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Research on mammal urination

By: Floro Mercene

The findings, released by the Proceedings of the National Academy of Science, shed light on the mysterious fluid dynamics of urination and could help engineers build better flow-regulating devices.

The research revealed that, regardless of size, all mammals above 3 kg in weight from the cat to the goat to the elephant, seem to empty in about the same duration of time: 21 seconds, roughly the same amount of time to urinate.

Some larger animals had really capacious bladders. For example, an elephant can fill a kitchen garbage can with pee, an elephant’s 18-liter bladder is nearly 3,600 times larger than a cat’s, but it doesn’t take 3,600 times longer to empty. This means that the elephant must be spewing out a massive torrent of liquid at high speeds in order to empty itself in the same time frame as a relatively tiny feline bladder. The secret seems to lie in the urethra’s design, which uses gravity to its advantage. The longer the urethra, the faster the flow. With a longer urethra, the effects of gravity increase, which creates more pressure in the bladder and pushes the urine out faster, researchers said.

Since the urethra is also proportionally wider in larger animals, it basically adds extra lanes to this fast-moving pee freeway – or “pee-way”.

The researchers found that animals lighter than 3 kg, such as small rodents and bats, couldn’t pee in streams; rather they urinated in a series of quick drops. Small animals don’t have enough liquid in their system to make it flow.

This study may help to diagnose urinary problems in animals as well as inspire the design of scalable hydrodynamic systems based on those in nature. David Hu, an assistant professor in the Woodruff School of Mechanical Engineering, has been awarded the Ig Nobel Prize for ‘Improbable Research’ in 2015.