Big or small, animals take 20 seconds to pee

"It turns out that you don't need external pressure to get rid of fluids quickly," says David Hu. "Nature has designed a way to use gravity instead of wasting the animal's energy." (Credit: "pug dog" via Shutterstock)

Researchers investigated how quickly 32 different kinds of animals urinate—and big or small, it’s remarkably the same.

Even though an elephant’s bladder is 3,600 times larger than a cat’s—just under five gallons vs. about one teaspoon—both animals relieve themselves in about 20 seconds.

In fact, all animals that weigh more than 6.6 pounds urinate in that same time span.

“It’s possible because larger animals have longer urethras,” says study leader David Hu, an assistant professor at Georgia Institute of Technology (Georgia Tech).


“The weight of the fluid in the urethra is pushing the fluid out. And because the urethra is long, flow rate is increased.”

For example, an elephant’s urethra is just over a yard long. The pressure of fluid in it is the same at the bottom of a swimming pool three feet deep. An elephant urinates about 13 feet per second, or the same volume per second as five showerheads.

“If its urethra were shorter, the elephant would urinate for a longer time and be more susceptible to predators,” Hu explains.

The findings conflict with studies that indicate urinary flow is controlled by bladder pressure generated by muscular contraction. The study appears in the Proceedings of the National Academy of Sciences.

Hu and graduate student Patricia Yang noticed that gravity allows larger animals to empty their bladders in jets or sheets of urine. Gravity’s effect on small animals is minimal.

“They urinate in small drops because of high viscous and capillary forces. It’s like peeing in space,” says Yang, a PhD student in the George Woodruff School of Mechanical Engineering. “Mice and rats go in less than two seconds. Bats are done in a fraction of a second.”

Using gravity

The research team went to a zoo to watch 16 animals relieve themselves, then watched 28 YouTube videos. They saw cows, horses, dogs, and more.

The more they watched, the more they realized their findings could help engineers.

“It turns out that you don’t need external pressure to get rid of fluids quickly,” says Hu. “Nature has designed a way to use gravity instead of wasting the animal’s energy.”

Hu envisions systems for water tanks, backpacks, and fire hoses that can be built for more efficiency. As an example, he and his students have created a demonstration that empties a teacup, quart, and gallon of water in the same duration using varying lengths of connected tubes.

In a second experiment, the team fills three cups with the same amount of water, then watches them empty at differing rates. The longer the tube, the faster it empties.

“Nature has shown us that no matter how big the fire truck, water can still come out in the same time as a tiny truck,” Hu adds.

Source: Georgia Tech