Provine, the pioneer in yawning research, wrote that  ”Yawning may have the dubious distinction of being the least understood, common human behavior “. Even though it is quite usual and is present across the animal kingdom, the origins and function of yawning have not completely been unraveled. Do we yawn to stay alert? Or when we are bored? why is it that when we see someone else do it a few seconds after we are yawning as well? Is it the same for other species? Do we share the same yawning characteristics with other primates?

Yawning is characterized by a powerful gaping of the jaw with inspiration, a brief period of peak muscle contraction, and a passive closure of the jaw with shorter expiration. The average duration of one is 5 s. Additionally, stretching of the limbs also frequently accompanies yawning. It has been often associated with boredom and drowsiness, but it may carry a more important function to the communication between subjects. It may seem like a trivial function and the literature is not vast, but its study can help elucidate an evolutionary link between species.

 

Why and How to study yawning

It is important to study yawning to understand evolutionary commons since it is a virtually ubiquitous behavior among all vertebrate species. There shall be an evolutionary advantage of yawning since it seems to be phylogenically old and frequent phenomenon. Besides occurring as a spontaneous event, it has the specific characteristic of being contagious in some animals. In the case of 40-60 percent of humans, hearing, reading, seeing another animal yawn or even thinking about it can trigger it.  Researches have different hypotheses for spontaneous and contagious yawning, and depending on this they use different tools to induce yawning.

Types of Yawning

To classify yawning behavior in clearly defined types is still under study. In human and nonhuman primates, two different types of yawn are generally distinguished according to the physiological state and social context. The true/rest yawns and the tension/aggressive or ’emotion yawn’. The physical manifestation of both is similar. True yawns are typically associated with states of drowsiness and relaxation (sleepiness or boredom), and are observed in transitions from rest to waking state.  The ’Emotion yawn’ could also be called the social yawn since it is elicited by a number of social signals, such as conflict, anxiety or threat situations.

They can also be classified as spontaneous or contagious. Spontaneous yawning is triggered due to physiological changes in internal temperature or arousal and is spread across vertebrates. On the other hand, contagious yawning (CY) is driven by sensing and involuntarily mimicking the action. Only humans and a limited number of non-human species have been documented to yawn contagiously. Those with pets at home must have noticed that following the dog’s or cat’s yawn we find ourselves yawning right after.

 

Contagious Yawning and Empathy

How CY relates to empathy requires more experimental evidence. In the study by Norscia, 2011, only social bonding predicted the occurrence, frequency, and latency of yawn contagion. The rate was higher in response to familiarity, leading them to conclude that it is mainly driven by a close emotional relationship with the other subjects. Since CY comes from witnessing someone or thinking about another person yawning, researchers use different kind of stimuli to induce the phenomenon. They typically show a video of another yawning subject or a live set-up subject (with different degrees of familiarity between the subjects). Taking into account that the social presence, of a real person or of a camera, can influence the results. CY would be a social model that concerns a communicative function. The functionality of yawning can be seen as a social one and/or a physiological one. Both views are up for test. According to Gallup, 2011 it is likely that instead of serving one purpose, yawning is multifunctional across a number of species, but as Guggisberg et al., 2011 responded, it is necessary more comparative data across species to get to a conclusion.

 

Why do we yawn?

Numerous hypotheses on the function of yawning have been proposed, usually derived from behavioral observations. In mammals, it has been observed that more than 90 percent of yawns occur at rest whereas the remaining yawns seem to be triggered by social or emotional stimuli. Researchers have not reached a consensus on the functions of yawning. The hypotheses formulated in the literature can be separated in the ones centered in physiological phenomena and the social/communicative hypothesis. All hypotheses postulating a physiological role of yawning share the common assumption that yawning regulates a particular body function be it the blood oxygen level, the brain arousal level or internal temperature. In the review by Guggisberg et al., 2010, different physiological theories and their relationship to triggers and effects are examined. The triggers refer to the change in body states that lead to yawning and the effects of how yawning acts on the corresponding body function. It also mentions the social perspective on yawning, to which the authors favor the most since they claim it has more experimental evidence.

 

Hypotheses

An old explanation for why animals yawn was the need for oxygen-rich air to help us stay alert and awake. But there is no evidence of yawning affecting the level of oxygen in the bloodstream. So, given current results, it seems unlikely that yawning has respiratory or circulatory functions.

Yawning is mostly associated with the feeling of sleepiness and boredom. Experimental data suggest that indeed it occurs during a progressive state of drowsiness. In humans, yawning follows a circadian pattern and occurs with the greatest frequency within the hours just after and right before waking up. In the review by Baenninger,1997, the author names experimental support to the state change hypothesis by Provine,1986 (for example sleep/wake state changes, boredom to alertness, etc.), saying that it facilitates arousal during environmental transitions.

Research on humans, chimpanzees, and rats, show that yawns precede an increase in activity from the attentional network in the brain. The arousing effect of yawns come following stressful events, threats, and increases in anxiety and some effects are changes in heart rate and skin conductance.

Another physiological function of yawning has been proposed, one that suggests that it regulates the temperature of the brain. Research shows that yawning frequency is altered by fluctuations in ambient temperature show a specific correlation between temperature and behavior. The thermoregulatory theory proposes that yawns should increase in frequency as the temperature rises so the regulatory mechanisms take place and balance the temperature and that yawns should decrease as ambient temperature draw near or exceed body temperature.

Yawning can be seen as a communicative function occurring in social contexts. The  Social/Communication Hypothesis states that yawning is a non-verbal form of communication that synchronizes the behavior of a group. The majority of the evidence in support for the social/communication hypothesis hinges on the contagious nature of this behavior in humans, the susceptibility to contagious yawning correlates with empathic skills in healthy humans.

The relationship of CY with empathy has not been thoroughly reported, though its link with social closeness (dependent of age) has been reported in several studies. Individuals scoring higher on a psychopathic personality inventory showed reduced contagious yawning.  

Initial reports on the absence of contagious yawning in children with autism spectrum disorder support the connection of CY to empathy. More controlled empirical studies on the social effects of yawns in different species are needed to describe the theory. Gallup, 2011 is inclined to assume that the communicative value in primates is a derived feature that appeared later in evolution from a basic communicative function. While Guggisberg et al., 2011 argues for yawning as a non-verbal form of communication that could have evolved in vertebrates independent of a physiological function, besides the mechanical component of it. The neural basis of contagious yawning activates a complex network of brain regions related to motor imitation, empathy, and social behaviour, it activates the right posterior inferior frontal gyrus which is part of the mirror neuron system responsible for action observation and imitation.

 

Come yawn with me

Many have hypothesized the function of yawning, and it is still up for debate whether it serves a physiological or social function or both. More studies are necessary and with a higher number of subjects, especially in the study of contagious yawning in non-human primates. It is hard with a small sample size to detect contagious yawning at the population level. To study the relationship between empathy and contagious yawning, given its commonality in multiple species and the low-cost of the behavioural data acquisition could lead to new insights in the mechanisms underlying the phenomenon of contagious yawning in humans and our nearest evolutionary neighbors. Perhaps in the future, it could lead to finding a precursor of a theory of mind that relates human and non-human primates.

 

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