Researchers find that most mammals, big and small, need 21 seconds to urinate

While changing his child's diaper, mechanical engineer David Hu got the idea to study how animals relieve themselves. What he discovered after watching rats, elephants, cats, and more urinate is that most mammals need the same amount of time: 21 seconds.

This discovery will "shed light on the mysterious fluid dynamics of urination," says Amina Khan at the Los Angeles Times. To get the data, Hu and the other researchers went to Zoo Atlanta, where they had to concoct a surefire plan to catch the animals in the act and measure the pee — not easy tasks.

That was only the beginning. After ascertaining the size of different animal bladders and the length and width of their urethras, Hu and his colleagues found that elephants could fill a household garbage can with urine, while some animals peed in tiny beads. All of the mammals, big and small, urinated for around 21 seconds, give or take 13 seconds — a very small amount of time.

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The finding was interesting, since not all of the bladders held the same amount of urine — the elephant had an 18-liter bladder, nearly 3,600 times larger than the cat's. They key is the urethra; the longer it is, the faster the flow. Urethras are wider in larger animals, which "basically adds extra lanes to the fast-moving pee freeways," Khan writes. The study, released by the journal Proceedings of the National Academy of Science, will be used in healthcare, and also to assist engineers with creating efficient toilets and above-ground water tanks.