Taking the Piss

Science always looks for patterns—it’s actually a human obsession. Games that line up cards, fill rows or columns, or match numbers are ubiquitous.

The deviation from patterns is also behind some of our fundamental discoveries, including electromagnetism—combining the two produces movement, and moving a magnet through a coil generates electricity.

Nature is rife with patterns—one of my favorites is the number of heartbeats over a mammal’s lifetime. From mouse to mongoose to moose, that number is similar—the heart muscle governs your lifespan, unless the gremlins get you first.

Another striking pattern is related to er… piss.

Your average peeing time is about twenty seconds. There is no evidence that this varies with gender, so my first question is ‘why are there always queues outside women’s toilets?’ A related question is whether the advent of unisex restrooms is discriminatory for men—there will be lots more guys hopping around waiting to relieve themselves.

The fascinating thing is that cats also take twenty seconds to have a pee, as apparently do elephants. Researchers at Georgia Tech, in the US, discovered a kind of ULU—the Universal Law of Urine. In experiments undertaken with thirty-two species, they found that 6.6 pounds is the weight cutoff threshold, if you excuse the pun.

Mammals above that threshold obey the twenty-second rule. Now, before I expound further on the golden delights, a caveat. I have a long experience in observing hound ablutions, and I assure you dogs are an exception to the ULU rule.

That’s not to say an uncommitted canine won’t micturate by the rules, but when it takes to the street, a hound is on a territorial mission—if dogs took twenty seconds to mark territory, given they are prone to doing so perhaps ten times, depending on the turnover of recently passed (sorry) competitors, their bladders would be capacious indeed.

So here’s the pattern and the paradox: a cat’s bladder is 3600 times smaller than an elephant’s—the pachyderm bladder holds thirty-eight (US) pints. So how in the world can Ellie have a five-gallon whiz in twenty seconds?

The answer lies in the length of the urethra. Fluid flow rates depend on the pressure gradient between the ends of the tube and on the resistance of the tube. This was analyzed in the early XIXth century by Poiseuille, who studied blood flow in humans. The French scientist considered horizontal flow, and a longer tube actually offers more resistance, but the key to the tale of the cat and the elephant is vertical discharge.

The larger bladder volume creates a greater head of pressure, and the length of the urethra creates a higher pressure gradient due to gravity.

The evolutionary consequences of this are far-reaching. In nature, a predator is always around the corner—it doesn’t pay to take too long to have a piss.

To stimulate your scientific quest, I am reproducing the first figure from the published article below. This, my friends, is serious piss!

Illustration of various aspects of the study, taken from the paper published by PNAS.

But fear not, much like the orange man (who may well graduate into an orange suit), my forays into the golden shower didn’t stop here—from mammal pee, I went way down the line to insects. How low can you get?

You guessed it, we’re back to Georgia Tech—with their penchant for piss, they should give Donald an honorary doctorate.

An expert in extreme biophysics discovered a bug that pees faster than a cheetah can sprint.

The insect is appropriately called a sharpshooter, and when a tree is suitably stocked, the resulting emission is known as ‘leafhopper rain.’ The little buggers (or is that piss artists) shoot at twenty times the acceleration of gravity. At two hundred meters per second squared, their jet also outsprints the cheetah by a factor of twenty.

There may be cutting-edge engineering lessons to be learned here, and then again there may be Ig Nobels waiting in the wings, if you excuse the pun.

Either way, that’s quite enough pissing about for one day.

The India Road, Atmos Fear, Clear Eyes, and Folk Tales For Future Dreamers. QR links for smartphones and tablets.

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