The Other Golden Rule

Did you know that most mammals, from a house cat to an elephant, take roughly the same amount of time to urinate? Researchers at Georgia Tech collected data from real-life and online video streams, and discovered that a combination of physiology and gravity enable this feat of fluid dynamics.

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IRA FLATOW, HOST:

Time now for our Video Pick of the Week with our video produce Luke Graskin. Hey, Luke.

LUKE GROSKIN, BYLINE: Hey, Ira.

FLATOW: What kind of video you got for us today?

GROSKIN: Well, Ira, you're in for quite a treat this week. This week we're looking at the science of peeing. Yes.

FLATOW: That's P-E-E-I-N-G.

GROSKIN: Yes. Peeing. Yes.

FLATOW: There's a science to this?

GROSKIN: Yes. Urination. There is a science to this. It's not as simple as you would think.

FLATOW: Mm-hmm.

GROSKIN: So a team of researchers at Georgia Tech, they discovered, gee whiz, there's a gold standard for the duration of urination. And that gold standard is roughly 20 to 30 seconds for most mammals.

FLATOW: Across the map? You mean they actually took out stopwatches and measured this?

GROSKIN: Yeah. They measured this across most mammals and it doesn't - it's not 20 to 30 seconds for all mammals. You know, small mammals like mice and bats and things like that, it's different. This law doesn't really - this rule, this gold standard doesn't really apply to those animals. This applies to animals from the size of a house cat to the size of an elephant.

FLATOW: Mm-hmm.

GROSKIN: And so when you think about it, I mean, consider this. You know, a house cat has a very small bladder.

FLATOW: Right.

GROSKIN: And an elephant has a really big bladder. And the two animals, you know, they pee for basically the same amount of time. Now that's pretty remarkable when you think about it. You know, it's like if you took a cup and you took a jug of water and then you tipped them over, which one would empty first?

FLATOW: Well, you'd think the cup.

GROSKIN: Yes.

FLATOW: Yes.

GROSKIN: Exactly. So, you know, physiology and gravity at work allows both animals to pee for the same amount of time.

FLATOW: I'm - wow.

GROSKIN: It's pretty incredible, right?

FLATOW: It is. Can you try this at home? I mean...

GROSKIN: You can. You can contribute to the data set.

(LAUGHTER)

GROSKIN: And I know what you're thinking. You're thinking, you know, why study this? Seriously. I mean, are we really solving one of life's greatest mysteries. But there is an engineer, a man named David Hu, and he recently became a father. And that got him thinking.

DR. DAVID HU: I started thinking about this when I was changing diapers a couple years ago. I was wondering, wow, how could my life be any worse right now? The only way it could be worse, if I was changing an elephant's diaper. And then that made me think, well, I wonder what would happen if, instead of this 10-pound baby it was an 8,000 pound baby and I'd have to deal with the mess. And I started thinking about it. And that's when we got started.

FLATOW: Wow.

GROSKIN: Yeah.

FLATOW: That's the way we got started. So the expression pee like a racehorse, is that valid then?

GROSKIN: No. I'm afraid to say it doesn't hold much water anymore.

FLATOW: Ooh.

GROSKIN: Because we take about the same amount of time. And, you know, it's kind of interesting because this is - it's not just a bladder pushing down. There's gravity at work here. Think about it this way. If you take a barrel and then you put a tube above it and you run that tube 30 stories up. Now you fill that tube with water and you fill the barrel with water.

You would think, OK, so the tube is just going to start to overflow.

FLATOW: Right. Right.

GROSKIN: But that's not what happens. The barrel bursts. So you flip the same thing and now the pressure of the water from the barrel's now pushing down that tube.

FLATOW: Mm-hmm.

GROSKIN: And that's kind of what's going on inside of a urethra and that's how you can get all that water out so fast.

FLATOW: Mm-hmm.

GROSKIN: And the video has a lot more details about this.

FLATOW: All right. So our Video Pick of the Week up there on our website.

GROSKIN: Yes. It's streaming. It's our number one video right now on the website.

(LAUGHTER)

GROSKIN: So go check it out.

FLATOW: Streaming video.

GROSKIN: Mm-hmm.

FLATOW: Any follow-up to this? You got anything going?

GROSKIN: As the French say, oui, oui. We will do a follow-up if there's enough interest. So go check it out.

FLATOW: All right. Can't top that one. It's up on our website at sciencefriday.com if you want to go see Luke Graskin's video. And something you can try at home yourself with the video. You can go to the website. It's sciencefriday.com. Thank you, Luke.

GROSKIN: No problem, Ira.

FLATOW: That's about all we have of time for today.

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