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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.
猫的膀胱容量只相当于一个高尔夫球的体积。人类膀胱大约与咖啡杯容量相同，而大象则相当于厨房垃圾桶的容量。
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?
大象膀胱的大小是猫膀胱大小的400倍，但是大象小便的时间可不是猫的400倍。这到底是为什么呢？
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.
答案在于科学家所谓的“另一条黄金法则”中。它适用于用尿液来排除身体中废物的哺乳动物们。
Other classes of animals use different systems and methods to excrete waste.
其他种类的动物通过不同的系统和方法来代谢废物。
The Rule describes how urinary organs and forces change with body size, enabling a range of mammals to achieve similarly rapid relief.
该规则解释了泌尿器官和压力是如何因为身体大小的不同而改变的，从而使一系列哺乳动物能够实现类似的快速排尿。
There are two main factors contributing to urination speed: bladder pressure and gravity.
影响排尿速度的主要因素有两个：膀胱压力和重力。
Bladder pressure is created when mammals contract their muscles to squeeze urine out.
当哺乳动物肌肉收缩时就会对膀胱产生压力从而将尿液挤出。
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.
这种压力等同于每单位面积内的肌肉的力量，但由于这些因素是成比例的，所以膀胱压力实际上并不随身体大小而变化。
Meanwhile, the effects of gravity do vary across species.
同时，重力的影响确实因物种而异。
Mammals have a pipe-like organ called the urethra that transports urine from a higher elevation to a lower one
哺乳动物有一个被称为尿道的管状器官，它将尿液从高处输送到低处，
specifically, from the bladder down to where urine exits the body.
特别是从膀胱输送到尿液流出身体的地方。
In doing so, the urethra harnesses the power of Torricelli's Law, which states that flow rate increases with container height.
这样，尿道遵循了托里切利定律，托里切利定律认为流速随容器高度的增加而增加。
Take these two containers holding the same volume of liquid. When identically punctured at the bottom, the vertical container drains nearly twice as fast.
这两个盛有相同体积液体的容器。当底部以同样形式被刺穿时，垂直容器的排水速度几乎是原来的两倍。
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.
这是因为水是从一个更大的高度差下降的。尿道的作用与其类似，通过管道将尿液从高处输送到低处。
The urethra is probably not perfectly vertical, so its length doesn't tell us exactly how far the urine falls.
尿道可能不是完全垂直的，所以它的长度并不能确切地告诉我们尿液下落的距离。
However, it's a good approximation. The urethra's diameter also affects flow rate.
然而，这可以提供一个很好的近似值。尿道的直径也会影响流速。
When that vertical container's exit hole has a larger diameter, the water drains out even faster.
当垂直容器的出口孔直径较大时，水排得会更快。
The length and diameter of an animal's urethra varies depending on its species and whether it has a penis or vagina.
动物尿道的长度和直径各不相同，这取决于它的物种以及它是否有阴茎或阴道。
For example, a female elephant has a meter-long urethra with a thirty-five millimeter diameter.
例如，一头雌象有一米长的尿道直径为35毫米。
A person with a vagina has a urethra that is more than 20 times shorter and 5 times thinner.
有阴道的人的尿道要比其短20倍薄5倍以上。
The elephant's longer, wider urethra is what allows it to urinate for a comparable amount of time as a human.
大象更长、更宽的尿道使它能够与人类排尿时间相同。
Indeed, if an elephant had the urethra of a housecat, it might take it about 2 hours to drain its bladder.
事实上，如果大象有家猫的尿道，它可能需要2个小时才能排空膀胱。
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.
据推测，另一条黄金法则则是保命用的，因为更快地排尿既可以减少被捕食者攻击的概率，也可以清除致病病原体。
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.
但这一规则有其局限性。这只被证明适用于非水生哺乳动物，且这条规则不适用于3公斤以下的哺乳动物。
They actually pee in droplets, not continuous streams, because surface tension breaks up their tiny jets of urine.
它们事实上是以水滴的形式排尿，而不是以水流的形式，因为表面张力破坏了它们微小的尿液喷射。
Even within non-aquatic mammals above three kilograms, different behaviors disrupt the Other Golden Rule.
即使在3公斤以上的非水生哺乳动物中，另一种行为破坏了这条黄金法则。
Male dogs don't always fully empty their bladders and will instead tinkle in short spurts to mark their territory.
雄性狗并不总是完全排空它们的膀胱相反，他们会在短时间内喷洒尿液来标记自己的领土。
Male pandas sometimes pee in handstands, marking bark higher up on trees in order to broadcast their scent to potential mates.
雄性大熊猫有时会倒立撒尿，在树上的高处留下记号以便向潜在的配偶传播他们的气味。
Handstand pees take longer because urine must work against gravity.
倒立排尿需要更长的时间因为尿液必须抵克服重力。
Underwater pees are also lengthier because the external water pressure counteracts the internal forces resulting from elevation difference.
水下排尿的时间也会更长，因为外部水压抵消了由高度差引起的内力。
Whether it's with the intensity of a fire hydrant or a squirt gun, this system swiftly delivers our urine to the outside world.
无论是消防栓的强度还是水枪的强度，这个系统都能迅速将我们的尿液输送到外部世界。
