Universal law of urination in mammals - everything pees for (approximately) 21 seconds

Related post on universal scaling: Does every species get around 1 billion heartbeats per lifetime?

You'll never look at Dumbo in the same way again. Elephants, cows, goats and dogs all take roughly 21 seconds to empty their bladders. A "law of urination" now explains the physics behind what happens when you just gotta go.

The team filmed rats, dogs, goats, cows and elephants urinating and gathered footage from YouTube of others relieving themselves. Combining this with data on mass, bladder pressure and urethra size, they were able to create a mathematical model of urinary systems to show why mammals take the same time to empty their bladder, despite the difference in bladder size.

Previous models have only considered the effects of bladder pressure, but the length of the urethra turns out to be important as well.

"Most of the research is on humans or animals smaller than humans," says Yang. In these species, the effect of gravity can be ignored. That's not true of elephants, whose urethral dimensions are comparable to a household pipe: a diameter of around 10 centimetres and a length of about 1 metre.

In this case size matters, as it means urine has time to reach higher speeds. This means that as it travels down the pipe, the urine accelerates and its flow rate rises, resulting in an elephant's large bladder being emptied in a similar time to those of smaller animals.

Medium-sized animals like dogs and goats have shorter urethras, so get less of a gravitational boost: their flow is slower. In addition, they have smaller bladders. The result of both effects is that they empty their bladders in roughly the same time as elephants.

Kind of related: German man went to court, won the right to pee standing up.