A new law of biology: All mammals pee for about 21 seconds

October 18, 2013 • 10:40 am

A new paper by P. J. Yang et al. at ArXiv.org (reference and free download of the first page below) has produced a new Law of Biology.  The whole paper isn’t online, and I don’t know where it was submitted, but. . . .well, the title speaks for itself:

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And what is that duration, exactly? The abstract says about 21 ± 13 (is that the standard deviation among species?) seconds:

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I don’t completely get this, but hey, it’s physics, Jake.  How does simply lengthening the urethra increase the flow. What about its width?  The penultimate sentence is ambiguous, perhaps smacking a bit of teleology, since a system can’t evolve for future contingencies, like being scaled up without hurting its function. But that may be some kind of shorthand.

A precis at Seriously, Science? on the Discover blogs just reiterates the paper, but adds a video showing micturation in mammals (you get a bonus defecation with the elephant), and then some geometry and algebra presented quickly, showing that the duration of urination increases only as the sixth root of body mass (mass^0.1666), i.e., very slowly, so that a large mammal will have pretty much the same duration as a small mammal.  Maybe you physics buffs can figure it out from the quick presentation on this video:

If you understand their conclusion based on the geometry and algebra, please explain in a comment.

I eagerly await the publication of the full paper. In the meantime, I think we should all start timing ourselves.


Yang, P. J., J. C. Pham, J. Choo, and D. L. Hu. 2013. Law of Urination: all mammals empty their bladders over the same duration. submitted

78 thoughts on “A new law of biology: All mammals pee for about 21 seconds

  1. …and now there will be a study to discover how many minutes over a lifetime a human spends at the end of a leash, waiting….

    1. I’ve just put one of my dogs on furosemide, a diuretic. She now pees for a significantly longer period of time after about 30 minutes. This evening I will place myself on a diuretic (diluted EtOH) and time myself.

  2. “nearly constant duration of 21 +- 13 seconds”.

    I read that as “anything between 8 and 34 seconds” which seems hardly constant, really. That’s more than a factor of four range.

    1. I think that’s still quite a surprisingly narrow range, if it’s true. Consider that (again, if true), this factoid covers everything from blue whales to the hog-nosed bat.


      On a different note, methinks someone is angling for a 2014 Ig Nobel prize.

      1. I agree with Eric. Anyone who walks a small, active terrier will know that this study is bollux, even if you add up all the half-second squirts on all sites that require information deposition. An IgNobel for sure!

        1. That’s scent-marking, not proper urination, so not a valid comparison.
          The correct spelling is bollocks (unless used as a verb, I think).

      2. On a different note, methinks someone is angling for a 2014 Ig Nobel prize.

        Don’t they have to measure the effect of the liquid on the dunk-strength of biscuits too? Or do you think that’s their strategy for getting two Igs in a row?

    1. That makes much more sense! I read the advice to do our own measurements, the video then cut quickly to some students interested in the underside of a goat. I was about to start preparing for a visit to Pennywell Farm.

  3. From the point of view of humans, it would be interesting to know whether 21 +/- 13 seconds is also the average length of a commercial break during a TV show.

    1. It is the length of one commercial (30 seconds, though there are 15-second spots here and there). The other commercials are to allow you to get to and from the toilet.

  4. The longest time a human has spent urinating is 36 minutes and 24 seconds by a British bloke called George Wingfield in 1986. He had drunk 45 pints of lager before. I hope this information has been useful.

  5. OK, I hadn’t thought of it before, but it’s not a big surprise. Most mammals (exceptions such as parrots and humans exist) live for about a billion heartbeats. If size of the bladder increases, increasing size of the urethra will allow it to empty more quickly, so that time is independent of size.

  6. A drinking contest back in the day in a college town: order a 60 oz pitcher of beer, one per contestant seated at the same table, to be consumed in a one ounce gulp, one shot of beer every 60 seconds on the clock. Begin with the first serving of alcohol of the day (highly recommended), or take your chances when you’re partway into the bag. Losers pay for the pitchers. Side bets allowed/encouraged.

