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Following a painstaking, large-scale animal study, researchers in the Netherlands now claim that, when it comes to yawning vertebrates, the larger and denser the brain, the longer the yawn.

In what could be considered the biological research equivalent of watching paint dry, the scientists collected data on 1,291 separate yawns by visiting zoos and poring over videos online, observing some 55 mammals and 46 species of bird.

"We went to several zoos with a camera and waited by the animal enclosures for the animals to yawn," says ethologist Jorg Massen, of Utrecht University in the Netherlands. "That was a pretty long haul."

The process was so tedious, in fact, that biologist Margarita Hartlieb, of the University of Vienna, Austria claims she is now immune to the socialized effect of yawning.

"Getting video footage of so many yawning animals requires quite some patience, and the subsequent coding of all these yawns has made me immune to the contagiousness of yawning," Hartlieb said.

Despite the painstaking and rather boring nature of the study, the researchers think they may have cracked several mysteries about yawning, including why some animals do it and others, such as giraffes, don't.

"This function seems to be conserved across a diverse range of animals, such that its evolutionary origin may be traced back to at least the common ancestor of birds and mammals, and potentially even further," the researchers write, suggesting that yawning co-evolved with brain size and neuron numbers across species.

The study was conducted to test the 2007 hypothesis, put forward by one of the research team, which proposed that yawning is a means of cooling down the brain. The logic was that, much like a supercomputer, the greater the size of the machine and the denser its processors (that is, neurons), the more cooling required to maintain its function.

This latest study shows that mammals yawn for longer periods than birds, which seems to support the theory.

Birds have a higher core temperature than mammals, and thus a greater difference in temperature with their surroundings, requiring shorter bursts of cooling air to keep their brains functioning. This is supported by previous research in the field.

"Through the simultaneous inhalation of cool air and the stretching of the muscles surrounding the oral cavities, yawning increases the flow of cooler blood to the brain, and thus has a thermoregulatory function," explains biopsychologist Andrew Gallup, of the State University of New York, who conducted his own smaller yawning study in 2016 and reached similar conclusions.