You know the feeling. It’s impossible to resist. You just need to yawn. A yawn consists of an extended gaping of the mouth followed by a more rapid closure.

In mammals and birds, a long intake of breath and shorter exhale follows the gaping of the mouth, but in other species such as fish, amphibians and snakes there is no intake of breath.

But why does it occur?

In the past, people have had many hypotheses. As far back as 400 B.C., Hippocrates thought yawning removed bad air from the lungs before a fever.

In the 17th and 18th centuries, doctors believed yawning increased oxygen in the blood, blood pressure, heart rate and blood flow itself.

More recently, consensus moved toward the idea that yawning cools down the brain, so when ambient conditions and temperature of the brain itself increase, yawning episodes increase.

Despite all these theories, the truth is that scientists do not know the true biological function of a yawn. What we do know is that yawning occurs in just about every species. It happens when an animal is tired. It can be used as a threat display in some species.

Yawning can occur during times of social conflict and stress, something researchers call a displacement behavior.

And that wide-open mouth can be contagious, especially in social species such as humans, chimpanzees, bonobos, macaques and wolves.

Watching someone yawn — even reading about yawns — can lead you to yawn yourself. Why?

Research on humans tell us that people who are more empathetic tend to be more susceptible to contagious yawning. When you see someone else yawn, the networks in your brain responsible for empathy and social skills are activated.

In 2013, cognitive and behavioral scientists at the University of Tokyo tested contagious yawning in canines while controlling for stress. The researchers found that dogs were more likely to yawn in response to a familiar person. They concluded that dogs can “catch” a yawn from humans and that yawning is a social rather than a stress-based behavior.

Try an experiment at home: Yawn and see if your pet yawns back.

Words to Learn 相关词汇

【会传染的】huì chuánrǎn de contagious spreading or tending to spread from one to another

【有同理心的】yǒu tónglǐxīn de empathetic able to understand and share others’ feelings

你知道这种感觉。那种冲动很难抗拒，你就是想打个哈欠。打哈欠时，人们张大嘴巴，之后以更快的速度闭上嘴。

哺乳动物和鸟类张嘴打哈欠会吸入一大口气，之后短暂呼气。但其他动物打哈欠时并不吸气，比如鱼类、两栖动物和蛇。

但哈欠是怎么来的呢？

过去人们有很多假想。在公元前400年，古希腊名医希波克拉底认为，发烧之前，哈欠会把废气从肺部排出。

在17和18世纪，医生们认为哈欠可以增加血液中的氧气、使血压和心率上升、促进血流。

近年来，更多人认为哈欠会让大脑冷静，所以当外界条件趋于紧张，大脑温度增加时，打哈欠的频率也会增加。

虽说有这么多的理论，但真相是，科学家并不知道哈欠真实的生物功能。

我们只知道，几乎每种动物都会打哈欠。动物疲劳的时候会打哈欠。一些动物用哈欠作为威胁。哈欠还可以在遇到社会冲突和压力时产生。一些研究人员称之为替代行为。

而且，打哈欠会传染，特别是在社会型动物中，比如人类、黑猩猩、倭黑猩猩、猕猴和狼。

看到别人打哈欠，甚至读到打哈欠的情节，自己也会打哈欠，这又是为什么呢？

关于人类的研究告诉我们，越有同理心的人越容易被哈欠传染。当你看到别人打哈欠，你的大脑中负责共鸣和社交技能的神经网络就会被激活。

2013年，东京大学认知和行为学家在控制压力的条件下测试了狗是否会被打哈欠传染。

研究人员发现，狗在见到熟悉的人打哈欠时更容易被传染。他们得出结论称，狗会被人打哈欠的行为传染，打哈欠是一种社会行为，而非基于压力的行为。

可以在家里做个小试验：打个哈欠，看看你的宠物会不会也打个哈欠。

(Chinadaily.com.cn)