There’s a strange correlation between brain size and yawning, study finds: ScienceAlert

A large-scale animal study from 2021 revealed something interesting about yawning: Vertebrates with bigger brains and more neurons tend to yawn longer.

The researchers collected data on 1,291 distinct yawns from trips to zoos and online videos, covering a total of 55 species of mammals and 46 species of birds. They found “strong positive correlations” between the duration of an animal’s yawn and the size of its brain.

“We went to several zoos with a camera and waited near the animal enclosures for the animals to yawn,” said ethologist Jorg Massen of Utrecht University in the Netherlands in a 2021 statement. “C was quite a long journey.”

The study could fill in some of the gaps in our knowledge about yawning, including why it happens in the first place and why animals such as giraffes don’t need to bother with yawning at all.

“Although the pattern of yawning is fixed, its duration co-evolved with brain size and number of neurons,” the researchers wrote in their paper.

“Furthermore, this function appears to be conserved across a diverse range of animals, such that its evolutionary origin may trace back at least to the common ancestor of birds and mammals and potentially even further.”

The analysis was set up to test a hypothesis put forward in 2007 by one of the researchers who worked on this study: that yawning is an essential way to cool the brain. It therefore follows that larger brains need longer yawns to cool down properly.

This seems to be confirmed by these data, which also show that mammals yawn longer than birds. Birds have a higher core temperature than mammals, which means a greater temperature difference with the surrounding air, which means that a shorter yawn is enough to draw in some cooler air.

Similar conclusions were reached in a 2016 study involving humans, although in this case only 205 yawns and 24 species were measured. He found the shortest yawns (0.8 seconds) came from mice, with the longest yawns (6.5 seconds) coming from humans.

“Through simultaneous inhalation of cool air and stretching of the muscles surrounding the oral cavities, yawning increases the flow of cooler blood to the brain, and therefore has a thermoregulatory function,” explained the ethologist. Andrew Gallup of the State University of New York (SUNY).

The researchers make no connection with intelligence, only the size of the brain and the number of neurons it contains; there is also no reference to the frequency of yawning. For example, we humans tend to yawn between 5 and 10 times a day.

It’s also contagious, as you may have noticed. One hypothesis about it is that it serves a social function, putting a group in the same frame of mind and possibly helping to synchronize sleep patterns. (More research will be needed to figure this one out, though.)

“Getting video footage of so many yawning animals takes a lot of patience, and the subsequent coding of all those yawns made me immune to the contagiousness of yawning,” observed biologist Margarita Hartlieb of the University of Vienna, in Austria.

Although there is more research to be done to uncover the reasons why we yawn, the study authors conclude that “these findings provide further support for distinct predictions derived from the brain cooling hypothesis.”

The research has been published in Communications Biology.

A version of this article was first published in May 2021.

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