Why do mammals yawn?

By  Jemima Frame

Warning: Reading this may cause excessive yawning

Yawning is a behavioural trait that is common between all vertebrate mammals and is seen as one of our most primitive behaviours, with Charles Darwin (1838) noting that “seeing a dog & horse & man yawn, makes me feel how much all animals are built on one structure”. Spontaneous yawning is thought to be a physiological function, occurring when animals need to be more alert. The act of yawning increases the blood flow to the head, which in turn oxygenates the brain (Ramirez et al, 2019), stimulating it into action. Yawning is often contagious, even the act of thinking about it (or reading about it) will cause someone to yawn, and the reason behind this is still largely debated about, with some scientists suggesting empathetic yawning to play a part. 

Research on humans have found that yawning is not only caused by fatigue, but also by boredom and hunger. All of these traits cause a lack of stimulation in the brain, and yawning is the body’s way of trying to restart the brain. In 1923 it was observed by Sir Francis Walshe that yawning allowed paralysed patients to temporarily regain their motor functions (Konnikova, 2014). This incredible revelation led to scientists determining that yawning is activated by a primal centre of the brain, rather than through conscious control. This means that yawning is spontaneous and is an unconscious behaviour; you cannot yawn on command and you often cannot stop a yawn once it has started. Contagious yawning is also a phenomenon that is very prevalent among humans. The reason for this is widely debated, with one suggestion being that it is related to empathy. Studies have shown that children younger than 5 do not yawn any more frequently when watching videos of people yawning than on average (Anderson and Meno, 2003), supporting this theory as empathy will only develop in the later stages of a child’s life. Another study showed that we were more likely to catch a yawn from a family member than a friend, and a friend than an acquaintance (Norscia and Palagi, 2011). Studies on chimpanzees also provided evidence for this theory, with chimpanzees significantly more likely to yawn when watching videos of familiar chimpanzees yawning, compared to stranger chimpanzees (Campbell and de Waal, 2011). However, scientists have also found that some humans are simply more prone to contagious yawning; and yawning more does not necessarily make them more empathetic (Bartholomew and Cirulli, 2014). There is still a debate to this day over whether contagious yawning and empathy are connected. 

A recent study on contagious yawning in lions bought about a new theory; contagious yawning is critical for developing advanced forms of sociality (Casetta, Nolfo & Palagi, 2021) . The study, observing 2 prides of lions, found that not only was a lion (B) more likely to yawn if they saw another lion (A) yawn, lion B would also mimic lion A’s subsequent movements after yawning. For example, if both lions were lying down and lion A yawned, lion B would also yawn, if lion A subsequently stood up, lion B would copy lion A and also stand up. This suggests that contagious yawning may have evolved to help animals living in social groups to synchronise their movements. The lions that caught a yawn from another lion were 11 times more likely to mirror the original yawners (Casetta, Nolfo & Palagi, 2021), a significant difference to lions who had not caught a yawn off the lion. This research provides evidence that yawning may play an important part in social cohesion, as yawning helps to harmonise a groups movements; a vital behaviour for animals who hunt and rear offspring cooperatively. 

Contagious yawning could serve multiple other functions in a social animal group that we still do not know about. For example, gelada baboons have been identified to have three distinct types of yawns, all of which convey different messages including friendliness and aggression (Leone, Ferrari & Palagi, 2014) however research on the importance of these yawns is not extensive enough. Yawning, despite it being one of the most common behavioural traits among mammals, is still widely misunderstood, with research only being carried out on a handful of species, and contagious yawning is only recently being accepted as a very important behavioural trait in group living species. This recent discovery will hopefully pave the way for future research to be carried out to discover any other functions contagious yawning might provide as well as the physiological science behind why animals catch yawns. 

References:

• Ramirez, V et al. (2019) Manipulating neck temperatures alters contagious yawning in humans. Physiology & Behaviour. 207, 86-89. Available from: https://doi.org/10.1016/j.physbeh.2019.04.016

• Konnikova, M. (14^th April 2014) The surprising science of yawning. The New Yorker. Available from: https://www.newyorker.com/science/maria-konnikova/the-surprising-science-of-yawning [Accessed 29/05/2021]
• Anderson, J. R. & Meno, P. (2003) Psychological Influences on Yawning in Children. Current Psychology Letters : Behaviour, Brain & Cognition. (11), Available from: doi: 10.4000/cpl.390.
• Norscia, I. & Palagi, E. (2011) Yawn contagion and empathy in Homo sapiens. PloS One; PLoS One. 6 (12), e28472. Available from: doi: 10.1371/journal.pone.0028472.
• Campbell, M. W. & de Waal, F.,B.M. (2011) Ingroup-outgroup bias in contagious yawning by chimpanzees supports link to empathy. PloS One; PLoS One. 6 (4), e18283. Available from: doi: 10.1371/journal.pone.0018283.
• Bartholomew, A. J. & Cirulli, E. T. (2014) Individual variation in contagious yawning susceptibility is highly stable and largely unexplained by empathy or other known factors. PloS One; PLoS One. 9 (3), e91773. Available from: doi: 10.1371/journal.pone.0091773.
• Casetta, G., Nolfo, A. & Palagi, E. (2021) Yawn contagion promotes motor synchrony in wild lions, Panthera leo. Animal Behaviour. 174, 149-159. Available from: https://doi.org/10.1016/j.anbehav.2021.02.010 
• Leone, A., Ferrari, P. F. & Palagi, E. (2014) Different yawns, different functions? Testing social hypotheses on spontaneous yawning in Theropithecus gelada. Scientific Reports; Sci Rep. 4 (1), 4010. Available from: doi: 10.1038/srep04010.