Friday, May 6, 2011
I'm yawing. Now you are picturing me yawning. Are you yawing yet?
Contagious yawing is well documented in humans. You probably don't need a scientific study to tell you that, just yawn in the presence of another person and watch them yawn right back. It isn't known exactly why people yawn. It has been suggested that yawning increases arousal and therefore reduces the probability of sleep. Or it could be some kind of social signal, one to synchronize a group's behavior, communicating drowsiness, social stress, or boredom. The contagiousness of yawning may simply be a fixed action pattern (an instinctive behavioral sequence produced by a neural network and consisting of a response to a releaser stimulus that triggers the sequence to run to completion) where the releaser is the yawn of another. It could also be the result of nonconscious mimicry, where we adopt postures, gestures, and mannerisms of others without really realizing it. Or perhaps it is a form of empathy, in this case understanding another's feelings by inferring their mental state.
Contagious yawning has been studied in non-human primates as well including chimpanzees (Pan troglodytes), stump-tailed macaques (Macaca arctoides), gelada baboons (Theropithecus gelada), and dogs (Canis familiaris). But, overall, little is known about the function and prevalence of contagious yawning in other animals. A new study in Current Zoology (note: formerly Acta Zoologica Sinica) looks at discriminating possible mechanisms controlling contagious yawning. They do this by picking a species that strikes out some of those possibilities for contagious yawning I listed above. Here they look at the red-footed tortoise (Geochelone carbonaria) because it is a species that is unlikely to show empathy, nonconscious mimicry, and social mimicry. This is mainly because these tortoises are solitary and so have no need for a sensitivity to social cues, but they are highly visual and so can observe the behaviors of other tortoises.
Before starting the study the researchers had to prep the tortoises. Seven tortoises were housed together in two groups for six months prior to the experiment so they could get used to living in a group. They picked one tortoise as a "demonstrator," and taught that tortoise to yawn. Now, how do you go about teaching a tortoise to yawn? Well, they used a training technique called successive approximation. Whenever the tortoise opened her mouth slightly in the presence of a red stimulus she was rewarded with food until the point where she performed a gape-like response that appeared highly similar to a naturally occurring tortoise yawn. Once the training was complete they broke their study down into three experiments:
Experiment 1: Tested the response to of others to a single yawn. To do this they exposed them to three yawning conditions: (1) a single conditioned yawn, (2) a control with a non-yawning individual, and (3) a control with just the red stimulus. Then they measured the number of yawns. The results of this experiment showed that there was no difference in the responses across conditions. Three of the six tortoises tested did not respond to any of the conditions, but of the three that did respond two responded more in the conditioned yawn than the controls.
Experiment 2: Tested if contagious yawning would occur if the tortoise was presented with multiple yawns. To do this they exposed the tortoises to the experimental conditions from Experiment 1 with 2-3 yawns per trial. They they counted the number of responding yawns. The results of this experiment again showed no difference in response between conditions.
Experiment 3: Tested the possibility that the lack of yawning in the first two experiments was because the conditioned yawn did not appear as a yawn to the tortoises. To do this they presented the observing tortoises with video stimuli of real yawns, fake yawns, or an empty background. Again they counted the number of yawns in the observers. The results here? Nobody yawned.
The take-home message of this study are that the red-footed tortoise does not yawn in response to the yawns of other red-footed tortoises. This suggests that yawning, at least in this species, is not a fixed action pattern. It, therefore, may be a social cue or higher level mechanism controlling this behavior. Basically, these researchers were able to cross one contagious yawing hypothesis off their list.
All right. So why post about a study that ultimately showed nothing at all? First, because it was a study about tortoise yawning and that's neat, even if they didn't end up yawning. Second, it is a great example of a study of negative results. In science you don't always get those significant differences you expect when you start an experiment, and it is difficult to get such results published. Kudos to Current Zoology for publishing a study that had results such as these. And lastly, because I just had to know how you train a tortoise to yawn.
Read the study for yourself (note, this link is to the uncorrected proof):
Wilkinson, Anna, Natalie Sebanz, Isabella Mand, and Ludwig Huber (2011) No evidence of contagious yawning in the red footed tortoise Geochelone carbonaria. Current Zoology: published online (Link)
Also, a story in The Guardian: http://www.guardian.co.uk/education/2011/may/02/improbable-research-yawning-contagious-tortoise
(image from tortoisetrust.org)