People tend to yawn when they are tired and bored but findings of a new study suggest a link between yawning and intelligence.

Andrew Gallup, from the State University of New York at Oneonta, and his team looked at the videos of 29 mammals yawning to calculate the average length of their yawns. The animals involved in the study have brain weight that have already been documented in an earlier research.

When the researchers crunched the numbers, they realized that brain weight and the number of neurons in the outer layer of the brain called cortex can reliably predict yawn length.

Gorillas, horses, walruses and African elephants, for instance, may have massive sizes but they have shorter yawns than humans because the size of their brains are smaller compared with ours. The findings suggest that the length of yawn does not correlate to the size of the body but to the size of the brain.

Earlier studies suggest that across species, relative large brains appear to be correlated with complex cognitive skills and more social strategies. Scientists however, consider the brain mass relative to the animal's body size when making speculations about cognitive abilities.

Gallup's research team also found that the animals with bigger brain were more variable in the length of their yawns. Liz Cirulli Rogers, from Duke University School of Medicine, said it is possible that more intelligent animals have different types of yawn in response to different stimuli while the less-intelligent ones only have one type of yawn.

"Using openly accessible data, we show that both the mean and variance in yawn duration are robust predictors of mammalian brain weight and cortical neuron number," the researchers wrote in their study, which was published in Biology Letters on Oct. 4

"Consistent with these effects, primates tend to have longer and more variable yawn durations compared with other mammals."

The correlation between brain size and yawn length may be explained by a theory that Gallup proposed in 2007. The idea posits that animals yawn to cool their brains. If this theory is correct, it can help explain why larger brains need a larger yawn to cool and regulate the brain temperature.

It also stands to reason why human yawns, which last about 6.5 seconds, are the longest in the animal kingdom beating those of other larger animals with known small brains.

Not all scientists, however, are convinced of the cooling function of yawning.

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