How do whales and dolphins sleep without drowning? | Hacker News Reader

How do whales and dolphins sleep without drowning?

scientificamerican.com

151 pointsfanf235 comments

Wow, what an interesting article. Here's a great tidbit:

when diving, marine mammals' blood travels only to the parts of the body that need oxygen--the heart, the brain and the swimming muscles. Digestion and any other processes have to wait.

I wonder if this is why people tell you not to go swimming after you eat? I remember my parents always telling me that. Would swimming/diving affect human digestion?

Yes but it's a myth: http://www.medicinenet.com/script/main/mobileart.asp?article...

Somewhat-relatedly, an interesting story showed up here a little while back about single-hemisphere sleep in humans: https://news.ycombinator.com/item?id=11545048

Humans detect drowning by elevated CO2 in the blood. This means people walking into downed weather balloons have no warning that they're running low on oxygen before passing out and dying.

IIRC most aquatic mammals detect low oxygen directly instead, allowing them to dive for over an hour without panicking. CO2 builds up in the blood, but they can feel their oxygen levels directly.

Since oxygen plays a rather vital role in our functioning, I'm curious why we can't detect low oxygen levels. Or perhaps we do detect them but only when it's dangerously low. How has it been confirmed that we detect CO2 levels instead of low oxygen?

Directly: https://en.m.wikipedia.org/wiki/Inert_gas_asphyxiation

You are totally unaware of your brain shutting down as the oxygen level decreases. People lose the ability to do arithmetic or simple puzzles including blocks in holes, but by the time that happens youre too far gone to recognize what that means.

perhaps its because we and our ancestors do not have the environmental stress to have that as one of our optimizations

As CO2 concentrations rise, it raises levels of carbonic acid in the blood (something CO2 does in aqueous environments; same thing that's bleaching corals in the sea). Our body tracks blood pH and when it dips too low from high CO2, that's when the body knows it needs to breathe.

(The above is just going off my memory of my physiology class from years ago. Apologies if I misremembered something.)

Just a guess, but CO2 levels in our body are much lower which would mean small changes are easier to detect. Also just a guess, when we were all just fish making a first few steps into land, atmosphere O2 concentration was lower, CO2 higher, so maybe we evolved to detect what was more abundant then.

Humans can detect both low CO2 and low O2, it's just that the CO2 'drive' is both stronger and happens faster.

> This means people walking into downed weather balloons have no warning that they're running low on oxygen before passing out and dying.

It also means that by hyperventilating, you can reducessubsequent feelings of asphyxiation, because hyperventilation (rapid, shallow breaths) purges CO2 from the blood. However, it is somewhat inefficient at building up oxygen (compared to slow, deep breaths) so it's easy to accidentally asphyxiate yourself that way.

A few observations from my freediving days (I've trained competitively for a few years).

The urge to breathe you get when you're holding your breath long enough is from elevated CO2 in your blood and the subsequent change in PH (as a comment here already stated). That urge is actually your diaphragm contracting involuntarily, but you can learn to endure that feeling which is the key for apnea attempts. But besides that, there are no almost no other indications before you pass out. Black-outs are quite common in freediving competitions because of that fact (freedivers just ignore the breathing reflex and nothing else tells them when to stop).

O2 saturation may fall 20-40% during your breath hold. You generally won't feel anything until it's too late. The problem is that you won't feel the difference between 99% and 65% saturation and you may black-out at 64% (those numbers are not the same for everyone). The signs you can detect before blacking out are blurred vision, tunnel vision, fingers tingling, metallic taste in your mouth... But, you have only a very small window of time between those symptoms starting and blacking out (literally a second or two if you're lucky).

Interestingly, the window of time depends on the discipline - the more physically engaged you are the window is bigger. The static apnea (just floating face down in water) gives you almost no warning at all while at longer dynamic apnea attempts you have a few seconds when you can "detect" something is funky with your body.

How could they have gradually evolved these abilities without drowning?

