An elephant’s bladder is more than 3000 times the size of a cat’s, yet the two animals take the same amount of time to urinate. That’s the surprising conclusion of a study previously released on the arXiv preprint server and set for publication this week in the Proceedings of the National Academy of Sciences. Videos shot of a range of different mammals reveal that, as long as the animals are larger than 3 kilograms, they take approximately 21 seconds to empty their bladders. To find out why, the researchers measured how quickly the urine flowed from their urethras. As an animal’s body size increases, so does the length of its urethra. Because an elephant’s urethra is longer than a cat’s, for example, gravity creates more pressure in the elephant’s urethra, pushing the urine through faster. The rule doesn’t hold for small animals like rats and bats, which take only 0.1 to 2 seconds to urinate. Their urethras are so thin that gravity doesn’t affect the flow of urine. Instead, surface tension pulls the urine through the urethra until it emerges in droplets (as in the video above). The researchers hope that their findings will help engineers build larger systems of pipes and reservoirs that don’t take as long to drain.
(Video credit: Kenneth Breuer and Sharon Swartz, Brown University)