Animals of different species rarely get it on, and when they do, they rarely produce offspring.

不同物種的動物很少會發生性關係，即使發生了，也很少會產生後代。

But even rarer than the ligers, zonkeys, camas, and beefalo of the world are hybrid animals capable of making babies of their own.

但是，比老虎、猿猴、駱駝和牛更罕見的是能夠自己生孩子的雜交動物。

In fact, the idea of a mule, the hybrid progeny of a female horse and a male donkey, having its own baby is so implausible that the ancient Romans used to employ the saying, when a mule falls, as a metaphor for the impossible.

事實上，騾子是母馬和公驢的雜交後代，騾子生下自己的孩子是如此不可思議，以至於古羅馬人曾用 "騾子跌倒 "來比喻不可能的事。

The same way that Americans say, when pigs fly, Turkish people say, when fish climb trees, and Nigerians say, when the chicken has teeth.

就像美國人說 "當豬飛起來的時候"，土耳其人說 "當魚爬上樹的時候"，奈及利亞人說 "當雞長出牙齒的時候"。

Here's why.

原因就在這裡。

In order to successfully reproduce, animals need to create viable sex cells.

為了成功繁殖，動物需要創造有活力的性細胞。

Normal body cells have two copies of each chromosome, one from the animal's mom and one from dad, sort of like two full decks of cards.

正常人體細胞的每條染色體都有兩個拷貝，一條來自動物的母親，一條來自父親，就像兩副完整的撲克牌。

And both decks can simply be duplicated and then split off to form a new cell.

這兩副牌都可以簡單地複製，然後分割開來，形成一個新的單元。

But sex cells are different.

但性細胞是不同的。

They're produced by taking each set of chromosomes, duplicating them, and then swapping a bunch of cards between decks before breaking them up to form four cells, each with just one complete deck of cards.

它們的產生過程是：提取每組染色體，複製它們，然後在每副牌之間交換一堆牌，再把它們打散，形成四個細胞，每個細胞只有一副完整的牌。

This card swap works when the decks in each set come from a mom and a dad of the same species, because they have all the same cards in the same order.

當每套牌中的牌來自同一物種的爸爸和媽媽時，這種換牌方法就會起作用，因為它們的順序都是一樣的。

So genes for eye color get swapped with other genes for eye color, and stripe pattern genes get swapped with stripe pattern genes, giving rise to perfectly healthy sex cells.

所以，眼睛顏色基因會與其他眼睛顏色基因互換，條紋圖案基因會與條紋圖案基因互換，從而產生完全健康的性細胞。

But in the cells of hybrid animals, the decks aren't identical because they came from parents of different species.

但在雜交動物的細胞中，由於它們的父母來自不同的物種，所以它們的基因組並不完全相同。

So an eye color card might get swapped for a paw size card, or a bone-making gene for a kidney-making gene, producing two really weird decks that give rise to totally non-functional sex cells.

眼睛的顏色基因可能會被換成爪子的大小基因，或者骨骼製造基因可能會被換成腎臟製造基因，從而產生兩副非常奇怪的牌，產生完全沒有功能的性細胞。

However, for reasons we really don't understand, it is sometimes possible for a hybrid animal to skip over the card swapping step and sort of just get rid of the dad's chromosomes to create new sex cells that contain only the mom's DNA.

然而，由於我們實在不明白的原因，有時雜交動物可以跳過換卡這一步，直接去掉爸爸的染色體，製造出只含有媽媽 DNA 的新性細胞。

We've actually only seen this happen in female mules.

實際上，我們只在雌性騾子身上見過這種情況。

There are a handful of documented cases in which mules have made babies from eggs containing only their mother's DNA.

有少數記錄在案的案例表明，騾子用只含有母親 DNA 的卵子生下了孩子。

Bizarrely, this means that each female mule's offspring was genetically also her half-sibling.

奇怪的是，這意味著每頭母騾的後代在基因上也是它的同父異母兄弟姐妹。

So while, as far as we know, pigs never fly, every once in a great while, a mule foals.

雖然據我們所知，豬是不會飛的，但偶爾也會有騾子產下小馬駒。