I use a Water-Pik after flossing (and so should you!), and I’ve noticed repeatedly that when I am squirting water between my teeth, I develop a sudden urge to urinate. Then I remembered the old summer-camp trick of putting a sleeping boy’s hands into a bowl of water, which supposedly made him wet his bed. I think asked a few friends if they also had an urge to micturate when they heard running water, and to a person they said “yes.” (One emphasized the need to pee in the shower.)
Well, immediately this brings up a question: “why does this happen?” There are two ways to approach this question.
First, there’s the physiological or “proximal” approach, which asks, “What is the neuronal/physiological basis of having to pee when you hear running water?” This question is in principle answerable, and, as you’ll see, appears to have been answered.
But there’s the evolutionary or “ultimate” approach. If one assumes this connection between water and urination arose directly via natural selection (and remember, it could be fortuitous: simply a byproduct of how our bodies evolved), why is it adaptive to respond to the sound of running water this way?
This question may not be answerable, as we weren’t around to see when it happens. (One could I suppose, at least see if the connection exists in other primates, which would bnttress the fact that it arose in a common ancestor and has persisted, but doesn’t answer whether the connection evolved directly by natural selection.)
These are two different ways of thinking about the question: the “how” approach versus the “why” approach. As I said, the “how” appears to have an answer in humans, as evidenced in this article from Australia’s Swinburn University (click to read):
First of all, the article asserts that the urge to pee when you hear running water is widespread, and occurs not just in the presence of water. (I haven’t had “nervous wees” before a date, though.)
We all know that feeling when nature calls – but what’s far less understood is the psychology behind it. Why, for example, do we get the urge to pee just before getting into the shower, or when we’re swimming? What brings on those “nervous wees” right before a date?
But let’s take a readers’ poll to see how widespread it is. Remember, your answer is anonymous, so please answer:
Loading ...Now, the “how” answer as given in the article above:
Research suggests our brain and bladder are in constant communication with each other via a neural network called the brain-bladder axis.
This complex web of circuitry is comprised of sensory neural activity, including the sympathetic and parasympathetic nervous systems. These neural connections allow information to be sent back and forth between the brain and bladder.
The brain-bladder axis not only facilitates the act of peeing, but is also responsible for telling us we need to go in the first place.
How do we know when we need to go?
As the bladder fills with urine and expands, this activates special receptors detecting stretch in the nerve-rich lining of the bladder wall. This information is then relayed to the “periaqueductal gray” – a part of the brain in the brainstem which constantly monitors the bladder’s filling status.
Once the bladder reaches a certain threshold (roughly 250-300ml of urine), another part of the brain called the “pontine micturition centre” is activated and signals that the bladder needs to be emptied. We, in turn, register this as that all-too-familiar feeling of fullness and pressure down below.
Beyond this, however, a range of situations can trigger or exacerbate our need to pee, by increasing the production of urine and/or stimulating reflexes in the bladder.
An illustration of where the brain’s “bladder control center”:
There’s more:
Peeing in the shower:
If you’ve ever felt the need to pee while in the shower (no judgement here) it may be due to the sight and sound of running water.
In a 2015 study, researchers demonstrated that males with urinary difficulties found it easier to initiate peeing when listening to the sound of running water being played on a smartphone.
Symptoms of overactive bladder, including urgency (a sudden need to pee), have also been linked to a range of environmental cues involving running water, including washing your hands and taking a shower.
This is likely due to both physiology and psychology. Firstly, the sound of running water may have a relaxing physiological effect, increasing activity of the parasympathetic nervous system. This would relax the bladder muscles and prepare the bladder for emptying.
At the same time, the sound of running water may also have a conditioned psychological effect. Due to the countless times in our lives where this sound has coincided with the actual act of peeing, it may trigger an instinctive reaction in us to urinate.
This would happen in the same way Pavlov’s dog learnt, through repeated pairing, to salivate when a bell was rung.
I’m not sure that a physiological effect differs from a psychological effect, except that the latter would be “learned” rather than inborn. But remember that any physiological effect like this has to come in through the senses and brain, which could be seen as “psychological”.
More “how” answers, involving different mechanisms:
But it’s not just the sight or sound of running water that makes us want to pee. Immersion in cold water has been shown to cause a “cold shock response”, which activates the sympathetic nervous system.
