When Nature Calls, Size Matters, New Research on Peeing Suggests

In his research on improving the designs of scalable hydrodynamics systems, David Hu, an engineer at Georgia Tech, looked to nature for inspiration, specifically when nature calls.

Hu and his team recently filmed the urination habits of 16 animals of various sizes. The team looked for differences in how mice, rats, dogs, goats, cows and an elephant go about peeing.

The crux of their findings? Size matters. The smaller the animal, the quicker it urinates. Mice and rats, for example, only spend about 2 seconds peeing, whereas animals larger than 5 kilograms take an average of 21 seconds to pee.

"An elephant has a large bladder and a urethra with dimensions comparable to a household pipe," said Patricia Yang, a graduate student who worked with Hu on the research. Yang explained that as gravity pulls fluid down to the bottom of the urethra, the flow speed increases, causing urine to be eliminated more quickly.

Yang said this explains why even though a dog has a much smaller bladder and urethra than an elephant, the two animals can empty their bladders in the same amount of time. The larger elephant gets more help from gravity than the smaller dog.

The upcoming meeting of the American Physical Society (APS) Division of Fluid Dynamics (DFD) meeting in Pittsburgh, Pa., will feature a presentation about the dynamics of peeing revealed in Hu's lab.

Another presentation at the APS DFD meeting will involve urinal dynamics. Fluid dynamicist Randy Hurd of Brigham Young University will present his findings on the ultimate angle for men to approach and use a urinal.

"For typical male urination, the stream breaks up into droplets before impacting the urinal wall or the water surface," Hurd said. "Significant splash-back occurs if that stream is angled perpendicular to the urinal wall, down to angles of about 45 degrees. But when this impact angle becomes very small, it is much easier for the droplets to only slightly change direction, and slide along the porcelain surface without generating large splashes," Hurd said.

In a press statement, Hurd said he hopes to eventually create an optimization function to find the ideal approach for urinal usage.

"Although reducing the impact angle would also work in traditional toilets, these angles tend to only present themselves around the rim of the bowl, simultaneously increasing the chances of missing the bowl entirely," said Hurd. "I wouldn't recommend this approach to anyone but military snipers."