科学美国人60秒:21-Second Rule Governs Mammal Micturition

In the time it takes you to listen to this story, an elephant could empty its bladder about three times. And, despite being much smaller, so could an adult cat. And a goat. Also a cow. Because researchers find that all mammals that weigh more than six-and-a-half pounds or so take about the same time to pee: 21 seconds, plus or minus 13 seconds.
 
The researchers videoed various mammals urinating at Zoo Atlanta. And they found that, once you get above the squirrel-mouse-shrew level, tinkle-time does not scale with body size. Good news for elephants, who could otherwise spend all day going number one.
 
Seems that larger animals have a longer urethra, the conduit that whisks the wee-wee away. That difference in size is big enough that gravity starts to really weigh in. As a result, when Jumbo answers nature’s call he has a much faster flow than does Mittens.
 
Given the urethra’s structure, the researchers say that the mammalian urinary system can be “scaled up by a factor of 3,600 in volume without compromising its function.” The study is in the Proceedings of the National Academy of Sciences. [Patricia J. Yang et al, Duration of urination does not change with body size, preprint]
 
This hydrodynamic finding could help engineers design systems that rapidly drain reservoirs, regardless of size. It also means if Mittens and Jumbo hit the head together, they can come out at the same time.
 
—Karen Hopkin