Just the sight of another person yawning causes many of us to open our mouths wide in mimicry. And we are not alone - other social animals, such as chimpanzees and lions, can also catch so-called contagious yawns. It is likely that all vertebrates yawn spontaneously in order to regulate internal processes in the body.
Yawning probably originated with the evolution of fish with jaws 400 million years ago, says Andrew Gallup, an evolutionary biologist at the State University Polytechnic Institute in New York who has been trying to understand why we yawn for years. In a paper published this month in Animal behaviorhe reports some evidence of how contagious yawning may have evolved to protect us. Science talked to Gallup about why yawning is ubiquitous — and useful. This interview is arranged for clarity and length.
Q: First, let’s deal with a long-standing myth: does yawning increase blood oxygen levels?
ABOUT: Not. Despite the constant belief, the research explicitly tested this hypothesis and the results concluded that breathing and yawning are controlled by different mechanisms. For example, there are really interesting cases of yawning in marine mammals, where yawning occurs while the animal is submerged under water and therefore does not breathe.
Q: So what does yawning actually do to the body?
ABOUT: Yawning is a rather complex reflex. It is triggered in different contexts and neurophysiological changes. It primarily occurs during the period of change of state, usually after the transition of sleep and waking. There are studies that also suggest that yawning is triggered by an increase in cortical arousal, so that only yawning can function to promote alertness. And there are a growing number of studies suggesting that yawning is caused by a rise in brain temperature. I have conducted numerous studies testing this on humans, non-human mammals, and even birds.
Q: What did those studies show?
ABOUT: These studies show that we can reliably manipulate the frequency of yawning by changing the ambient temperature and the temperature of the brain and body of an individual. We have conducted studies in rats that show that a rise in brain temperature reliably causes yawning, [and that yawning is] followed by a decrease in brain temperature.
Q: Do all animals yawn in the same way?
ABOUT: We did a series of large comparative studies, where we recorded the duration of yawning in over 100 species of mammals and birds. We find that even when body size is controlled, there are very strong positive relationships between how long animals yawn and how large and complex their brains are.
Q: One of the most interesting things about yawning is that it can be contagious. Do all animals yawn contagiously?
ABOUT: So far we have talked about spontaneous yawning - it is inside, physiologically guided. Infectious yawning is caused if you see or hear yawning in others, and [they] are documented only in highly social species, including humans. There is great variability in this response among individuals. Some individuals are very susceptible to yawning, while others are not.
Q: What explains this variability?
ABOUT: Some studies suggest that individual differences in empathy may contribute to this response. If we see someone yawning, and that reflexively triggers the same response in us, that could be in some way a basic indicator of empathic processing. However, other studies have failed to show this connection. I think the jury is still out of power.
Q: Then why do we yawn when we see someone else yawning?
ABOUT: Infectious yawning may have evolved to synchronize group behavior - yawning is often grouped at certain times of the day that coincide with transitions and activities. It may also have evolved to increase vigilance within the group. The basic rationale is that if yawning is an indication that one person is experiencing reduced arousal, then watching another person yawn could, in turn, increase observer alertness to compensate for low yawning alertness. Spreading infectious yawning throughout the group could then increase the alertness of the entire group.
Last year, I conducted a study that tested this. We showed people a series of images that included threatening stimuli — snake images — and non-threatening stimuli — frog images — and determined how quickly they could choose 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, threatening stimuli, quickly improved. However, after observing the yawn, the detection of frogs was not affected.
Q: You read, write and think about yawning all day. Is that why you yawn all the time?
ABOUT: When I first started studying this topic, I yawned excessively. I read literature, wrote notes and wrote papers, and I found myself yawning all the time. But over time, I’ve gotten used to the effects. I still yawn contagiously during social interactions, but watching the stimulants I use in the lab no longer produces an effect.