Researchers at the Georgia Institute of Technology have discovered a new golden rule: every mammal takes about 21 seconds to urinate. Patricia Yang and her co-authors dubbed it the "Law of Orientation" in a paper published this week, and they say it applies across a wide range of animal sizes.
Yang and her team discovered this commonality after observing male and female cows, dogs, elephants, goats, and rats at the Atlanta Zoo. They filmed these animals urinating, taking note of their size, bladder pressure, and urethra length, and then searched for videos of other peeing animals on YouTube. Based on mathematical models derived from these data, the scientists found that every animal took an average of 21 seconds to relieve itself, despite bladders that varied in volume from 100 milliliters to 100 liters. The video below offers a more visual explanation.
That's not to say that animal size doesn't matter. According to Yang, it just doesn't have a significant impact. Prior to this study, most urination models focused on bladder pressure and relatively small mammals. As a result, they didn't take gravity into account.
As New Scientist explains, an elephant's long urethra — one meter, with a diameter of ten centimeters — gives its urine more time to pick up speed. Because of this, the animal can empty its enormous bladder in about the same time as goats, dogs, and other medium-sized mammals. For small mammals like rats and bats, gravity doesn't exert much influence on flow rates, which are instead determined by viscosity and surface tension. That's why they urinate in small drops, Yang says, finishing the job in less than a second.
The researchers will present their findings next month at the American Physical Society Division of Fluid Dynamics meeting.