Brain size drives differences in yawn duration across mammals and birds | Hacker News Reader

Brain size drives differences in yawn duration across mammals and birds

nature.com

136 pointsvnglst61 comments

I don't understand why I would need to cool my brain more when I'm tired. I'm not buying the theory that yawns are for cooling brains without more details.

I'm not buying the brain-cooling theory because a couple of yawns won't lead to anywhere near enough evaporative cooling to chill the brain by a few degrees. Also, the brain has a very very good blood flow with the rest of the body, so you really have the cool the whole organism.

The only area of the body yawning could have any kind of immediate cooling effect is the soft palate in the rear upper portion of the mouth—the part responsible for brain freezes when eating ice cream. I don’t think ambient air passing over that palate would create a measurable change in brain temperature.

Yawning always seemed, to me, to go hand in hand with stretching. I believe the reason we yawn is one in the same with the reason we stretch. By yawning, we are effectively stretching one of the few internal organs under our direct control: the lungs. It stretches the muscles involved in breathing, releasing trapped lactic acid built up over a long day of automated breathing. It improves blood oxygen concentration, which improves wakefulness and attention, which is exactly what you need when you are fatigued. Deep breathing also reduces anxiety, which is why breathwork is heavily incorporated in meditation.

I think the contagious aspect could be an evolutionary behavior. Social mammals tend to work in groups, and “contagious” yawning was likely naturally selected for since all members of an exhausted group who yawn would be more awake and alert in the event of an attack or when hunting prey. These are physically strenuous (and tiring) activities that directly affect the survival of the organism.

It changes air flow by stretching the fold between mouth and sinus, maybe it is a pre-sleep sinus stretch.

I guess because your brain overheating is what's making you feel tired in the first place?

That seems pretty easy to falsify. I still get tired in cold environments, do you?

I actually have an easier time maintaining flow in cold environments. I keep my house pretty chill and warm the rest of my body with warm clothes. Summer in South Texas was miserable for me, so I moved father north and could not be happier with the change.

It's not that I don't still get tired, of course I do. More that I'm less affected by the ambient heat.

Doesnt adenosine improve bloodflow to the brain when we are tired? If that is true, increased bloodflow should increase the brain temperature as more blood circulates and exchanges heat

Sometimes there are aspects of the brain designed for one purpose that get used for a total different one. For example, neurons that detect smell will trigger when we are thinking about purely moral issues, possible because it helps to boost the emotional reaction.

The body do cool down when it sleeps, and many people do feel aided in falling asleep if they lower their body temperature a bit. Yawning might just be that, a small trick to get the body to do a short temporary jump towards a more sleepy state, and a social ques with a minor biological aspect to trigger similar state in a social group. It could also be the other way around, that being a social trigger with a minor biological aspect attached that happens to have a real use in other animals.

To go to sleep your body temp needs to drop a couple degrees.

https://www.sleep.org/does-your-body-temperature-change-whil...

Maybe yawning helps you get to the right temperature.

Could just act as a little nudge in the right direction.

I think sighing, rather than yawning, makes more sense with the brain cooling theory.

When we are distressed, we release cortisol, which is a pro-oxidizing free radical that raises body temperature and blood pressure. Signing involves a fast and prolonged exhalation of air. We tend to sigh when we are distressed, so it possibly serves as a way to cool ourselves down similar to an exhaust fan on a desktop chassis.

I read somewhere that when we fall asleep our body temperature is lowered a bit, maybe that’s what’s happening?

Probably because the brain is what takes the most energy for us

There's also the social aspect of yawning - seeing someone yawn triggers a yawn... Maybe brains like to maintain a similar temperature to nearby brains.

Or yawning is just a communication mechanism, formed in a time before using words became commonplace and maintained because it works.

It might also be a type of communication which goes against the typical 'pack' animal communication hierarchy. It might make sense for the pack to sleep when any of the pack is tired, rather than waiting for the alpha-dog to feel tired. 'contagious' yawning allows that.

Hell, for me seeing a video of someone yawning can make me yawn. Even reading these comments... I seriously yawned three times just thinking about people yawning.

It's quite possible that it is a social signal, I think. Something designed to facilitate a non-spoken syn-ack handshake between members in a social group. I see you, you see me, I see that you see me, end of transmit.

Just to clarify, the hypothesis in the post above was intended to be humorous.

For me, even reading the word "yawning" so many times triggers yawning.

Yawns are caused by changes in the souls position relative to others.... why it is contagious, and why someone who is cognitively your superior will cause you to yawn in their presence. I know it sounds nuts, but they know more than they let on.... just look for the well seal ;)

Is this a reference to something that I'm not picking up on?

This is hilarious

If you really understood what was inferred you would be terrified, or you do and there is truth in your humor. Being intentionally dismissive or flippant is not cognitively challenging nor particularly original. Thanks for the consideration though.