This so-called “fight or flight” response drives up our blood pressure which, in turn, causes our kidneys to filter out more fluid from the bloodstream to stabilise our blood pressure, in a process called “immersion diuresis”. When this happens, our bladder fills up faster than normal, triggering the urge to pee.
Interestingly, immersion in very warm water (such as a relaxing bath) may also increase urine production. In this case, however, it’s due to activation of the parasympathetic nervous system. One study demonstrated an increase in water temperature from 40℃ to 50℃ reduced the time it took for participants to start urinating.
or:
But it’s not just the sight or sound of running water that makes us want to pee. Immersion in cold water has been shown to cause a “cold shock response”, which activates the sympathetic nervous system.
This so-called “fight or flight” response drives up our blood pressure which, in turn, causes our kidneys to filter out more fluid from the bloodstream to stabilise our blood pressure, in a process called “immersion diuresis”. When this happens, our bladder fills up faster than normal, triggering the urge to pee.
Interestingly, immersion in very warm water (such as a relaxing bath) may also increase urine production. In this case, however, it’s due to activation of the parasympathetic nervous system. One study demonstrated an increase in water temperature from 40℃ to 50℃ reduced the time it took for participants to start urinating.
Conclusions:
We all pee (most of us several times a day). Yet research has shown about 75% of adults know little about how this process actually works – and even less about the brain-bladdder axis and its role in urination.
Well, you know now! More:
Most Australians will experience urinary difficulties at some point in their lives, so if you ever have concerns about your urinary health, it’s extremely important to consult a healthcare professional.
And should you ever find yourself unable to pee, perhaps the sight or sound of running water, a relaxing bath or a nice swim will help with getting that stream to flow.
This article was originally published on The Conversation.
The mechanistic explanation also predicts that if you’re in a restroom and you hear other people peeing, that would increase your urge to join them.
As I said, the water-urination response may not be a direct response to natural selection. That is, there may be no reproductive advantage to having to pee when you encounter water. It could simply be, as Gould and Lewontin called it, a “spandrel.” But let’s engage in some “adaptive storytelling” here and think up ways the connection might have been adaptive.
There don’t seem to be many. The first one that struck me was that, as noted above, a lot of people have bladder issues (only ones that occur before reproduction ceases can be considered). If this is the case, and if retaining urine is bad for you. which it is, then anything that facilitates peeing when you have bladder issues would be adaptive. If you already have a physiological system in place for peeing when your bladder’s full, it might be easier to hijack this system in those with bladder issues by using the same stimulus: the sound of running fluid. (This presumes that the sound stimulates urination even in people without bladder issues, which is apparently does.) But somehow I’m not satisfied with this
I asked a colleague, who gave a response that sounded good at the time but now seems dubious as well. He said that if you hear running water, you have an opportunity to hydrate yourself by drinking, and running water is more likely to be clean water that is good to drink. But the connection between having to drink and having to pee is obscure to me.
A question, then, for readers:
So, if one assumes that the connection between water and urination is the result of natural selection, please tender your own theory. Even crazy theories should be given, because, after all, “evolution is cleverer than you are.”
Remember this old joke?
Generally speaking, I often have the urge to pee before starting almost any task that will require at least some concentration. For example, before giving a lecture; prior to assembling a product; before cooking; before sitting down to watch a movie, etc. “Let me pee first,” is a common utterance, so I’ve always assumed it was somewhat universal.
Whether this is adaptive or not would seem to be related to the act of reproduction itself. Before doing the deed, pee, seems like a simple enough rule. This may have fitness benefits related to successful reproduction itself or health. I suppose I’d want to know if the phenomenon is the same in both males and females. The question about running water would need adequate controls to answer, meaning, the person wouldn’t be doing any other task. It may be that the urge to pee is related to the task at hand and the running water is a correlation.
I too would like to know if males and females differ.
They do not differ (results from personal experiments with a sample size of one).
Perhaps it was advantageous to pee near running water as the water would carry the urine away, leaving a cleaner ‘nest’, plus it would make bathing and personal clean-up possible.
Maybe, but AIUI urine is generally more sterile than most surface water.
The authors claim that the urge to urinate when you hear running water is widespread, which may be true, but is it more widespread than when engaged in other activities—such as waiting for the dentist to call your name in the dentist’s office? It seems to me that establishing the validity of the premise requires demonstrating that the urge is greater when around flowing water than it is when encountering other sensory stimuli. I wonder if the rigor is there to support the premise. I doubt it.
