Recently, scientists discovered a correlation between yawning and brains: the longer the average duration of a specie’s yawn, the bigger that specie’s brain size,  as measured by brain weight and total number of cortical neurons.

The study was conducted on 109 individuals from across 19 different species, including cats, humans, mice, camels, and more. The investigators found that the duration of yawns was shortest in mice, who averaged 0.8 seconds, and longest in humans, who averaged 6.5 seconds. The scientists plan on investigating whether this correlation holds true amongst individual members of a species.

The study was created in response to the ideas set forth in Gallup’s 2007 paper on the thermoregulatory theory of yawning, one of the strongest theories about why we yawn (we do not yet definitively know the biological purpose of yawning). The thermoregulatory theory indicates that yawning cools down the brain in homeotherms via three potential mechanisms. But whether or not this brain-cooling is simply a side effect or the primary function of yawning is up for debate.

Based on Gallup’s paper, the investigators of this study hypothesized that longer yawns would produce greater physiological responses, in terms of blood flow and circulation to the brain– which would be evolutionarily necessary for species with larger, more complex brains.

There are other theories about why we yawn, such as a 2014 paper stating that yawning stimulates cerebrospinal fluid circulation, which in turn increases species’ alertness. A common theory that yawning increases blood oxygen levels has largely been disproved. How would such alternate theories have different implications for the discovered correlation between yawning and brain size?