Larger brains mean longer yawns, study of 101 animal species reveals

A team of researchers has documented something straightforward but revealing about the animal kingdom: the bigger the brain, the longer the yawn. The finding comes from an unusually patient study in which scientists collected footage of 1,291 separate yawns across 101 species—55 mammals and 46 birds—and measured them against the size and neuron density of each animal's brain. The correlation held up consistently. Mammals yawned longer than birds. Humans, with their relatively large brains, produced yawns lasting around 6.5 seconds, while mice managed only 0.8 seconds.

The work began with a hypothesis. In 2007, one of the researchers on this study proposed that yawning serves a cooling function for the brain. If that were true, it would follow logically that larger brains—which generate more heat and have greater cooling demands—would require longer yawns to do the job. The new data supported the idea. "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," explained Andrew Gallup, an ethologist at the State University of New York.

The practical work of gathering this evidence was less glamorous than the findings suggest. Jorg Massen, an ethologist from Utrecht University in the Netherlands, described the process plainly: researchers went to zoos with cameras and waited by animal enclosures for yawns to happen. "That was a pretty long haul," he said. Margarita Hartlieb, a biologist from the University of Vienna, noted that coding all those yawns afterward had an unexpected side effect—she had become immune to contagious yawning, the phenomenon where seeing someone else yawn triggers your own.

The pattern explains some puzzles in animal behavior. Birds, which maintain higher core body temperatures than mammals, face less of a temperature gap with their surroundings. A shorter yawn is therefore sufficient to draw in cooler air. Larger-brained mammals need more time. The researchers emphasized that they were measuring yawn duration, not frequency or intelligence. Humans yawn between 5 and 10 times daily, but brain size alone predicted how long each yawn would last, not how often we did it.

The study also hints at why some animals don't yawn at all. Giraffes, for instance, appear to have no yawning behavior—a gap the researchers suggest their work might eventually help explain. The findings point toward an evolutionary origin for yawning that reaches back at least to the common ancestor of birds and mammals, possibly further still. The research, published in Communications Biology, represents a substantial expansion of earlier work. A 2016 study had measured only 205 yawns across 24 species and reached similar conclusions, but this new dataset provides far more robust evidence. The authors conclude their findings "provide further support for distinct predictions derived from the brain cooling hypothesis," leaving open the question of what other functions yawning might serve and why contagious yawning exists at all.