The other "Golden Rule" - David L. Hu

譯者: Lilian Chiu 審譯者: Helen Chang

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

貓的膀胱能貯存的尿液量 只等同一個高爾夫球的大小。

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

人類則等同一個咖啡馬克杯， 大象等同一個廚房垃圾筒。

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

大象的膀胱尺寸是貓的四百倍，

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

但大象尿尿的時間並沒有長四百倍。

So, how does this work?

所以，這是怎麼回事？

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

答案在於科學家所謂的 「其他黃金定律」。

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

該定律適用於將排泄物以黃色 液體形式尿出來的哺乳類動物。

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

其他類型的動物用不同的 系統和方法來排出排泄物。

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

這條定律講的是泌尿器官和力量

會如何隨著身體大小而改變，

enabling a range of mammals to achieve similarly rapid relief.

讓許多哺乳類動物 能達到差不多的快速解放。

There are two main factors contributing to urination speed:

排尿速度背後有兩個主要的因子：

bladder pressure and gravity.

膀胱壓力和地心引力。

Bladder pressure is created when mammals contract their muscles

哺乳類動物收縮肌肉 將尿液擠出，這就是膀胱壓力。

to squeeze urine out.

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

這種壓力等同於每單位面積 所施加的肌肉力量，

但因為這些因子會成比例增加，

since these factors scale proportionally,

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

不同身體大小對應的 膀胱壓力不會差很多。

Meanwhile, the effects of gravity do vary across species.

而地心引力的影響則會因物種而異。

Mammals have a pipe-like organ called the urethra

哺乳類動物有種 管狀的器官，叫做尿道，

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

它會將尿液從較高的高度 輸送到較低的高度——

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

明確來說，就是從膀胱向下 輸送到身體的尿液出口。

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

此時尿道所利用的原力 是「托里切利定理」，

which states that flow rate increases with container height.

該定理出，容器高度 越高時流速會越快。

Take these two containers holding the same volume of liquid.

這兩個容器裝了等量的液體。

When identically punctured at the bottom,

在底部打一個完全相同的洞，

the vertical container drains nearly twice as fast.

垂直容器流光的速度幾乎是兩倍。

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

這是因為水從更大的高度差向下流。

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

尿道將尿液從高處輸送 到低處時也有類似的效應。

The urethra is probably not perfectly vertical,

尿道未必完全垂直，

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

所以它的長度無法代表 尿液向下落的高度差，

However, it’s a good approximation.

然而是個很不錯的近似值。

The urethra’s diameter also affects flow rate.

尿道的直徑也會影響流速。

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

若垂直容器的出口洞孔直徑比較大，

the water drains out even faster.

水流光的速度會更快。

The length and diameter of an animal’s urethra varies

動物的尿道長度和直徑各有不同，

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

要看物種以及牠們具有陰莖或陰道。

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

比如，母象的尿道有一公尺長，

with a thirty-five millimeter diameter.

直徑為三十五毫米。

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

有陰道的人類，

尿道短了至少二十倍，

and 5 times thinner.

窄了五倍。

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

大象的尿道較長、較寬，

讓牠們的排尿時間和人類不相上下。

for a comparable amount of time as a human.

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

的確，如果大象的尿道和家貓一樣，

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

把膀胱中的尿液排完 可能就要花上兩個小時。

It’s hypothesized that the Other Golden Rule

「其他黃金定律」 之所以被保存的推理是

is conserved because quicker pees may both reduce vulnerability to predators

排尿較快或可減少 被捕食者獵殺的機會，

and flush out disease-causing pathogens.

並將會造成疾病的病原體快速排出。

But the Rule has limitations.

但這條定律也有侷限。

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

目前只證明它適用於 非水生哺乳類動物。

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

且這條定律並不適用於 體重三公斤以下的哺乳類動物。

They actually pee in droplets, not continuous streams,

牠們排出的尿是小滴狀， 不是連續的水流，

because surface tension breaks up their tiny jets of urine.

因為表面張力會破壞 牠們射出的小小尿注。

Even within non-aquatic mammals above three kilograms,

即使在三公斤以上的 非水生哺乳類動物中，

different behaviors disrupt the Other Golden Rule.

不同的行為也會打破 「其他黃金定律」。

Male dogs don’t always fully empty their bladders

公狗不見得一定會把膀胱清空，

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

反而會分次短暫排放，以標記地盤。

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

公熊貓有時會以前肢站立的姿勢排尿，

以標記樹木更高處的樹皮，

in order to broadcast their scent to potential mates.

為要把氣味散佈給潛在的配偶。

Handstand pees take longer because urine must work against gravity.

倒立排尿要花的時間更長，

因為尿液必須要對抗地心引力。

Underwater pees are also lengthier

水底的排尿也比較花時間，

because the external water pressure counteracts the internal forces

因為外部的水壓會抵消

內部由高度差產生的力量。

resulting from elevation difference.

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

不論強度是消防栓等級 或玩具水槍等級，

this system swiftly delivers our urine to the outside world.

這個系統都能快速地 將我們的尿液送到外面的世界。