Large or small, most animals take the same amount of time to empty their bladders. (Oppenheim Bernhard/Getty Images)

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. For example, enlarged prostates in males can constrict the urethra, which slows the flow of urine. Obesity can increase abdominal pressure, which can lead to incontinence.

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, suspected there might be more to it. What he found was 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 — from Fido to Dumbo — 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. “Consider the pressure at the bottom of a swimming pool,” he said. If you sat on the bottom of a 50-foot-long pool that was 10 feet deep, you would feel the same pressure from the water pushing down on you as you would at the bottom of a 500-foot-long pool that was also 10 feet deep. That’s because pressure is determined only by the height of the water above it, not the width of the pool.

An elephant’s bladder is about 11 / 2 feet tall. But its urethra is three feet long. If you could fit inside the end of the elephant’s urethra, you’d feel the pressure of 41 / 2 feet of urine above you: the three-foot urethra plus the 11 / 2-foot bladder.

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.

Steven Vogel, a biology professor at Duke University who was not involved in the research, was surprised. “Who would have expected urination time to be so nearly constant?” he said. “I wrote a whole book on biological fluid mechanics, even giving some space to one aspect of urination, and I never imagined.”

The researchers think this information may be useful for the design of drainage systems that have different sizes but need to empty at equal rates. “If you ask an engineer how to design a water tank that drains at a constant time” no matter how large it is, “she will probably be stumped,” Hu said. “Animals have figured out how to do it.”

This study may induce giggles, but it shows that what we learn from animals can lead to innovations. “We need more people in this field, those willing to straddle both biology and engineering, to make these discoveries,” Hu said.

Goldman writes for Scientific American and other publications.