than humans. Instead, it seems that brain size is key, say the researchers from the State University of New York.

Creatures with big brains, particularly relative to their body size, yawn for longer, they report in the Royal Society journal Biology Letters.

Mice have the shortest yawns, averaging just 0.8 seconds, camels manage 4.8 seconds and people 6.5 seconds.

The suggestion that yawning is linked to brain size could help shed light on why we yawn at all.

Some scientists say that yawning is a sign of empathy. It is argued that ‘contagious’ yawning after someone else does is a sign of being keenly interested in their thoughts and feelings.

Others say that we simply yawn when we are tired and need more oxygen to wake up.

A third theory claims that yawning cools the brain. A large and hard-working organ, the brain eats up a lot of calories and is prone to overheating.

The latest research fits with this – because a bigger brain would take more cooling.

Elainie Madsen, who researches yawning at Lund University in Sweden, said: ‘If yawning cools certain parts of the brain, it makes sense that larger brains with more neurons require longer yawns and more influx of air. It’s a very nice and intuitive finding.’