    Anyone who left the table to whiz before the pitcher emptied and the end of the hour is a loser.

    Some cylindrical beer steins with concave bottoms on the base hold exactly an ounce, and the main place I knew of where the game was frequently played, in a town with dozens of bars, was a particular bar that used these mugs. People challenged others to meet them: Mel’s on Tuesday night? Happy to take you on, sucker, I’ll be there.

    I witnessed few people able to empty their pitcher before leaving to empty their bladder, but that may have been because many contestants primed for the event with beer. I can’t speak to urination times for participants in these contests, but personal anecdotal experience is volume and pressure are both quite high.

    1. Sounds a bit like the 100 club. Try to drink 100 shots of beer in 100 minutes. Sounds easy, a shot of beer every minute, how hard can that be? I’ve seen people on their knees throwing up before they’ve got to 10.

        1. I think the worst is if you have to have an ultrasound – pregnant women have this and I had to have it to look at my ovaries – they tell you to drink a bunch of water beforehand then not pee as the tech runs an ultra sound wand over your bladder! Torture! I actually begged them to let me pee some out first (and I’m proud to say I had to control to do so but enough was in for the procedure). I was really cranky that day.

          1. Well, I’ve never been pregnant, but I went through the same procedure to check my kidney function after I was diagnosed hypertensive (193/smth).

            It was not fun.


            PS. The ultrasound staff are, of course, the most dependable people in a hospital.

            PPS. On meds, I’m now around 13x/8x.

        2. No longer true. I had one last June, and it was like drinking fruit juice, only less sugary. A huge improvement over earlier concoctions.

          1. “Had one” referring to a colonoscopy, of course. I’ll take Diana’s word for pregnancy and ultrasound treatment.

        3. Years ago that stuff was torture but it is much much nicer now. Not something I would drink by choice but vastly better than eight years ago.

          1. Ugh. I had to get a colonoscopy about a year ago, and the drink was horrible. If it was worse before, I feel sorry for the people who had to drink that.

            As far as the drinking games, we used to call the power hours and century clubs. Definitely harder than it sounds like (for me, the carbonation was the worst).

          2. I’ve had the older stuff (a gallon of really, really, awful stuff). I’ve had the newer stuff (a half gallon of Gatorade mixed with powdered awful). The newer stuff is not nearly as bad as the older stuff. It is still awful.

  7. For a full load, so to speak, I normally have a time of about 52 seconds.

    My personal best was 1m 23s. This occurred while inebriated during college.

    1. My worst experience was actually having a hard time peeing because of holding it for so long. Poor muscle couldn’t relax! It was too late of an intermission during a Shakespeare play (I think it was The Tempest) & I stupidly had had an alcoholic beverage before the show started.

        1. I’d have to go to the trouble of looking it up. Mostly I use “ban ban caliban” to talk about the Taliban or kanban boards.

  8. According to Wikipedia the term pissing contest can refer to futile or puroposeless and excessively agressive verbal disputes or as a pejorative metaphorical description of aggresive or vulgar egoistic behavior. This tactic was tried by the Republicans recently but apparently their more refined opponents prevailed.

  9. Never mind about the time spent having a Jimmy Riddle – do all mammals take the same time having a dump?

    I’d imagine that the curry factor might play a part in the latter…

  10. River otters have specific dump sites on the shore. Evidently this serves a communication
    function. Any otter can sniff it and get the straight poop who is in the neighborhood and deposit a message about its arrival. A study in Alaska showed that otter poop was a significant source of nitrogen for near shore vegetation. Of course pissing around the neighborhood serves the same function. We fed a neighborhood tom cat for a while which performed magnificent squirts, but after it killed off a number of potted plants we trapped it and had it chopped and channeled (as the saying goes) and he eventually became a tame house cat.

