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Tuesday, March 19, 2024
March 19, 2024

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Why a rhino urinates at the same rate as a raccoon

The Columbian
Published:

You might think that elephants take longer to empty their bladders than humans do, because pachyderms are so much larger. But you’d be wrong. Recent research shows that most animals, including humans, take the same amount of time to pee.

When Georgia Institute of Technology researcher Patricia J. Yang filmed zoo animals urinating, she wasn’t doing it for fun. It turns out that a good way to assess urinary health is to measure flow rate.

Changes in the speed at which urine exits the body can be used to diagnose problems.

To understand what happens when urination goes wrong, researchers and clinicians need a grasp of how urination ordinarily happens. “Despite the wide range of animals used in urological studies,” write Yang and her colleagues in a recent study, “the consequences of body size on urination remain poorly understood.”

So Yang went to Zoo Atlanta. She collected videos of mammals ranging from the tiniest, such as bats and rodents, to the largest, elephants. She also rounded up YouTube videos of animal urination to complete her collection.

Until now, the urethra was thought to be simply a pipe from the bladder to the genitals that expels urine from the body. But David Hu, an assistant professor of mechanical engineering and biology at Georgia Tech, found that it is indeed a pipe, but with important size considerations. The key was in its length.

The researchers led by Hu discovered that the smallest critters can’t produce streams of urine. Instead, they pee in drops. But as soon as an animal tops about 6.6 pounds, it can produce jets. And no matter how big that animal is it takes the same amount of time to do it as other animals: roughly 21 seconds. Hu’s team reported its findings in June’s Proceedings of the National Academy of Sciences.

It’s all thanks to the urethra. According to Hu, the time it takes to pee is constant, no matter the animal’s size, because of gravity.

That’s why a rhino or an elephant can expel urine in roughly the same time as a raccoon or a ferret, even though the larger animals have a lot more urine. “Our model shows that differences in bladder capacity are offset by differences in flow rate,” Yang writes, “resulting in a bladder emptying time that does not change.” In other words, as the bladder size increases, the urethra becomes longer to compensate, allowing gravity to produce a higher flow speed.

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