Anyway, I can tell an adaptation story. Here you go:
Miring in urine is probably unhealthy. Urinating in flowing water carries urine away from the vicinity, limiting contact with it and potentially increasing your probability of living long enough to reproduce. Hence, peeing when one hears flowing water is favored by natural selection.
I’m not saying that I believe it, but that’s my story and I’m sticking with it.
Yes to Janet! I just saw her succinct and very plausible response.
I had about that. Technically, fresh urine is sterile although it does support bacterial growth, and animals do stay away from drinking it.
As Mark says, urine is sterile, and in fact there are cultures in which drinking urine is common, as is bathing in it. So I would say that idea is “busted”.
Even if it is sterile, urine does contain a lot of urea -> ammonia, and as any baby will tell you, prolonged skin contact with ammonia is very irritating. Also, even though urine may come out sterile, if it stays on the skin it will feed bacteria and hence invite skin infections.
Sure, but it is relatively harmless and I don’t see any danger from casual contact with soil or plants that have been pissed on. And as I said, people in some cultures deliberately bathe in it.
As further evidence for this, I note that currently there are popular skin rejuvenation treatments based, believe it or not, on urea.
https://www.clinikally.com/blogs/news/the-dermatologists-guide-to-using-urea-cream-effectively
So I think we can dismiss theories of urination that assume urine is dangerous in casual contact.
There are popular skin rejuvenation treatments based on all sorts of BS, maybe even literally (microbiome and all that).
Harumph.
Whether it works or not is irrelevant; the point is that it is not very dangerous.
Many prey animals – including humans – pee when frightened or when they see a predator. Probably lightens the load and makes running away easier. It might be adaptive to pee when you hear others peeing, rather than having to find the predator first.
Clever!
The sound of running water is an audible signal of drinking water and other resources that promise sustenance. This is a territory to hold, and urination marks territory, so the autonomic nervous system has evolved to persuade mammals to urinate in order to mark territory.
There, that is my natural selection hypothesis.
Are (or were) humans sufficiently sensitive to urine smells to be able to notice ours / not ours? It should be easy enough to test this experimentally, but maybe not so easy to get a grant for it.
When I see a chocolate soft-serve ice cream machine, I…never mind.
On a related point, when I see someone on television have an alcoholic drink, my inhibition for having a nice cold beer goes way down. I have to resist.
I’m like that with cigarettes. Even though I quit 30 years ago, if I’m in proximity to someone smoking a cigarette, I suddenly want one desperately. Thank goodness smoking is verboten almost everywhere these days, or god forbid, I might pick up that filthy habit again.
ou812 is talking about chocolate ice cream being squeezed out of a machine stimulating excretion of a different bodily waste product.
“cold shock response”
Interesting. I always thought fast filling bladder in cold water is simply due to the change of our blood distribution to preserve body heat. In warm environment our body directs blood towards to skin to transfer the excess heat to the surface, while in cold environment the opposite happens. Since the kidneys are internal organs, they came under higher blood pressure when blood is directed away from the surface, so they remove more water.
I like your idea but I suspect the timing is off. Water diuresis is fast. I would expect the lag between getting cold -> shunting blood flow away from the shell and to the core -> and then that translating into more urine production and movement of urine through the ureters into the bladder to be orders of magnitude longer.
The other thing is that the urge is likely at equivalent volumes that an instant before did not produce an urge. Probably a change in elasticity (thank you thoracic sympathetic outflow) not in urine volume.
Fascinating.
Gentlemen here of my age, 55, and older no doubt spend a lot of time on this topic – much of it in the bathroom several times a night.
From NHK a few years ago – a Japanese urologist worked out that all mammals have a bladder capacity of 22 seconds – from elephant to mouse to humans. (Evacuation). Which seems not far from the truth, though obviously that time can be “curated” – like by my dog who pees every few seconds – often only a few drops – around 8th Ave to mark his presence.
The periaqueductal gray is an interesting brain part. One of its main jobs is in endogenous and exogenous opiate reception and pain recognition. Busy little nub.
D.A.
NYC
Rats pee in 6 s. Just saying.
