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Down in the forested streams of South America lives a curious little mammal called the yapok.
在南美洲溪流深处住着一种古怪的小型哺乳动物—蹼足负鼠。
Also known as the "water opossum," it's the only marsupial adapted for an aquatic lifestyle.
也被称为“蹼足负鼠属”，它们是唯一一种适应水中生活方式的有袋类动物。
And to be so at home in the water, it's developed a curious trait: the males have pouches, too.
为了在水中舒适生活，蹼足负鼠进化出了一种怪异的特质：雄性蹼足负鼠也有育儿袋。
And what's a male yapok do with his pouch? Tuck his junk, of course.
那么雄性蹼足负鼠要育儿袋干啥呢？当然是偷藏自己的垃圾。
Yapoks are nocturnal, spending their nights diving in streams in search of small fish, crustaceans, insects, and frogs to eat.
蹼足负鼠是夜行动物，它们在夜间跳入水中寻找小鱼、甲壳纲动物、昆虫和青蛙吃。
They're sort of like an otter or muskrat, but cousin to the good 'ole Virginia opossum.
它们有点像是水獭或是麝鼠，但是它们是弗吉尼亚负鼠的老亲戚。
And they have a lot of similar aquatic adaptations—webbed feet, water-repellent fur, and tails that they use as rudders.
它们有很多相似的适应性—蹼足、防水的皮毛以及被它们用作方向舵的尾巴。
But one of their water-adapted traits stands out.
但是它们适水性的特征中，有一点非常突出。
They're the only living marsupial species in which both sexes have pouches.
它们是唯一一种雌雄都有育儿袋的有袋类动物。
The females use their pouch like other marsupial ladies do—as a nice protective case for their developing young.
可其他有袋类雌性一样，雌性蹼足负鼠将它们的袋子作为培育下一代的有效保护。
But because they enjoy such a wet lifestyle, a female yapok's pouch is watertight.
但是因为它们喜欢这种潮湿的生活方式，所以雌性蹼足负鼠的袋子是防水的。
That way, it keeps the babies safe and dry while mom's swimming.
这样在妈妈游泳时能够保护宝宝的安全和环境干燥。
The male's pouch isn't watertight, but it's definitely there, which is more than you can say for any other adult male possum—
雄性的育儿袋并不防水，但是它们就是有，和任何其他成年雄性负鼠都不一样—
or any other adult male marsupial, for that matter.
或者其实是任何其他成年雄性有袋动物。
Some young male marsupials have pouches, but they lose them by the time they mature—except for yapoks.
一些年轻的雄性有袋动物都有育儿袋，但是随它们长到成熟期，育儿袋就会消失—除了蹼足负鼠。
And like a female's pouch, the male yapok's pouch keeps something safe and warm while the animal swims: his scrotum.
和雌性的育儿袋一样，在游泳时，雄性蹼足负鼠的育儿袋能够保护袋内物品的安全和温度—他的阴囊。
And that's probably in part to help keep his sensitive sac safe, as dangly bits are thought to be a bit inconvenient when swimming
在这某种程度上是为了帮助保护敏感阴囊的安全，因为在游泳时，晃动的小部分被认为有点不方便，
because they create drag and could snag on an unseen object. Youch.
因为游泳时，它们成了阻碍，还会抓住某些看不见的物品。
Perhaps more importantly, a nice pouch ensures the sperm-makers stay at just the right temperature.
也许最重要的是，好的育儿袋能够保证阴囊待在温度适当的地方。
One of the reasons mammals have an external sac for the testes is actually to keep them cool.
哺乳类动物为睾丸进化出一个额外囊袋的原因之一就是保证它们的清爽。
Overheating damages sperm, so hot testicles can hamper a male's fertility.
过热会损伤精子，热热的睾丸可能会妨碍雄性的生育能力。
But it's not good for sperm to get too cold, either, and getting wet is a sure way to catch a chill.
但是精子太冷也不好，而被淋湿肯定会感到寒冷。
That's because water acts as a better heat conductor than air, so you lose heat more quickly in it.
这是因为相较于空气，水是一种更好的导热体，所以在水中，散热更快。
Hence the yapok's unique system. Other water-dwelling mammals face the same issue.
因此蹼足负鼠有独特系统。其他在水中生活的哺乳动物都面临同样的问题。
But since they lack pouches, they've come up with other ways to hit that Goldilocks temperature.
但是由于它们没有育儿袋，它们也有其他方式对抗金凤花区温度。
In whales, dolphins, manatees, and seals, the testicles are internal,
鲸鱼、海豚、海牛和海豹的睾丸是生长在体内的，
but they're hooked up to a kind of cooling system that uses chilled blood from the extremities to prevent overheating.
但是它们连接着一种冷却系统—利用骨端冰冷的血液预防过热。
Sea lions do have scrotums, but they can be a bit flexible about the degree of dangling.
海狮确实有阴囊，但是它们对于摇晃程度要灵活一点。
They're able to tuck their testicles up and in, and only poke them out into their scrotums as needed to cool off or during the breeding season.
它们能够卷起收入它们的睾丸，只在需要降温或在繁殖季节才将它们的睾丸释放出来。
And otters are considered seasonally scrotal—their testes are so small when they're not breeding that they don't really protrude from the body,
水獭的阴囊被认为是季节性的—当不在繁殖季节时，它们的睾丸非常小，这样它们就不需要从身体向外凸出，
so they don't create much drag or catch on things.
因此就不会变成阻碍或碰到其他物体。
But since yapoks come from an evolutionary lineage where pouches are a thing,
但是由于蹼足负鼠来自于一种重视育儿袋的演化谱系，
natural selection apparently co-opted a part already at hand to help males with their tender testes when they took to the water.
很显然自然的选择为它们分派了这个必要的特征帮助维护雄性脆弱的睾丸。
Because if you've got a pocket right there, you might as well use it!
因为如果你正好有这么个口袋的话，你也会用的！
Thanks for watching this episode of SciShow, a Complexly production.
感谢收看本期《科学秀》，本期节目由Complexly出品。
If you really dig stories about amazing animals and their pouches,
如果你喜欢动物和它们育儿袋的故事，
you might want to check out our sister channel, Animal Wonders, over at youtube.com/animalwondersmontana.
请登录youtube.com/animalwondersmontana，观看我们的姐妹频道《动物奇迹》。
On Animal Wonders, resident animal specialist Jessi Knudsen Castaneda talks about some of the most incredible creatures
在《动物奇迹》中，当地动物专家 Jessi Knudsen Castaneda将讲述一些
that share this planet with us and the challenges of caring for them as a wildlife rescuer.
和我们共享地球的不可思议的生物以及作为野生动物救助者的挑战。
