More than you might think.

可能比你想像中來得多。

Along with over 5,000 other species, they're mammals, or members of class mammalia.

哺乳類動物中有超過五千多個物種，或者是「哺乳動物綱」成員。

All mammals are vertebrates, meaning they have backbones.

所有的哺乳類動物皆為脊椎動物，代表牠們都有脊骨

But mammals are distinguished from other vertebrates by a number of shared features.

但除了身為脊椎動物的共同特質外，哺乳類還有一些不同之處

That includes warm blood,

共同之處包含恆溫

body hair or fur,

身體有毛髮

the ability to breathe using lungs,

可用肺呼吸

and nourishing their young with milk.

雌性動物能哺乳幼子

But despite these similarities,

但儘管有這些類似的特質

these creatures also have many biological differences,

哺乳內動物也有許多生物性質上的差異

and one of the most remarkable is how they give birth.

而其中最顯而易見的就是生育方式

Let's start with the most familiar, placental mammals.

讓我們從最熟悉的胎盤哺乳動物開始介紹

This group includes humans,

胎盤哺乳動物包含人類

cats,

貓

dogs,

狗

giraffes,

長頸鹿

and even the blue whale, the biggest animal on Earth.

甚至藍鯨，世上最大的動物

Its placenta, a solid disk of blood-rich tissue,

胎盤，一固狀圓盤，富含血液的組織

attaches to the wall of the uterus to support the developing embryo.

附著在子宮壁上，以支撐胚胎發育

The placenta is what keeps the calf alive during pregnancy.

胎盤能維持幼子得以在懷孕的雌體內生存

Directly connected to the mother's blood supply,

與母親的血液供應直接相連

it funnels nutrients and oxygen straight into the calf's body via the umbilical cord,

透過臍帶，運送養分與氧氣直接到幼子的身體

and also exports its waste.

並且同時排出體內代謝廢物

Placental mammals can spend far longer inside the womb than other mammals.

胎盤哺乳動物比其他哺乳類動物能待在子宮的時間還要長

Baby blue whales, for instance, spend almost a full year inside their mother.

舉例來說，藍鯨寶寶近乎一年都待在母親體內

The placenta keeps the calf alive right up until its birth,

胎盤能維持幼鯨生命機能直到出生

when the umbilical cord breaks

當臍帶斷裂

and the newborn's own respiratory,

新生兒能依靠自己呼吸

circulatory,

依靠自己的體內循環系統

and waste disposal systems take over.

排泄系統開始作用

Measuring about 23 feet, a newborn calf is already able to swim.

長約二十三尺的新生幼鯨，已有游泳能力

It will spend the next six months

接下來的六個月

drinking 225 liters of its mothers’ thick, fatty milk per day.

幼鯨每天會喝將近二百二十五公升的營養母奶

Meanwhile, in Australia, you can find a second type of mammal -

同時，在澳洲，你能找到第二種類型的哺乳類

marsupials.

有袋動物

Marsupial babies are so tiny and delicate when they're born

有袋哺乳類寶寶剛出生時，既小又脆弱

that they must continue developing in the mother's pouch.

牠們必須持續留在母親的育兒袋中發育

Take the quoll, one of the world's smallest marsupials,

舉袋鼬為例，全世界最小的有袋哺乳類動物

which weighs only 18 milligrams at birth,

出生時只有十八毫克重

the equivalent of about 30 sugar grains.

約莫30格令的糖（譯注：格令為重量單位）

The kangaroo, another marsupial,

袋鼠，另一種有袋動物

gives birth to a single jelly bean-sized baby at a time.

一次生出一胎約約豆型軟糖大小的嬰兒

The baby crawls down the middle of the mother's three vaginas,

嬰兒必須爬過母親的三管陰道

then must climb up to the pouch,

且一定要爬到育兒袋內

where she spends the next 6-11 months suckling.

在育兒袋內，嬰兒接下來會花六到十一個月吸吮

Even after the baby kangaroo leaves this warm haven,

甚至在袋鼠寶寶離開這溫暖的避風港後

she'll return to suckle milk.

仍是繼續吸位於育兒袋中的母奶

Sometimes, she's just one of three babies her mother is caring for.

有時候，牠只是母親所照顧的三個小孩其中之一

A female kangaroo can often simultaneously support one inside her uterus

雌性袋鼠可以同時維持一位在子宮內的袋鼠

and another in her pouch.

與另一位在育兒袋中的袋鼠

In unfavorable conditions,

在不甚理想的狀況下

female kangaroos can pause their pregnancies.

雌性袋鼠可以停止懷孕週期

When that happens, she's able to produce two different kinds of milk,

當此狀況發生時，雌性袋鼠可以製造兩種不同的母乳

one for her newborn,

一種供給給新生兒

and one for her older joey.

另一種則是出生一段時間的袋鼠幼兒

The word mammalia means of the breast,

「有袋哺乳類動物」一字含義為乳房

which is a bit of a misnomer

似乎有點用詞不當

because while kangaroos do produce milk from nipples in their pouches,

原因是當袋鼠真的從育兒袋中的乳頭分泌乳汁時

they don't actually have breasts.

牠們並不是真的有乳房

Nor do monotremes, the third and arguably strangest example of mammalian birth.

單孔類哺乳動物也沒有乳房，這是第三種且可能是最奇怪的哺乳動物出生方式

There were once hundreds of monotreme species,

以前曾有上百種的單孔類哺乳動物

but there are only five left:

但如今只剩五種倖存

four species of echidnas and the duck-billed platypus.

四種針鼴鼠與一種鴨嘴獸

The name monotreme means one hole

「 monotreme」這個字代表「單孔」

referring to the single orifice they use for reproduction,

代表牠們只有一個孔洞能用來繁殖

excretion,

排泄

and egg-laying.

並且為卵生

Like birds,

像是鳥類

reptiles,

爬蟲類

fish,

魚類

dinosaurs,

恐龍類

and others,

與其他動物

these species lay eggs instead of giving birth to live young.

這些物種都是卵生動物，而不是胎生幼體

Their eggs are soft-shelled,

牠們的蛋為軟殼

and when their babies hatch, they suckle milk from pores on their mother's body

而寶寶孵化出來時，寶寶會從母體的小孔吸吮母乳

until they're large enough to feed themselves.

直到牠們大到能夠自行覓食

Despite laying eggs and other adaptations that we associate more with non-mammals,

儘管下蛋和其他適應方式會讓我們聯想到非哺乳類動物

like the duck-bill platypus's webbed feet,

像是鴨嘴獸的蹼

bill,

嘴

and the venomous spur males have on their feet,

以及雄性鴨嘴獸腳上的毒刺

they are, in fact, mammals.

但事實上，牠們是哺乳類動物

That's because they share the defining characteristics of mammalia

那是因為牠們共享哺乳類動物所定義的特質

and are evolutionarily linked to the rest of the class.

並且在演化上和其他類動物有所連結

Whether placental,

無論是否有胎盤

marsupial,

有育兒袋

or monotreme,

或是單孔動物

each of these creatures and its unique birthing methods, however bizarre,

這些生物與牠獨特的生育方式，不管有多特殊

have succeeded for many millennia in bringing new life and diversity

都承接了許多千年物種的特性，生育下一代與促成生物多樣化

into the mammal kingdom.

造就出哺乳類動物王國