Sir Isaac Newton probably wasn't thinking about how animals urinate when he was developing his laws of gravity. But they are connected - by the urethra, to be specific.
A new Georgia Institute of Technology study investigated how quickly 32 animals urinate. It turns out that it's all about the same. Even though an elephant's bladder is 3,600 times larger than a cat's (18 liters vs. 5 milliliters), both animals relieve themselves in about 20 seconds. In fact, all animals that weigh more than 3 kilograms (6.6 pounds) urinate in that same time span.
"It's possible because larger animals have longer urethras," said David Hu, the Georgia Tech assistant professor who led the study. "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 one meter in length. The pressure of fluid in it is the same at the bottom of a swimming pool three feet deep. An elephant urinates four meters 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 explained.
The findings conflict with studies that indicate urinary flow is controlled on bladder pressure generated by muscular contraction. The study has just been published in the Proceedings of the National Academy of Sciences (PNAS).
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," said Yang, who is pursuing her doctoral degree 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."
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," said 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 added.
The trick is gravity. Newton would be proud.