Call it the other Golden Rule: Scientists have found that all mammals weighing more than 2.2 pounds (a kilogram) empty their full bladders in about 20 seconds.

Like many new parents, David Hu, a mechanical engineer at the Georgia Institute of Technology, has changed a lot of diapers. Unlike many new parents, however, these soggy diapers caused Hu to think about the physics of urination.

“While I was changing these diapers, I was wondering how it would be different for different animals. How much fluid would they create and how long would it take to leave the body?” Hu said. (Also see “Growing Teeth and Four More Odd Uses for Urine.”)

“The physics of urination—what the forces are and how they affect how quickly urine comes out, is not totally understood, even though it’s a really old problem.”

Although it might sound silly on the surface, urination is actually serious business in the medical and veterinary worlds, especially during the aging process. Many men get enlarged prostates as they get older, which can narrow the urethra and impede urine flow. Veterinarians have been looking for a quick and easy way to identify problems with animals’ urinary tract.

Pee Cam

First, Hu wanted to know how urination varied from species to species. The bladder of a large domestic dog can hold 1.4 liters (about 0.4 of a gallon) of fluid, or roughly the amount of a large bottle of soda. An elephant’s bladder, however, can hold 160 liters of fluid, or enough to fill three large garbage cans. Hu wanted to know how this size difference affects the urinary tract and urine flow.

Enter the pee cam. Three of Hu’s graduate students at Georgia Tech used high-speed cameras to record peeing animals at Zoo Atlanta and elsewhere.

They also measured how much pee was produced by each of these animals, which ranged from rats to jaguars to elephants. The scientists supplemented their research with YouTube videos from zoo visitors. Last, the researchers obtained measurements of the animals’ bladder and urethra widths and lengths from other researchers.

Hu and colleagues suspected that bigger animals would take longer pee breaks than smaller animals, since they had to expel larger volumes of urine. But when they began determining the urination duration of each of these animal species, that wasn’t what they found.

“Even though you have thousands of times more urine, it’s coming out in the same amount of time, which is around 20 seconds,” said Hu, whose study appeared recently on the Cornell University website arXiv. (Also see “Urine Battery Turns Pee Into Power.”)

Urination Law Explained

Urinary tract measurements helped to solve the mystery of how animals of such different sizes all pee for the same amount of time.

Larger animals not only had larger bladders, they also had longer and wider urethras. The length of the urethra increases the force of gravity on the urine, which in turn increases how fast pee flows out of the body.

A wider urethra also increases flow rate by increasing the volume of urine that can leave the body at the same time. These increases correlate with the increase in body mass, such that an elephant can empty its bladder in the same amount of time as a cat. (Also see “Turtles Urinate Via Their Mouths—A First.”)

Saying an elephant pees for the same amount of time as a domestic cat seems impressive enough, but it gets even more impressive when you consider just how much urine the elephant produces: several bathtubs’ worth. It’s like having 60 showerheads all going at full blast, Hu said.

For small mammals—those weighing less than 2.2 pounds (a kilogram)—Hu’s Universal Law of Urination wasn’t as accurate. Their urethras were so narrow and short that the surface tension of the urine slowed flow down to mere droplets. Their bladders were also much smaller, so a few drops could successfully empty the bladder. These animals, like rats and mice, can successfully urinate in less than a second.

Animals benefit from such relatively short pee breaks because such pit stops increase risk of predation, Hu noted. Faster urine flow may also help prevent urinary tract infections by flushing out the system.

Now I just have to fight the urge to bring a stopwatch with me into the bathroom.

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