Do larger animals take longer to pee? - David L. Hu

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A cat’s bladder can only store a golf ball’s worth of urine. For humans, it’s a coffee mug and for elephants, a kitchen trash can. An elephant’s bladder is 400 times the size of a cat’s, but it doesn’t take an elephant 400 times longer to pee. So, how does this work? David L. Hu digs into what scientists call the “Other Golden Rule.”

Lesson by David L. Hu, directed by Anton Bogaty.

A cat’s bladder can only store
a golf ball’s worth of urine.

For humans, it’s a coffee mug
and for elephants, a kitchen trash can.

An elephant’s bladder is 400 times
the size of a cat’s,

but it doesn’t take an elephant 400 times
longer to pee.

So, how does this work?

The answer lies in what scientists call
the “Other Golden Rule.”

It applies to mammals, which pee out
some of their wastes in a yellowy liquid.

Other classes of animals use different
systems and methods to excrete waste.

The Rule describes how urinary organs
and forces change with body size,

enabling a range of mammals
to achieve similarly rapid relief.

There are two main factors
contributing to urination speed:

bladder pressure and gravity.

Bladder pressure is created
when mammals contract their muscles

to squeeze urine out.

This pressure is equal to the applied
muscular force per unit area but,

since these factors scale proportionally,

bladder pressure doesn’t actually
change much depending on body size.

Meanwhile, the effects of gravity
do vary across species.

Mammals have a pipe-like organ
called the urethra

that transports urine
from a higher elevation to a lower one—

specifically, from the bladder
down to where urine exits the body.

In doing so, the urethra harnesses
the power of Torricelli’s Law,

which states that flow rate
increases with container height.

Take these two containers holding
the same volume of liquid.

When identically punctured at the bottom,

the vertical container drains
nearly twice as fast.

This is because the water’s falling
from a greater elevation difference.

By transporting urine from high to low,
the urethra has a similar effect.

The urethra is probably not perfectly
vertical,

so its length doesn’t tell us exactly
how far the urine falls.

However, it’s a good approximation.

The urethra’s diameter
also affects flow rate.

When that vertical container’s exit hole
has a larger diameter,

the water drains out even faster.

The length and diameter
of an animal’s urethra varies

depending on its species
and whether it has a penis or vagina.

For example, a female elephant
has a meter-long urethra

with a thirty-five millimeter diameter.

A person with a vagina has a urethra
that is more than 20 times shorter

and 5 times thinner.

The elephant’s longer, wider urethra
is what allows it to urinate

for a comparable amount
of time as a human.

Indeed, if an elephant had the urethra
of a housecat,

it might take it about 2 hours
to drain its bladder.

It’s hypothesized
that the Other Golden Rule

is conserved because quicker pees
may both reduce vulnerability to predators

and flush out disease-causing pathogens.

But the Rule has limitations.

It’s only been shown to apply
to non-aquatic mammals.

And the Rule doesn’t apply to mammals
under three kilograms.

They actually pee in droplets,
not continuous streams,

because surface tension
breaks up their tiny jets of urine.

Even within non-aquatic mammals
above three kilograms,

different behaviors disrupt
the Other Golden Rule.

Male dogs don’t always fully empty
their bladders

and will instead tinkle in short spurts
to mark their territory.

Male pandas sometimes pee in handstands,
marking bark higher up on trees

in order to broadcast their scent
to potential mates.

Handstand pees take longer
because urine must work against gravity.

Underwater pees are also lengthier

because the external water pressure
counteracts the internal forces

resulting from elevation difference.

Whether it’s with the intensity
of a fire hydrant or a squirt gun,

this system swiftly delivers our urine
to the outside world.