I'm guessing, some lower form of life, pre dolphin must have mutated the ability, but useless to them. Down the line, the dolphin was finally mutated, still having the same.

If a mutation is useless it is unlikely to proliferate.

It could, assuming it was not overtly harmful

Is it not possible for a combination of positive attributes to produce a useless attribute as a consequence, but the surplus attribute can't disappear as it's a manifestation of positive evolutionary traits that give you an advantage?

Fixation of a neutral allele within the population is proportional to its frequency in the population. So, even something that is initially useless can linger in the population, and even become fixed within the population. If that neutral allele is lingering in the population at a low frequency and suddenly becomes advantageous, there will subsequently be strong signals of selection at that locus.

That's not true at all.

http://www.medicaldaily.com/10-useless-human-body-parts-what...

Cetaceans are thought to have evolved from semi-aquatic land mammals. (Prehistoric hippos, more or less.) It seems reasonable to assume that traits like larger lung capacity and higher CO2 tolerance were gradual, incremental gains that allowed these animals to venture further and further off-shore to hunt, or allowed them to stay submerged for longer and longer periods of time. Those with the mutant traits outcompeted the rest, and this process was cumulative. Whether concurrently or shortly thereafter, evolution transformed hands and feet into flippers.

To this day, whales have vestigial arm, wrist, hand, and finger bones in their front flippers. Baby whales of certain species are also born with a very light amount of fur, which they shed a few weeks after birth.

Cetacean evolution is a fascinating topic.

I'd be cautious of answers to questions like that that sound like a story. But it's worth pointing out that the ancestors of whales started out on land and it's not necessary to imagine they jumped straight from that to fully aquatic.

This isn't really an answer, but likely the same way we've evolved systems to raise us from sleep when we need to switch positions in the night or stop breathing due to sleep apnea.

If a dolphin were studying a human, they'd probably be asking what sort of brain function can magically wake us up when someone pinches our nose closed in our sleep.

Per 2nd para, likely very similar system - just when we're roused we're not under water so are more likely to be able to act to recover our air supply.

I would argue that drowning is likely a pretty important form of selective pressure in the evolution of these animals. Marine mammals that are able to avoid predation and hunt for extended periods without surfacing would outcompete peers who lack those abilities. It's not hard to imagine incrementally larger lung capacities and incrementally better CO2 tolerances happening over millions of years and arriving at the current state.

Also, they would not have just become swimming mamals one day. The transition from land to water would have taken a while.

The same way any other adaptive trait evolved.

Those offspring with random mutations drowned less, and and more reproductive success.

I wonder if this is a limiting factor for the development of their cognition.

I'm not sure why it would be; as far as I understand it, they don't have a hemisphere asleep all the time.

That said, if I recall correctly, the architecture that supports this capability is more heavily bicameralized than our own. I can easily see that imposing the kind of limit you describe. But I'm not sure we know enough about their connectome to really say one way or another.

It'd be interesting to find that whales are left-finned when one hemisphere is asleep, and right finned when the other hemisphere is asleep.

No, actually it seems to enchance their abilities, one recent study discussed how dolphins can click and process received information while sleeping, or can use sonar to search for fish, make decisions about echoes received, while simultaneously whistling, communicating with others. This is a level of complexity that is completely alien to humans.

http://jeb.biologists.org/content/218/24/3987

What definition of sleep is being used here - full cognition (though in a limited domain), motion, conscious response to external stimulii ... doesn't appear to resemble my naive model of 'sleep', in what way is it sleep?

Physiological definition- unihemispheric slow wave sleep, EEG measures each hemisphere's activity, where the sleeping one shows low frequency, high amplitude EEG readings.This is stage 3 sleep. Birds and manatees have it too.

I am also not sure if this a limiting or enhancing factor. But in general, I think, sleep, and 'how much of it' -- was reason (not consequence) of human's cognitive development.

I will admit, I have not found a set of repeatable studies to verify the above assertion. So just a thought.

One hemisphere at a time.

Reply on news.ycombinator.com