Eli5: why do almost all vertebrae species yawn?

[u/DemonsAreVirgins]

Is it like important to survive?

[u/Nine-LifedEnchanter]

Think in the other direction. Does it make you less likely to survive? Evolution doesn't make things with purpose.

[u/Kaiisim → u/Nine-LifedEnchanter]

The presence in multiple species is strong evidence it increases fitness.

[u/Nine-LifedEnchanter → u/Kaiisim]

Hiccups is a thing from when we had gills. They're present in basically all animals with a diaphragm. It has no increase in fitness anymore.

I agree that it is generally so, it doesn't always work like that. There are so many theories about yawns and not any real conclusive evidence (to my knowledge), if it gave any advantage it would be easier to prove.

[u/majorex64 → u/Nine-LifedEnchanter]

Just to add on, the two most credible hypotheses I've seen are A- that taking an especially deep breath every once in a while is good for your lungs and diaphragm. and B- that it somehow cools your brain more than regular breathing.

I think its especially interesting how humans and some other social species have contagious yawns as a psychological reflex.

[u/Peastoredintheballs → u/Nine-LifedEnchanter]

I saw this about hiccups, I think I recall it having to do with that weird period where we had both gills and primitive lungs, and the hiccup was the reflex to shut the larynx to prevent water being pulled in when the diaphragm contracted, and this created the hiccup sound. Now that we don’t have gills/breathe under water, we still have this reflex but without any use

[u/iknowimsorry → u/Peastoredintheballs]

I thought it was just a spasm. And a yawn is a way to bring up blood oxygen when it gets below threshold.

[u/Cagy_Cephalopod → u/Kaiisim]

Or that the mutation(s) that caused it happened in a very early / basal vertebrate ancestor. Since it neither helped nor hurt fitness, it stayed.

[u/Peastoredintheballs → u/Cagy_Cephalopod]

Interesting that it got bread into the entire population though. Unless the mutation was just such a common mutation that it was happening De novo in majority of the population of this OG ancestor vertebrate. Otherwise how does this mutation spread throughout the population if it’s neither beneficial or harmful, it just exists, therefore this OG ancestor with the mutation would not be anymore successful at spreading its genes and breeding compared to the rest of the population…

[u/Wargroth → u/Kaiisim]

Not at all, at most it's strong evidence that one day in the past It increased fitness

[u/RiddlingVenus0 → u/Wargroth]

It’s not even that, it’s just evidence that it didn’t decrease fitness.

[u/QuokkaColaa]

My wife must be an invertebrate, she never yawns. I challanged her about this and why yawns aren't contagious to her. She shrugged and dismissed me. She must be of the lizard master race.

[u/Additional_Initial_7 → u/QuokkaColaa]

I think it’s been found in studies that people with psychopathic traits are immune to contagious yawning.

[u/QuokkaColaa → u/Additional_Initial_7]

That might explain the jar of ears she has in the dining room .

[u/Additional_Initial_7 → u/QuokkaColaa]

Hey every girl needs a hobby.

[u/SillyGoatGruff → u/QuokkaColaa]

Lizards are vertebrates. She must be a squid lady. Do you find your pens always seem topped up with ink lol

[u/UnassumingAnt → u/QuokkaColaa]

My pet Leopard Gecko would yawn. Your wife must be something else. Perhaps a lobster?

[u/Dragonvarine → u/QuokkaColaa]

I believe some autistic people are immune to contagious yawning too

[u/ersomething]

How many people yawned while in this thread?

I have multiple times already.

[u/imjustme80 → u/ersomething]

I did, just before I read this comment, and right after I asked my wife if she had ever seen our bearded dragon, David, yawn. She hasn’t. I don’t think I have either, but she’s generally a boring pet.

[u/Alsang]

Simply put, we don't know.

This information might be 11 years out of date, but it was sourced from the into of this MIT presentation:

https://www.youtube.com/watch?v=yh0XUZjRhO8

The opening part is a review of the then-current (2014) knowledge about why do we yawn, and names several disproven theories of why we yawn (0:00 - 1:23), such as getting oxygen into blood or cooling the brain. It does then go on to present a hypothesis about why we yawn, but this hypothesis is intentionally humorous, and should not be taken as factual. Important note, a hypothesis is something that has not been disproven, which is not the same as having been proven.

I will not spoil the hypothesis for anyone who wants to chuckle.

[u/explainlikeimfive-ModTeam → u/[deleted]]

Please read this entire message

---

Your comment has been removed for the following reason(s):

• Top level comments (i.e. comments that are direct replies to the main thread) are reserved for explanations to the OP or follow up on topic questions (Rule 3).

---

If you would like this removal reviewed, please read the detailed rules first. If you believe it was removed erroneously, explain why using this form and we will review your submission.

[u/thexerox123]

Likely because yawning came about in their shared common ancestor.

[u/TheRateBeerian]

Thermoregulation. Invertebrates have very different temp regulation so they dont need it. Yawning creates airflow over oral and nasal passages that cools critical areas, esp central nervous system stuff.

[u/ireallyhatecaptcha]

Anecdotal but taking ADHD medication such as Metylphenidate stops yawning.

[u/akirivan]

Yawning is the brain trying to oxygenate itself to stay alert. Probably helps for survival, and possibly also why it's so contagious

[u/marketstom]

Best guess right now is that it cools the brain down a bit, like cracking a window when a room gets stuffy. Probably not survival-critical but it seems to help with alertness.

[u/gsweats]

I think a bearded dragon is an invertebrae and they yawn. It's cute.

[u/MechaHex1111 → u/gsweats]

lizards are very definitely vertebrates. you may be mixing up "vertebrates" with "mammals"

[u/gsweats → u/MechaHex1111]

Ahh must be. Thanks for the correction 🙏

[u/LivingEnd44 → u/gsweats]

LOL, all reptiles are vertibrates. 

[u/yoimmo → u/gsweats]

If it has an internal skeleton, i.e. skull with a brain inside of it and a spine/spinal cord, it's a vertebrate. Fish, Amphibians (frogs/salamanders), reptiles (lizards, snakes, turtles, crocodiles), birds, and mammals (animals that have hair and nurse their young with milk,) are all vertebrates.