Scientists love yawning. No, that's not quite right. Scientists love doing research on yawning. It seems to be of interest to folks in fields ranging from primatology to developmental psychology to psychopathology to animal behavior. If the notion of scientifically investigation the purpose of yawning makes you, well, yawn, then you're missing one of the more interesting debates in the social cognition literature. To understand why yawning is about more than feeling tired or bored, we have to go back a few years.

Once upon a time, scientists thought that yawning might be process through which the brain keeps itself cool (PDF). Yawning is associated with increases in blood pressure, and the consequential increase in blood flow might mean that the vascular system acts as a radiator, replacing the warm blood in the brain with cooler blood. It could also be that the deep inhalation of cold air during a yawn can, through convection, alter blood temperature which in turn could cool the brain.

Even if it turns out that some yawns can be explained through purely physiological means, yawning is also contagious for humans and other species. If someone watches someone else yawning, they'll be likely to yawn as well. That means that there is social component to yawning, and it might be related to empathy. It turns out that there's a correlation between a person's self-reported empathy and their susceptibility to reacting to a yawn contagion, and those who are more skilled at theory of mind tasks are also more likely (PDF) to yawn contagiously.

In 2009, psychologist Ramiro Joly-Mascheroni showed that human yawns are contagious for dogs. That is, the yawn contagion can be passed from one species, humans, to another, dogs. Perhaps this is unsurprising. If dogs are uniquely skilled at comprehending human social cues, and yawning is itself a sort of social cue, then it stands to reason that they might have also developed the ability to "catch" our yawns. In that experiment, 21 of 29 dogs yawned in response to watching a human yawn.

But then, a study that same year by Aimee Harr and colleagues did not replicate Joly-Mascheroni's findings. On the other hand, a 2010 study by Sean O'Hara and Amy Reeve did find evidence for contagious yawning in dogs, it was significantly weaker than what Joly-Mascheroni reported. In 2012, a study by Karine Silva and colleagues found that dogs were four times more likely to yawn after hearing the yawn of a familiar human than an unfamiliar human. While the Joly-Mascheroni, O'Hara, and Silva studies showed that dogs were susceptible to contagious yawns, and that the yawn contagion could spread between different species, they didn't serve to elucidate the possible role of empathy in contagious yawning. And why didn't Harr find evidence for contagious yawning?

While these sorts of confusing piles of evidence are commonplace in science, especially for relatively new lines of inquiry, that doesn't make them less confusing. Whose data will stand, and whose will fall?

In 2011, Matthew Campbell and Frans de Waal added a new wrinkle to the story by looking at chimpanzees. Chimpanzee culture is defined by the social group. Other research has provided plenty of evidence that chimps empathize with their own familiar group members, but not with strangers. So, Campbell and de Waal reasoned, if empathy explains the yawn contagion, then chimps should be more likely to yawn when watching a fellow group member yawn, but not when watching an unfamiliar chimp yawn. That's exactly what they found. And, since the chimps actually paid more attention when viewing videos of outgroup members, the results can't be explained by attentional differences. As for dogs (maybe), contagious yawning in chimpanzees seems to be modulated by social factors.

But just because yawning is social doesn't by necessity invoke empathy. In a review paper, animal behaviorists Jennifer Yoon and Claudio Tennie point out that dogs and chimps could be engaging in non-conscious mimicry, a simpler phenomenon. Indeed, when it comes to animal behavior the explanation that requires the least cognitive sophistication is usually best. And there's a third possibilitity. Yoon and Tennie say that contagious yawning could be due to the activation of a fixed action pattern. That would mean that contagious yawning is simply a mindless reflex, just like kicking your leg when the doctor hammers on your kneecap. Given the social nature of yawning for dogs and chimps, it is safe to reject the this explanation.

The latest contribution in the debate over contagious yawning was published today in the journal PLOS ONE. In this study, Elainie Alenkær Madsen and colleagues from Lund University in Sweden turned back to chimps. In particular, they were interested in determining how a chimpanzee's susceptibility to contagious yawning changes with age, and whether it depends on familiarity with the individual who they watch yawn.

But they also did something else that's new: they used human models to yawn instead of showing the chimps videos or audio playbacks of yawns. They reasoned that they could create more naturalistic conditions in which contagious yawning might occur by using live models rather than ones on video screens. "For practical reasons we consequently used human models," they write, "and the study thus represents the first test of interspecies yawn contagion in primates."

Thirty-three orphaned chimpanzees - twelve infants between 1 and 4 years old, and twenty one juveniles between 5 and 8 years old - participated in the study, which was conducted at the Tacugama Chimpanzee Sanctuary in Sierra Leone. In accordance with the guidelines of the Pan-African Sanctuary Alliance, chimps at Tacugama are provided with enriched physical and social environments, where they might adequately express their own species-typical behaviors. All the orphans are cared for with the hope that they might be released back into the wild.