Maybe it’s a “safety in numbers” effect? If the men are out hunting and only one has to go off and pee, he’s very vulnerable to predation. But if there is a whole “pee pack” a would-be predator might hold off…? So hearing to collective flow helps us go.
Hearing THE collective fow
Here’s my take. Likely wrong but “submitted for your approval.” The brain/body relationship works well but circumstances can sometimes fool it. My primary example of this is paradoxical undressing. I am sure that most of the readers are familiar with the phenomenon, but here is a brief summary: people who have frozen to death are often found to have taken off clothing as their final act. Odd behavior, indeed. What happens is that the body initially draws blood into the core and the individual shivers to make muscles produce more heat. But they can do this only so long. As they run out of energy, muscles used to sequester blood in the core lose strength and relax. Blood rushes out to the extremities. This is where the brain gets things backwards. The normal process of thermoregulation works thusly: the body gets too hot, the brain senses this and sends blood to the extremities to shed heat by radiation and convection. At the same time we sweat, the evaporation of which cools the skin which in turn cools the blood. Paradoxical undressing happens because the brain gets the cause and effect backwards. It interprets the sudden flushing of the skin with blood, which in this case happens because muscles have weakened from fatigue, to mean that you are overheated. Normally the brain detects overheating and responds by flushing the skin with blood. But now the flushing of the skin with blood is interpreted as overheating and the freezing person suddenly feels unbearably hot and disrobes to cool off. The brain associates temperature and blood distribution but gets the cause and effect backwards. Could something akin to this be happening here? The urge to pee becomes associated with the sound of a stream of water running into the toilet bowl (or making a puddle on the ground or splashing on a rock or whatever) and that association is flipped upside down by the brain. The urge to pee commonly results in the sound of splashing water, so the sound of splashing water results in the urge to pee. In the case of thermoregulation, the system has a clear adaptive advantage and it works automatically for a lifetime. Only the rare instance of freezing to death brings the brain’s misinterpretation of sensory information into play. But, maybe the brain can be fooled into backwards interpretations in other, less extreme, situations. Think this idea has any traction?
I was thinking similar thoughts. In humans and some other animals controlling when you pee is a learned behaviour. Perhaps we learn to hold our pee and we also learn to let our pee go – with the sound of running pee providing extra feedback to the hold/release processes?
When we are in a modern bathroom we perceive super stimuli which would be rare in nature. The sound of water running into a basin is rare in nature, and the sound of pee falling into a toilet bowl is rare in nature. Maybe it is in the nature of super stimuli to overwhelm the temporal order of our learned responses?
A question – do populations who use squat toilets experience the same ‘running water’ trigger to pee?
Very interesting Steve, I’ve often wondered about that undressing thing.
When it comes to evolution I think we might be a bit askew here with the sound aspect of urination. It is only since flush toilets that the familiar liquid on liquid sound impacts us.
Before toilets presumably the familiar tinkle was only heard if you pee’d in streams or still water. Which I’m guessing.. species wide… used to be rare. And the toilet era isn’t old enough to have impacted selection.
(Just guessing here).
D.A.
NYC
Interesting that one learns to pee, but this study suggests that one poops to learn.
doi.org/10.1016/j.smhs.2024.04.001
https://www.cbc.ca/listen/live-radio/1-51-quirks-and-quarks/clip/16125198-voiding-bowels-improve-cognitive-performance
Obviously. It clears the head.
There are much more general questions about urination that deserve some thought by evolutionary scientists. Here is a question that occurred to me as a kid. Why do so many animals have so much bladder control? Humans certainly do and anybody who has ever had a dog knows that dogs can have excellent discipline to hold the bladder even when obviously uncomfortable. Why do animals have that much capability to control where, when, and how they pee?
I assume that in the wild, animals are picky about when and where to pee because they must control how they do or don’t advertise their presence to other animals with keen noses. An herbivore does not want local predators to notice its scent. Indeed some herbivores that live in herds have interesting latrine habits. For example, I have read that wildebeest, zebras, and certain other herbivores typically urinate simultaneously so that everybody’s pee mingles together. Perhaps that way no individual’s scent is easily identifiable to predators. Carnivores, of course, have immensely complex urinary habits as everyone knows from walking a dog. Dogs sniff and sniff make delicate decisions about where they wish to pee. Carnivores pee to scent mark territory and to announce their presence to potential mates and for who knows how many other purposes. Carnivores have special preputial glands (or clittoral glands in females) that can add strong musky smelling substances to the urine. Thus canines and felids can optionally make their pee strong smelling or comparatively odorless.
