Australasian Science: Australia's authority on science since 1938

The Law of Urination

By Magdeline Lum

Why do bats, dogs and elephants take the same time to urinate?

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Dogs, goats, cows and elephants are all mammals that can empty their bladders in around 21 seconds, even though their bladders range in size from 100 mL to 100 L. Researchers at the Georgia Institute of Technology in Atlanta wanted to know why mammals of varying size empty their bladders in the same amount of time, and used high-speed video to study how urine is ejected from various animals. The team also gathered footage of animals urinating from YouTube.

Data from the footage was combined with information of the animals’ mass, bladder pressure and urethra size to create a mathematical model of urinary systems of mammals. The resulting “Law of Urination” applies to both males and females.

Previous mathematical model of urinary systems had only considered the effect of bladder pressure. It turns out that the length of the urethra also plays a role in the call of nature.

The team found that the longer the length of the urethra, the greater the effect of gravity and the flow rate of urine, particularly in larger animals. They also suggest that the urethra evolved as a flow-enhancing device that allows the urinary system to be scaled up without compromising its function.

Medium-sized animals like dogs and goats have shorter urethras. Their system isn’t affected by gravity as much, so the flow of urine is slower. However, their bladders are...

The full text of this article can be purchased from Informit.