Larger brains yawn longer, study of 101 animal species reveals

Researchers armed with cameras and considerable patience discovered something unexpected while watching animals at zoos: the bigger the brain, the longer the yawn. The finding emerged from a 2021 study that catalogued 1,291 separate yawns across 101 species—55 mammals and 46 birds—collected through zoo visits and online video footage. The data revealed what the researchers called robust positive correlations between brain size and yawn duration, suggesting that vertebrates with larger brains and more neurons simply hold their yawns open longer than their smaller-brained counterparts.

The work was designed to test a hypothesis first proposed in 2007: that yawning serves as a brain-cooling mechanism. If that's true, then larger brains would logically require longer yawns to achieve the same thermoregulatory effect. The pattern held across the animal kingdom. Mice produced the shortest yawns at 0.8 seconds; humans generated the longest at 6.5 seconds. Mammals, which have lower core body temperatures than birds, yawned longer overall—a difference explained by the fact that birds' higher internal heat means they need less time to draw in cooler air.

The mechanics are straightforward. When an animal yawns, it simultaneously inhales cool air and stretches the muscles surrounding the mouth and jaw. This combination increases blood flow to the brain and allows cooler air to circulate through the nasal passages, effectively lowering brain temperature. For a small-brained creature, a brief yawn suffices. For a human or an elephant, a longer duration is necessary to achieve meaningful cooling. The researchers found no correlation between yawn frequency and brain size—humans yawn between five and ten times daily regardless—only duration.

Jorg Massen, an ethologist from Utrecht University in the Netherlands, described the data-collection process with understated weariness: waiting by animal enclosures at multiple zoos, camera in hand, for the moment an animal would yawn. "That was a pretty long haul," he noted. The payoff was substantial. The study's findings suggested that yawning's thermoregulatory function is ancient and conserved, likely originating in the common ancestor of birds and mammals, possibly reaching back even further in evolutionary time. The pattern itself—the act of yawning—appears fixed across species, but its duration has co-evolved with brain size and neuron count.

A 2016 study involving humans reached similar conclusions, though it worked with a smaller dataset of 205 yawns across 24 species. Andrew Gallup, an ethologist at the State University of New York, explained the mechanism in detail: yawning increases cooler blood flow to the brain through simultaneous air inhalation and oral muscle stretching, serving a clear thermoregulatory function. The researchers were careful to note they found no link between yawn duration and intelligence—only brain size and neuron density mattered.

One curiosity remains unresolved: contagious yawning. The phenomenon is well documented—one person yawns, and others nearby follow—but its purpose is unclear. Some researchers hypothesize it serves a social function, synchronizing groups into a shared physiological state or coordinating sleep patterns. Margarita Hartlieb, a biologist from the University of Vienna who coded the yawns for the study, noted with dry humor that her exposure to so many yawning animals had rendered her immune to the contagion. The study's authors concluded their findings provide further support for the brain-cooling hypothesis, though they acknowledged that more research is needed to fully understand yawning's evolutionary origins and its social dimensions. The work was published in Communications Biology.