Researchers have found evidence that the length of a yawn can indicate how big your brain is, and how many neurons you have, which could explain why humans yawn longer than other mammals.

The discovery is based on the fact that animals, such as gorillas, hippos, and elephants - which are bigger than us but have a proportionately smaller brain - all yawn for a shorter duration than humans. This lead to the hypothesis that brain size - rather than head or body size - is likely linked to yawn length.

"Importantly, neither the size of the body nor the anatomical structures specific to yawning (cranium and mandible) are driving these effects, because gorillas, camels, horses, lions, walruses, and African elephants all have shorter average yawns than humans," the State University of New York team reports.

The researchers scanned YouTube to find a bunch of different animals yawning. In the end, they studied over 205 yawns from 177 individuals across 24 taxa. 

They found that mice had the shortest yawns, lasting, on average, 0.8 seconds, and humans had the longest, lasting an average of 6.5 seconds. Camels came in second, and dogs in third.

After recording all of the yawn times, the team investigated why certain animals yawned longer than others, finding that factors like body size and jaw shape had little to do with the length of a yawn.

The only correlation the team found was brain size, which actually fits into an existing hypothesis on why creatures yawn in the first place.

Despite the fact that nearly all animals yawn - except a few like the giraffe - scientists aren't entirely sure why. Some say that yawning happens because we are tired and need more oxygen to wake up, though the amount of sleep we get is not tied to how often we yawn.

One of the most convincing hypotheses came from Andrew Gallup, one of the researchers who worked on the latest study, who in 2007 proposed that yawning might act as a natural way to cool the brain - an idea that was later backed up by separate research in 2014.

Now, with the latest findings pointing to brain size being a leading factor into how long a yawn lasts, the evidence suggests that Gallup might have been right all along, because a larger brain would need a longer yawn to cool down, as compared to a smaller brain.

It's important to make a distinction here that the team isn't saying that if you yawn longer than another person, you are somehow smarter than they are. They are simply showing that - as humans - we have the largest brains and, therefore, need to yawn longer. They never make the jump to actual intelligence.

But the team says there's need for further study to find out why adults yawn longer than children, and also how this cooling might work for different species.

So scientists might finally be on the verge of understanding yawning. Knowing how yawning might cool the brain could also open up a better understanding of how our bodies work in general, which could have a major impact on many fields.

The team's work was published in Biology Letters.