It takes around 21 seconds to empty your bladder. And that applies whether you are a dog, a human or an elephant.

Dr Patricia Yang of Georgia Institute of Technology will be addressing the 66th Annual Meeting of the American Physical Society of Division of Fluid Dynamics next month. While many of the talks at the event will be on issues such as bubbles and surface tension, or magnetohydrodynamics, Yang will be talking about the somewhat fundamental fluid dynamic that is urination.

Her team have combined their own filming in a Georgia zoo with YouTube footage of other animals urinating, which does make you wonder what IT staff made of their search history.

Their goal was to figure out what the 21 second rule, which seems to apply to most mammals of cat size or above, should be valid even though bladder size varies immensely.

They discovered that it’s not simply a case of bladder pressure, but rather that the size of the urethra is important. Elephants have a larger urethra than many other mammals at around a meter long and ten centimeters in diameter. That extra length gives more room for gravity to boost acceleration and thus eject at a faster rate.

Looking at other mammals, the researchers found that the relationship between bladder size and urethra size is consistent. Smaller animals have smaller urethras and less boost from gravity. In turn they urinate at a slower rate, but because they have smaller bladders to empty, it still works out at 21 seconds.

Why exactly it is 21 seconds isn’t yet clear. (The less authoritative source that is my wife speculates that it developed as the best compromise of safely and comfortably emptying the bladder but minimizing the time during which evading predator attack becomes more awkward.)

Smaller mammals such as rats don’t fit into the 21 second rule. That’s because they don’t rely on gravity so much, instead urinating in a series of drops rather than a continuous stream and thus doing the job in around a second.