What do these animals have in common? More than you might think.
这些动物有什么共同之处？比你能想到的多。
Along with over 5,000 other species, they're mammals, or members of class mammalia.
有5000多种物种是哺乳动物，或者属于哺乳动物纲。
All mammals are vertebrates, meaning they have backbones.
所有哺乳动物都是脊椎动物，也意味着它们都有脊椎。
But mammals are distinguished from other vertebrates by a number of shared features.
但是哺乳动物与其他脊椎动物的不同之处在于以下这些共同特征。
That includes warm blood, body hair or fur, the ability to breathe using lungs, and nourishing their young with milk.
这包括体温恒温，拥有体毛或者毛皮，能够使用肺部呼吸以及给幼崽喂奶。
But despite these similarities, these creatures also have many biological differences,
但除了这些共同点，这些动物在生理上也有差异，
and one of the most remarkable is how they give birth.
其中一个最显著的差异是生育方式。
Let's start with the most familiar, placental mammals.
我们从最熟悉的胎盘类哺乳动物开始。
This group includes humans, cats, dogs, giraffes, and even the blue whale, the biggest animal on Earth.
这一类别包括人类、猫、狗、长颈鹿，以及地球上最大的哺乳动物蓝鲸。
Its placenta, a solid disk of blood-rich tissue, attaches to the wall of the uterus to support the developing embryo.
胎盘是充满血液的盘状固体组织，贴在子宫内膜上，来帮助胚胎发育。
The placenta is what keeps the calf alive during pregnancy.
胎盘在孕期能维持幼崽的生命。
Directly connected to the mother's blood supply,
它直接与母体的供血系统相连，
it funnels nutrients and oxygen straight into the calf's body via the umbilical cord, and also exports its waste.
胎盘通过脐带将营养和氧气直接输送到幼崽的身体，同时排出废弃物。
Placental mammals can spend far longer inside the womb than other mammals.
胎盘类哺乳动物比其他哺乳动物在子宫内待的时间要长很多。
Baby blue whales, for instance, spend almost a full year inside their mother.
比如蓝鲸宝宝在母体内几乎要待近一年的时间。
The placenta keeps the calf alive right up until its birth,
胎盘保证幼崽在出生前的生命，
when the umbilical cord breaks and the newborn's own respiratory, circulatory, and waste disposal systems take over.
当脐带被剪断，新生儿开始使用自己的呼吸系统、循环系统以及废物排泄系统。
Measuring about 23 feet, a newborn calf is already able to swim.
刚出生的蓝鲸宝宝体长约23英尺，一出生就会游泳了。
It will spend the next six months drinking 225 liters of its mothers thick, fatty milk per day.
在出生之后的6个月中，它每天会喝掉225公升浓稠高脂的母乳。
Meanwhile, in Australia, you can find a second type of mammal -- marsupials.
同时，在澳大利亚，你会找到第二类哺乳动物--有袋目哺乳动物。

Marsupial babies are so tiny and delicate when they're born that they must continue developing in the mother's pouch.
有袋目哺乳动物宝宝刚出生时候个头娇小且体弱多病，以致于它们必须继续待在妈妈的育儿袋里面。
Take the quoll, one of the world's smallest marsupials,
比如袋鼬，它是世界上最小的有袋目哺乳动物之一，
which weighs only 18 milligrams at birth, the equivalent of about 30 sugar grains.
刚出生的时候只有18毫克重，差不多相当于30颗砂糖颗粒的重量。
The kangaroo, another marsupial, gives birth to a single jelly bean-sized baby at a time.
而另一种有袋目哺乳动物，袋鼠每次仅生育一个软糖大小的幼崽。
The baby crawls down the middle of the mother's three vaginas,
幼崽从妈妈的第三个阴道出生，
then must climb up to the pouch, where she spends the next 6-11 months suckling.
然后必须爬到育儿袋里面，这是它在接下来6-11个月中喝奶的地方。
Even after the baby kangaroo leaves this warm haven, she'll return to suckle milk.
即便在袋鼠幼崽离开育儿袋后，它还会回来吃奶。
Sometimes, she's just one of three babies her mother is caring for.
有时候，袋鼠妈妈需要照顾3个宝宝。
A female kangaroo can often simultaneously support one inside her uterus and another in her pouch.
通常袋鼠妈妈在怀孕的同时，育儿袋中还可以哺乳一个幼崽。
In unfavorable conditions, female kangaroos can pause their pregnancies.
在条件不利时，雌性袋鼠可以暂停怀孕。
When that happens, she's able to produce two different kinds of milk, one for her newborn, and one for her older joey.
当这种情形发生时，它能产生两种不同的乳汁，一种给新生儿，另一种给幼袋鼠。
The word mammalia means of the breast, which is a bit of a misnomer
哺乳这个词意味着乳房，但其实有点名不副实，
because while kangaroos do produce milk from nipples in their pouches, they don't actually have breasts.
因为尽管袋鼠育儿袋中的乳头确实产生母乳，但是它们并没有乳房。
Nor do monotremes, the third and arguably strangest example of mammalian birth.
单孔目哺乳动物也没有乳房，它们是第三类以及哺乳动物生育方式中最奇怪的一类。
There were once hundreds of monotreme species, but there are only five left:
曾经有好几百种单孔目哺乳动物，现在仅存5种：
four species of echidnas and the duck-billed platypus.
4种针鼹和鸭嘴兽。
The name monotreme means one hole referring to the single orifice they use for reproduction, excretion, and egg-laying.
单孔目这个词指一个孔，指的是一个用于生育、排泄和下蛋的孔。
Like birds, reptiles, fish, dinosaurs, and others, these species lay eggs instead of giving birth to live young.
跟鸟类、两栖类、鱼类、恐龙以及其他动物一样，单孔目动物是卵生，而非直接生育幼崽。
Their eggs are soft-shelled, and when their babies hatch,
它们的蛋壳很软，当幼崽孵出后，
they suckle milk from pores on their mother's body until they're large enough to feed themselves.
它们通过母体的细孔吮吸母乳，直到它们长大能够独立生存。
Despite laying eggs and other adaptations that we associate more with non-mammals,
尽管卵生以及其他进化特征会让我们更多联系到非哺乳动物，
like the duck-bill platypus's webbed feet, bill,
比如鸭嘴兽的蹼足、鸭嘴，
and the venomous spur males have on their feet, they are, in fact, mammals.
以及雄性脚上才有的毒刺，事实上，它们是哺乳动物。
That's because they share the defining characteristics of mammalia and are evolutionarily linked to the rest of the class.
这是因为它们拥有哺乳动物的界定性征，而且在进化上与其他哺乳动物有千丝万缕的联系。
Whether placental, marsupial, or monotreme, each of these creatures and its unique birthing methods, however bizarre,
无论是胎盘类，有袋类或是单孔目类，虽然各自有着既奇怪又独特的生育方式，
have succeeded for many millennia in bringing new life and diversity into the mammal kingdom.
但在过去的几千年，它们都成功地在哺乳动物的王国繁衍生息。