  11. That comment about lengthening increasing gravitational force AND flow rate is nonsense. Increasing the length (assuming same diameter) increases the head differential (‘gravitational force’) but also the total viscous ‘friction’ in the fluid. Assuming a vertical pipe, you have the entry loss, the exit loss, and the ‘friction’ loss in between. For a long thin pipe, the first two of those are negligible compared with the third; the flow rate on lengthening will stay the same. Of course, IF the diameter increases proportionately with the length, THEN the flow rate will increase.
    This all applies of course to an ideal straight uniform rigid pipe (probably attached to a spherical cow…)

  12. Ponder the fate of Danish astronomer Tycho Brahe. From his Wikipedia page: “Tycho suddenly contracted a bladder or kidney ailment after attending a banquet in Prague, and died eleven days later, on 24 October 1601. According to Kepler’s first hand account, Tycho had refused to leave the banquet to relieve himself because it would have been a breach of etiquette.[24][25] After he had returned home he was no longer able to urinate, except eventually in very small quantities and with excruciating pain. The night before he died he suffered from a delirium”

  13. Emptying its bladder can eventually be a time during which the mammal becomes vulnerable to predation and independently of this hypothesis, emptying its bladder seems to exhibit few functions: to eject the urine and in some cases to mark territory.
    It seems intuitive that the action of territory marking by ejecting some urine must be as short and efficient as possible in order not to be vulnerable to predation or to concurrent males and it seems intuitive that a long time of urine ejection cannot exhibit any kind of advantage.
    Thus maybe the shorter possible time is selectioned because it is also the less expensive in terms of natural selection and maybe inside the mammalian bladder structure there is a physiological and physical limit in this race to shortening the urination duration, so that independently of the mammal species (and their difference in body measurements) this time is converging toward an inferior limit, so that we have the impression of a constant.
    And I can add that there can be a great variability of this duration in function of the emotional state of an individual, so that I don’t believe that this report shows a new law of biology but only one of the consequence of natural pression of selection, which seems here the main biological law.

  14. I am intimately acquainted with a small Pomeranian mammal who purposefully retains his urine so he can dribble out a few drops every now and then, here and there, all over the damn place.

    1. I have a book on dogs that says dog bladders cannot completely empty on a single pee. Dont know if thats true but if so, you cant blame the Pomeranian mammal for purposefully retaining.

    1. Science, of course!
      There are zoos, and animals in zoos, and millions of people who would queue up to volunteer to spend any amount of time watching animals without having to feel they should be doing something more boring. Hand out a few stopwatches (or these days, smartphones) and you’re doing Science.

      00:22.25 🙂

  15. I have been peed on by many mice and they usually manage to deliver the whole load – it wouldn’t take 21±13 seconds to drop them.

    Seriously though, in mice micturation patterns are strain specific, some strains pee large volumes (large by mouse standards anyhow) some do a drip here and a drip there. It’s a confounding factor when comparing results of, for example, partial bladder outlet obstruction studies. You have to know your strain. There is also variation in patterns between dominant and submissive rodents – submissive rats often pee furtively and quickly in one corner of the cage while dominant ones pee wherever and for as long as they please.

    They didn’t even need to go to the zoo to do the studies there is a whole literature on this, they just need to look. This looks like a “law” that will be defined by its many exceptions

  16. As one of my fellow grad students used to say, “You don’t say? How very interesting.”

    And then there are the studies on ejaculation performance with terms like muzzle velocity. A colleague once showed me one such paper. Single au, and from the footnotes it was clear that the experimental subject was himself.

  17. And to think this paper comes right on the heels of the announcement in the Bulletin of the American Physical Society of the upcoming presentation on urinal dynamics at the 66th Annual Meeting of the APS Division of Fluid Dynamics (biofuids session).

    These are heady days for urine scientists.

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