The mere sight of another person yawning causes many of us to open our mouths wide in mimicry. And we’re not alone—other social animals, such aschimpanzees and lions, can also catch so-called contagious yawns. It’s likely that all vertebrates yawn spontaneously to regulate inner body processes.
Yawning probably arose with the evolution of jawed fishes 400 million or so years ago, says Andrew Gallup, an evolutionary biologist at State University of New York Polytechnic Institute who has spent years trying to figure out why we yawn. In a paper published this month in Animal Behavior, he reports some evidence for how contagious yawns might have evolved to keep us safe. Science chatted with Gallup about why yawning is ubiquitous—and useful. This interview has been edited for clarity and length.
Q: First, let’s address a long-standing myth: Does yawning increase blood oxygen levels?
A: No. Despite continued belief, research has explicitly tested that hypothesis and the results have concluded that breathing and yawning are controlled by different mechanisms. For example, there are really interesting cases of yawning in marine mammals, where the yawning occurs while the animal is submerged underwater and therefore not breathing.
Q: So what does yawning actually do to the body?
A: Yawning is a rather complex reflex. It’s triggered under a variety of contexts and neurophysiological changes. It primarily occurs during periods of state change, commonly following transitions of sleeping and waking. There’s research that also suggests that yawns are initiated alongside increases in cortical arousal, so yawns themselves may function to promote alertness. And there’s a growing body of research that suggests that yawning is triggered by rises in brain temperature. I’ve conducted a number of studies testing this in humans, nonhuman mammals, and even birds.
Q: What have those studies shown?
A: These studies show that we can reliably manipulate yawn frequency by changing ambient temperature and the brain and body temperature of the individual. In rats, we’ve conducted studies showing that rises in brain temperature reliably trigger yawning, [and that yawning is] followed by decreases in brain temperature.
Q: Do all animals yawn the same way?
A: We’ve done a number of large-scale comparative studies, where we’ve recorded the yawn durations from over 100 mammalian and avian species. We find that even when controlling for body size, there are very strong positive relationships between how long an animal yawns and how large and complex their brain is.
Q: One of the most curious things about yawning is that it can be contagious. Do all animals yawn contagiously?
A: We’ve been talking about spontaneous yawns up to this point—those are internally, physiologically driven. Contagious yawns are elicited by seeing or hearing yawns in others, and [they] have only been documented in highly social species, humans included. There’s a wide variability in that response among individuals. Some individuals are very susceptible to yawn contagion, while others are not.
Q: What accounts for that variability?
A: Some studies suggest that individual differences in empathy may be contributing to this response. If we see someone yawn, and that reflexively triggers the same response in us, that could be kind of a basic indicator of empathetic processing. However, other studies have failed to show that relationship. I’m of the view that the jury’s still out.
Q: So why do we yawn when we see someone else yawning?
A: Contagious yawning may have evolved to synchronize group behavior—yawns often cluster during particular times of day that coincide with transitions and activity. It also may have evolved to increase vigilance within a group. The basic rationale is that if yawning is an indicator that one individual is experiencing diminished arousal, then seeing another person yawn might, in turn, increase the observer’s vigilance to compensate for the low vigilance of the yawner. The spreading throughout the group of contagious yawns might then increase the vigilance of the entire group.
I conducted a study last year that tested this. We showed people arrays of images that included threatening stimuli—images of snakes—and nonthreatening stimuli—images of frogs—and timed how fast they could pick out those images after seeing videos of people yawning or moving their mouths in other ways. After seeing other people yawn, their ability to identify and detect snakes, the threatening stimuli, rapidly improved. However, following the observation of yawning, frog detection was unaffected.
Q: You read, write, and think about yawns all day. Are you yawning all the time as a result?
A: When I first started studying this subject, I was yawning excessively. I was reading the literature and writing notes and writing papers, and I found that I was just yawning all the time. But over time, I became kind of habituated to the effects. I still yawn contagiously during social interactions, but seeing the stimuli that I use in the lab no longer produces the effect.
Source: Science Mag