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A cat’s bladder can only store a golf
ball’s worth of urine.
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For humans, it’s a coffee mug and for
elephants, a kitchen trash can.
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An elephant’s bladder is 400 times the
size of a cat’s,
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but it doesn’t take an elephant 400 times
longer to pee.
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So, how does this work?
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The answer lies in what scientists call
the “Other Golden Rule.”
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It applies to mammals, which pee out
some of their wastes in a yellowy liquid.
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Other classes of animals use different
systems and methods to excrete waste.
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The Rule describes how urinary organs
and forces change with body size,
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enabling a range of mammals
to achieve similarly rapid relief.
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There are two main factors contributing
to urination speed:
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bladder pressure and gravity.
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Bladder pressure is created when
mammals contract their muscles
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to squeeze urine out.
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This pressure is equal to the applied
muscular force per unit area but,
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since these factors scale proportionally,
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bladder pressure doesn’t actually
change much depending on body size.
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Meanwhile, the effects of gravity
do vary across species.
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Mammals have a pipe-like organ called
the urethra
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that transports urine from a higher
elevation to a lower one –
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specifically, from the bladder down to
where urine exits the body.
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In doing so, the urethra harnesses
the power of Torricelli’s Law,
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which states that flow rate increases
with container height.
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Take these two containers holding
the same volume of liquid.
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When identically punctured at the bottom,
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the vertical container drains nearly
twice as fast.
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This is because the water’s falling
a greater elevation difference.
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By transporting urine from high
to low, the urethra has a similar effect.
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The urethra is probably not perfectly
vertical,
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so its length doesn’t tell us exactly
how far the urine falls.
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However, its’s a good approximation.
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The urethra’s diameter also affects
flow rate.
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When that vertical container’s exit
hole has a larger diameter,
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the water drains out even faster.
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The length and diameter of an animal’s
urethra varies
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depending on its species and whether
it has a penis or vagina.
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For example, a female elephant has
a meter-long urethra
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with a thirty-five-millimeter diameter.
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A person with a vagina has a urethra
that is more than 20 times shorter
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and 5 times thinner.
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The elephant’s longer, wider urethra
is what allows it to urinate
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for a comparable amount
of time as a human.
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Indeed, if an elephant had the urethra
of a housecat,
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it might take it about 2 hours
to drain its bladder.
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It’s hypothesized that the Other
Golden Rule
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is conserved because quicker pees may
both reduce vulnerability to predators
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and flush out disease-causing pathogens.
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But the Rule has limitations.
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It’s only been shown to apply to
non-aquatic mammals.
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And the Rule doesn’t apply to mammals
under three kilograms.
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They actually pee in droplets,
not continuous streams,
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because surface tension breaks up
their tiny jets of urine.
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Even within non-aquatic mammals above
three kilograms,
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different behaviors disrupt the
Other Golden Rule.
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Male dogs don’t always fully empty their
bladders
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and will instead tinkle in short spurts to
mark their territory.
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Male pandas sometimes pee in
handstands,
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marking bark higher up on trees
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in order to broadcast their scent
to potential mates.
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Handstand pees take longer because
urine must work against gravity.
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Underwater pees are also lengthier
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because the external water pressure
counteracts the internal forces
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resulting from elevation difference.
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Whether it’s with the intensity of a fire
hydrant or a squirt gun,
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this system swiftly delivers our urine
to the outside world.