Truly pee is a fascinating subject worthy of scientific investigation. I don’t have any clever thoughts about Jerry’s original question.
It is very interesting Michael. I’m always amazed by (my) dogs’ proclivities given we live in a high dog area. (In my – very gay – building there more dogs than kids!).
For dogs they have 100x the sensors (our 1m. to their 100m receptors) we have so they smell the world entirely differently to us.
Various urban structures become like internet bulletin boards for dogs with each pooch examining, almost reading the smells before their leave their own “emails”.
D.A.
NYC
pmails?
Good one!
Ig Nobel Prize level stuff! Makes you laugh, then makes you think! Some thoughts :
• The exact sound of the water could matter
• How about arriving at home too – sort of a territorial reaction?
• Maybe the sound is relaxing – it signsls being home.
Yes, maybe include a mathematician in fluid dynamics in the study, the kind who designs public fountains and water features, since surely we’d want to know whether the sounds of, eg, gurgling versus tinkling water affect the results, and we’d want to include formulae to describe how to generate the different sounds. As I read this discussion I too thought it has the makings of a lovely Ig Nobel study! The funding grant (if they still exist) could include a travel budget to visit Niagara Falls, the Trevi Fountain,….
One of the indicators of an older man having a swollen prostate is that when he returns home he often has an immediate need to urinate.
Not so much a territorial reaction as the stimulus of returning home (or a handy toilet?) makes the bladder relax enough to overcome the restriction of a swollen prostate?
This caused a small brainstorm -added a comment below.
For a memorable but short period of my life I was in Beirut, Lebanon. There were daily blackouts as their electricity, like everything there, was intermittent.
You’d always make sure to go to the bathroom before taking an elevator lest you be caught short.
D.A.
NYC
Classical conditioning explanation (pairing of water sound & peeing / urge to pee?) suggests possible sex and cultural differences. Perhaps women should have a weaker association on the premise that urinating when seated is quieter? And I believe that males across cultures differ in whether they pee standing or sitting. Similar prediction? And just individual differences within cultures?
There is this thing about serving food in a restaurant. When the meal arrives at the table, how its presented, the first impressions, are important to what’s before ye and sets up the guest enjoyment and experience. i.e. salivating over just what it the meal LOOKS like.
I contend here that the audio aka sound of running water does the same it relaxes the bladder because we know peeing is very rewarding, like I had yesterday, it was so tense after delaying that peeing nearly made my eyes role backward in relief. 😅.
My theory: peeing makes noise. Running water masks the noise, Ideal time to pee. The Sabre Tooth tiger could not detect dinner was being served.
Lucky for me, I don’t pee. I was born on Krypton, all I need is Earth’s yellow sun, and all my excrements are burned up by my alien mitokryptochondria.
The reason I brought up Superman is because I never have the piss-urge from running water. My wife does, I think many/most people do, but I really don’t get pee-triggered by that. As others have mentioned, I do respond when I know I’m going on an extended trip, regardless of vehicle. But running water just doesn’t work on me. I’m evolutionarily advanced, as Darwin used to say. Citation needed.
Fun post Jerry, thanks! great comments too.
Too easy. The brain associates running water with lower temperatures. Cold diuresis ensue.
Oh – a dumb but biologically-relevant observation :
Reproductive activity is impossible unless the call from Nature is answered…. if you follow…
Not to argue with any of the excellent scientific theories already posted…but I don’t see any commenters referring to the fact that they were taught to pee to the sound of running water during potty training. I’m 74yo, and have VERY clear memories of my mother turning on the bathroom sink faucet after she had placed me on the toilet and urged me to tinkle. Before she died, she admitted many times to doing so. Maybe the idea was in the very popular Dr. Spock books of the 50’s?
I’ve always laughed about being a victim of my potty training. If I have even a teeny need to pee, and hear running water in a sink such as when brushing my teeth…I suddenly have an overwhelming urge to do so. No debate or wait – just do it. Standing next to a fast-flowing mountain stream or in a shower doesn’t have the same effect – only faucet water.
That’s such an interesting topic! Amazing how our brains connect sounds and body responses—nature really is fascinating.