Honestly the charts in the paper is not very convincing. The outliers are all over the place and there is hardly an association judging from naked eye.

(e.g: imagine seeing figure 4 without the model prediction in the background)

Agreed, it looks like they may have even missed a U-shaped quadratic association. If were doing pure curve fitting it'd work way better with those data. For real though, very unconvincing graphs.

The model hardly predicts anything, or then the data is super noisy. It's not very convincing to express things with certainty when your model hardly fits the data.

> Our species-level measures of average brain mass, body mass, total neuron count, and cortical and pallium neuron counts were highly skewed. We therefore log-transformed these measures prior to statistical analysis.

Most of the yawns were collected in non-controlled conditions, and then the rest of the data is completely junk, so this whole paper is garbage.

I just saw my unborn son yawn in the womb on an ultrasound today so this brings me a bit of silly joy - knowing his brain folds are just developing and he might be cooling them! That said, the stats nerd in me doesn’t buy this study outright.

I’d personally hypothesize that yawns, like almost all other emotional displays (smiling, laughing, worrying, eye dilation) are simple pre-language communications. It’s useful for a pack of animals to know when one animal is tired - yawning is contagious because it’s advantageous for the pack to have a similar sleep schedule - and to be aware of how each other feels. Our faces display our emotions clearly as a form of communication to others. We developed whites in our eyes, for example, so we can track the attention of our pack-mates. It's easy to forget that biologically, spoken language is a brand new invention - but communication is extremely ancient.

> I’d personally hypothesize that yawns, like almost all other emotional displays are simple pre-language communications

The trouble with that hypothesis that even very primitive vertebrates that have or have had almost no social history also yawn.

Agreed - but it’s a chicken/egg issue there. It’s not clear that behavior is a precursor to social groups. Social groups with no communication behavior would be very hard to maintain. Hard to say tho for sure which comes first

> yawning is contagious [...] to be aware of how each other feels

To this point, fascinatingly, psychopaths are immune to “contagious” yawns, highlighting their inability to empathize with others.

https://www.psychologytoday.com/us/blog/your-online-secrets/...

I'm definitely citing this study next time I'm sleepy and someone comments on my constant yawning

title doesn't state if larger size is positively correlated with more frequent yawns :)

You're giving people far more credit than I would! You can get a ton of mileage out of the most spurious of correlations...

Doesn't read like sound science when all of my 6 pet rats yawn longer than I do.

Read The Hitchhiker's Guide to the Galaxy, and post back here.

Minor spoiler: mice (okay, not rats) are projections into our universe of immensely powerful trans-dimensional beings, or something like that.

Scared me for a moment there, I’ve killed upwards of 40 rats during this pandemic (outside). They seem to have gone rampant with the onset of COVID.

Wait until you see the mouse plague in Australia

Makes sense as I really seem to be their pet most of the time or worse a glorified janitor: the "poo shoveler".

I don't think they found absolute brain mass was correlated with yawn duration. They scaled brain mass in some complicated way that might put rats above humans for all I know:

"we calculated residual brain mass measures from a phylogenetically controlled linear regression of the respective brain measure on body mass"

There is a chart: https://www.nature.com/articles/s42003-021-02019-y/figures/2

Humans are above rats in this chart in both yawn duration and brain mass.

Rats are above dogs though.

Yawning cools the brain?!

I researched this independently when I started yawning madly at the gym

Ok, glad to know I'm not the only one to do this. However, the plot thickens... why is this happening?

Yes, that’s it’s purpose. Hence why the results of this study is not really surprising.

We don't know if that's the purpose of yawning.

now explain why when you see a cat yawn, you have a compulsion to say "big yawn!"

Ig-nobel worthy??

Doesn't explain why yawn is contagious

The word “yawn” triggered me and now I am yawning constantly.

Why is yawning highly correlated with fatigue then?

Why is yawning socially contagious?

Can't the thermal hypothesis be measured in lab?

The brain size correlation has a co-founder variable: Lung size/capacitance.

I thought it was commonly known that yawning promotes cerebrospinal fluid movement in the neck.

Ha los are a real thing, praise be.

drives -> correlates with

Drives? Correlates! "Scientists" are literally brain dead.

But how long are their yawns?

Why do we need to cool our brains when we are tired and not for example when we are exercising or thinking hard or in a hot environment?

My totally made up guess is when we sleep our brains operate at a lower temperature than awake, regardless of activity. So to quickly transition from waking temperature to sleeping temperature, maybe yawning does a rapid cooling that's then maintained by normal slower-acting processes.

Maybe you are tired, and thus need to cool your brain.

I find it a bit sad that whimsical/mundane science interests/amuses people. It's also sad that educationalists mandate that science taught in schools should be "relevant to students' lives". So a bunch of apes on a vegetated rock hurtling through space learn how to read the nutritional info on the back of cereal boxes instead of thinking about the genesis of time, matter, energy, information, and life from fucking nothing.

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