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Shocking discovery: All mammals take about 21 seconds to go pee

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That's right, all of them — and the shape or size of the animal doesn't matter. Scientists are calling it the Law of Urination, and it's a process driven by the way mammalian urinary systems evolved to eject fluids from the body in the quickest and most efficient way allowable by physics.

To make this discovery, Patricia Yang and colleagues brought their high-speed camera to the Atlanta Zoo. They filmed male and female rats, dogs, goats, cows, and elephants taking a whiz. The researchers produced this rather delightful video:

Later, they studied YouTube videos of the same. This allowed them to create a mathematical model of urinary systems — a model showing that mammals take the same time to empty their bladders despite considerable differences in the size of their bladders — differences in volume than can range from 100 mL to 100 L.


New Scientist explains what's going on:

According to the team's model, an animal's size does make a difference to urination time – but only very slightly. Their law of urination says that the time a mammal takes to empty a full bladder is proportional to the animal's mass raised to the power of a sixth, meaning even very large changes in mass have little effect on the time.

There are limits to this scaling. Gravity only plays a small role in the urination of very small mammals like rats and bats, which urinate in under a second. Instead, viscosity and surface tension dominate, which explains why their urine is released as a stream of individual drops rather than the continuous jet seen in larger mammals.


You can check out the entire study at the preprint archive arXiv: "Law of Urination: all mammals empty their bladders over the same duration."

Top image: An elephant on a toilet, origin unknown.