Why do humans and many animals yawn?

When you get up early in the morning or have a full meal, you may yawn lazily. But have you ever thought about why people yawn? In fact, not only humans, but also many mammals and birds yawn.

Some studies believe that yawning may have social attributes. For example, when you see someone else yawning, you will unconsciously open your mouth, which is a manifestation of empathy.

In 2011, an experiment at Emory University confirmed this theory. Researcher Matthew Campbell and renowned primatologist Franz deWaal divided 23 chimpanzees into two groups and asked them to watch videos of chimpanzees in the same group and in different groups yawning or resting. The results showed that chimpanzees yawned 50% more often when they saw their own group yawning than when they watched strangers yawn. This shows that even if yawning is contagious, the "contagiousness" of yawning is different between familiar people and strangers.

In 2021, new progress has been made in the field of biology on the study of "yawning". Previously, there was a saying that yawning is a manifestation of brain hypoxia, and opening the mouth wide can allow more oxygen to enter the body. However, biologist Jorg Massen from Utrecht University in the Netherlands and biologist Andrew Gallup from the State University of New York Polytechnic Institute in the United States came to different conclusions.

They believe that yawning is to cool down the brain. Experiments have found that when the mouth is open, more cool blood flows to the brain due to the inhalation of cool air and the stretching of the muscles around the mouth, which achieves the function of temperature regulation. Further research also shows that yawning once can cool the brain by 0.1℃, and yawning several times in a row can cool it by as much as 0.3℃.

There is also a lot of evidence to prove this point. For example, if you put an ice pack on your head or neck, or cool your brain in other ways, or if you are in a relatively low temperature environment, the frequency of yawning will be greatly reduced.

On this basis, Masson and Gallup also found that the length of yawning in different animals seemed to have some relationship with the size of their brains. To this end, they held up cameras and went to the zoo to wait for the animals to yawn. In the end, they collected 1,250 yawns from 55 species of mammals and 46 species of birds, and linked the yawning duration data of different species with the brain and neuron data provided by the research team of Charles University in Prague, Czech Republic, and came to a conclusion: the length of yawning is positively correlated with the size of the species' brain and the number of neurons in the brain.

This is actually easy to understand. The larger the brain size and the more neurons there are, the more cold blood needs to flow to the brain, so yawns will naturally last longer.

How about this? Are you feeling a little confused after reading this? Yawn quickly to cool down your brain.

This article is a work supported by Science Popularization China Starry Sky Project

Team Name: The Nutcracker

Review: Tao Ning

Produced by: China Association for Science and Technology Department of Science Popularization