The experiment was fairly simple: each chimpanzee participated in three seven five-minute experimental sessions in which the human models either yawned, opened their mouths wide (but did not yawn), or wiped their noses. Then, there were three post-experimental sessions when the humans continued to socially interact with the chimps, but without any yawning, mouth gaping, or nose-wiping. Each chimp participated in the study twice, once with a familiar human and once with an unfamiliar human.

Madsen's group found that yawning, but not nose-wiping, was contagious for the juveniles, and nothing was contagious for the infants. More specifically, forty-eight percent of the juveniles yawned after watching a human yawn, but none of the infants did. And there were no statistical differences in their responses to the familiar and unfamiliar yawns.

Weighed against all the prior research, the new findings accomplish at least one thing: they suggest that domestic dogs' susceptibility to catching a yawn contagion from humans probably isn't associated with their domestication, since juvenile chimpanzees show the same susceptibility.

But can this study shed any light on the question of empathy? As before, the waters are still muddy. The researchers write that the infants' apparent immunity to the yawn contagion could reflect the "developmental immaturity of [their] socio-cognitive skills and/or neural networks involved in processing social information." That is, contagious yawning may be tied with the emergence of skills which underlie empathy, like perspective-taking and the ability to identify others' emotions, which would explain its absence in the youngest chimps. The obvious follow-up to this might be to look for contagious yawning in the youngest human children, before they've completely developed those skills.

On the other hand, if contagious yawning was tied with empathy, then why did the juveniles yawn in response to unfamiliar humans, while the Campbell and de Waal study only found evidence for an ingroup contagion? It could be that the juveniles were motivated to bond with the unfamiliar human, and therefore engaged in a form of non-conscious social mimicry. Or, it could be that the familiarity effect doesn't emerge for chimpanzee-human contagious yawning, only for contagious yawning between chimpanzees. Among chimps, strangers are automatically enemies, and their relationship is inherently competitive. But chimps don't compete with unfamiliar humans like they do with each other. Comparing this study with the earlier one by Campbell and de Waal may be inappropriate, as Madsen herself acknowledges writing, "adult chimpanzees have only been tested on yawn contagion when viewing the yawns of conspecifics, while young chimpanzees have only been explicitly tested with respect to [human] yawn contagion." In other words, it might be apples and oranges.

If any answers are going to be found in the search for the social roots of contagious yawning, it's going to require that researchers better align their research methodologies. Some experiments have looked at within-species yawning, and other at between-species yawning. Some have used video stimuli, others used audio playbacks, and still others use live models. As researchers continue to probe the cognitive and emotional factors that provide a foundation for contagious yawning, it would be prudent to converge on a standard set of tools to do it.

A nice video from Lund University describes the new research:

Portions of this post were adapted from this post, originally published May 17, 2012.

Madsen EA, Persson T, Sayehli S, Lenninger S, Sonesson G (2013) Chimpanzees Show a Developmental Increase in Susceptibility to Contagious Yawning: A Test of the Effect of Ontogeny and Emotional Closeness on Yawn Contagion. PLoS ONE 8(10): e76266. doi:10.1371/journal.pone.0076266

Joly-Mascheroni, R., Senju, A., & Shepherd, A. (2008). Dogs catch human yawns Biology Letters, 4 (5), 446-448. DOI: 10.1098/rsbl.2008.0333

Matthew W. Campbell, & Frans B. M. de Waal (2011). Ingroup-Outgroup Bias in Contagious Yawning by Chimpanzees Supports Link to Empathy PLoS ONE, 6 (4) : 10.1371/journal.pone.0018283

Harr, A., Gilbert, V., & Phillips, K. (2009). Do dogs (Canis familiaris) show contagious yawning? Animal Cognition, 12 (6), 833-837 DOI: 10.1007/s10071-009-0233-0

O’Hara, S., & Reeve, A. (2011). A test of the yawning contagion and emotional connectedness hypothesis in dogs, Canis familiaris Animal Behaviour, 81 (1), 335-340 DOI: 10.1016/j.anbehav.2010.11.005

Yoon, J., & Tennie, C. (2010). Contagious yawning: a reflection of empathy, mimicry, or contagion? Animal Behaviour, 79 (5) DOI: 10.1016/j.anbehav.2010.02.011

Silva, K., Bessa, J., & de Sousa, L. (2012). Auditory contagious yawning in domestic dogs (Canis familiaris): first evidence for social modulation Animal Cognition DOI: 10.1007/s10071-012-0473-2

Chimp yawn via Flickr/Pelican; Baby yawning via Flickr/fumanch00. Dog photo copyright the author.