This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
It's been a pretty long stressful week around here, and not just because of Pepsipocalypse and the resulting fallout. But, well, I'm back, and I have an awesome paper to tell you about. When I saw it I just KNEW it had to be blogged.
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Did watching that video make you yawn? Chances are it did, and you can thank contagious yawning for it. What is contagious yawning? Contagious yawning is a very well-dcoumented phonemenon wherein yawning is triggered by the perception of others yawning. Originally, scientists thought that contagious yawning was the result of a releasing mechanism - in other words, seeing someone yawn flipped the yawning-switch in the brain, and it makes you yawn. But it actually turns out that there is a correlation between the susceptibility for contagious yawning and self-reported empathy. Converging evidence for the empathy hypothesis comes from the fact that contagious yawning is absent in individuals with autism spectrum disorder.
Spontaneous yawning is known to occur through a ton of vertebrate species, but until recently, contagious yawning only seen in humans, chimpanzees, and potentially in stumptail macaques (though this is still unclear). But domesticated dogs, since they are so good at reading human social and communicative cues, might show contagious yawning as well, and that is exactly what this study was designed to investigate.
As we've discussed before, dogs' unique social skills in interacting with humans is probably the result of selection pressures during the domestication process. So it's possible that as a result of that process, they may have developed the capacity of empathy towards humans. And if so, it is further possible that they may yawn when perceiving humans yawn.
Twenty-nine adult dogs (12 females, 17 males) were tested at home during the middle of the day, in two conditions. In one condition, the experimenter, who was a stranger to the dogs, attracted the dog's attention and then initiated a yawn comprising both physical movement and vocalization. The yawn was repeated for five minutes after re-establishing eye contact with the dog, which meant that the number of yawns varied between ten and nineteen per individual. The control condition was identical, except that the experimenter displayed a fake yawn, which mimicked the mouth opening and closing actions, but not the vocalization or other subtle muscular changes.
The yawning condition made the dogs yawn for 21 out of 29 individuals, and none of the dogs yawned in the control condition. Further, no yawns were observed in the interval between the two conditions (there was a five minute break between each of the five-minute-long conditions). During the yawning condition, dogs yawned an average of 1.9 times, and there were no sex or age differences, and there was no effect of the order in which the conditions were presented.
That's pretty amazing when you stop to think about it! Human yawns are possibly contagious to dogs! Seventy-two percent of the dogs tested yawning in response to human yawns. Amazingly, this is even higher than rates reported for humans (which range from 45-60%) and in chimpanzees (33%). Since no dogs yawned during the control condition, the results can't be attributed to the presence of the stranger, or human mouth movements in general, or anything like that. This is also the first recorded example of contagious yawning between two species!
So what might explain why human yawns make dogs yawn? In accordance with the current theory for human contagious yawning, it could reflect dogs' empathy towards humans. Dogs are really good at reading human social signals (e.g. finger pointing, eye gaze, etc), and it is possible that they can represent humans' actions in their minds and consequently adjust their own behavioral and autonomic responses.
Another possibility is that the dogs' yawns could be the result of mild tension or stress (this is the alternative explanation for contagious yawning in stumptail macaques, as well). Unfortunately, physiological data (such as cortisol levels or heart rate) were not available in this study, so this can't be entirely ruled out. However, the fact that the dogs did not yawn during the control condition suggests that the results are probably not attributable to stress more generally, such as the presence of the unfamiliar experimenter.
So I think this is a pretty cool study, aside from the fact that I've been yawning for the entire time I've been writing this. But the book is not yet closed on this question. For one thing, someone should do a study that collects both behavioral responses as well as physiological data to account for acute stress responses. Also, someone should do a study looking at contagious yawning between dogs, as opposed to between dogs and humans. If they acquired contagious yawning due to domestication, then it is possible that dogs are actually more sensitive to human yawns than dog yawns. So if I was going to do a follow-up study, one thing I'd do is look at infant puppies, to see if contagious yawning is innate (it appears to be innate in humans), or if it requires experience. I would also compare infant and adult dogs to infant and adult domesticated foxes, as well as undomesticated wolves (perhaps both wild, as well as hand-reared). I might also look at other domesticated animals, such as horses (do horses even yawn?)
I wonder if I'm really tired now because I've been yawning so much, or simply because it's pretty late?
Joly-Mascheroni, R., Senju, A., & Shepherd, A. (2008). Dogs catch human yawns Biology Letters, 4 (5), 446-448. DOI: 10.1098/rsbl.2008.0333