"Is it a boy or is it a girl?"

「是男生還是女生？」

Now, there are some assumptions behind that question that we take for granted because of our familiarity with our own human biology.

我們總是理所當然地問這個問題，是因為我們熟悉人類生物學。

For human babies, we take it for granted that there's a 50/50 chance of either answer, boy or girl.

我們想當然認為嬰兒有各半的機會是男生或女生。

But why is it that way?

但為什麼？

Well, the answer depends on the sex determination system that has evolved for our species.

答案與我們的性別決定系統有關。

You see, for most mammals, the sex of a baby is determined genetically with the XY chromosome system.

大多數哺乳類動物的嬰兒性別都取決於 XY 遺傳染色體系統。

Mammals have a pair of sex chromosomes, one passed down from Mom, and one from Dad.

哺乳動物有一對性別染色體，一條來自母親，一條來自父親。

A pair of X's gives us a girl, and an X and a Y together gives us a boy.

XX 會得到女兒，而 XY 配對則會得到兒子。

Since females only have X's to pass on in their egg cells, and males can give either an X or a Y in their sperm cells,

既然雌性卵子中只有Ｘ染色體，而雄性可以藉由精子傳送 X 或 Y 染色體，

the sex is determined by the father and the chance of producing a male or a female is 50/50.

所以性別由父親決定，而孩子是雄性或雌性的機會為各半。

This system has worked well for mammals, but throughout the tree of life, we can see other systems that have worked just as well for other animals.

對哺乳動物而言，這個系統運作良好，但在整個演化樹上，還有別種系統在其它動物身上一樣成功運作。

There are other groups of animals that also have genetic sex determination, but their systems can be pretty different from ours.

其他種類動物也有遺傳性別決定法，但與我們的大庭相逕。

Birds and some reptiles have their sex genetically determined, but instead of the sex being determined by Dad, their sex is determined by Mom.

鳥類及某些爬行類動物，也有性別遺傳決定法，但其性別不是由爸爸決定，而是由媽媽。

In those groups, a pair of Z sex chromosomes produces a male, so these males only have Z's to give.

以這些動物來說，一對 ZZ 染色體會生出雄性，所以雄性只能給出 Z 染色體。

However, in these animals, one Z and one W chromosome together, as a pair, produces a female.

然而，如果是一條Ｚ及一條 Ｗ 染色體配對，則會產出雌性。

In this system, the chance of a male or a female is still 50/50, it just depends on whether Mom puts a Z or a W into her egg.

這種系統中，生出雄性或雌性的機會仍是各半，差別僅在於是由母親放 Z 還是 W 到卵子裡決定性別。

Certain groups have taken genetic sex determination in completely other directions.

但某些動物則以完全不同的方法來決定性別。

Ants, for example, have one of the most interesting systems for determining sex, and because of it, if you are a male ant, you do not have a father.

舉例來說，螞蟻就有一套非常有趣的性別決定系統，且正因那套系統，如果你是隻雄蟻，那你不會有父親。

In an ant colony, there are dramatic divisions of labor.

在蟻巢裡，每隻螞蟻的工作劃分非常明確。

There are soldiers that defend the colony, there are workers that collect food, clean the nest, and care for the young.

兵蟻專門守禦蟻巢、工蟻收集食物、清理巢穴並照顧幼蟻。

And there's a queen and a small group of male reproductives.

還有一隻蟻后及一小群負責生殖的雄蟻。

Now, the queen will mate and then store sperm from the males.

蟻后會與雄蟻交配，並儲存精子。

And this is where the system gets really interesting.

而這就是有趣的地方。

If the queen uses the stored sperm to fertilize an egg, then that egg will grow up to become female.

如果蟻后拿儲存的精子受精卵子，那麼這個卵將會長成一隻雌蟻。

However, if she lays an egg without fertilizing it, then that egg will still grow up to be an ant, but it will always be a male.

但如果牠下了卵，沒有受精，那這個卵子仍會長成螞蟻，但一定會是一隻雄蟻。

So you see, it's impossible for male ants to have fathers.

所以，雄蟻不可能有父親。

And male ants live their life like this, with only one copy of every gene, much like a walking sex cell.

而且雄蟻每種基因都只有單個，就好像活生生的生殖細胞一樣。

This system is called a haplodiploid system, and we see it not only in ants, but also in other highly social insects like bees and wasps.

這個系統叫「染色體倍性性別決定系統」，不只在螞蟻，在其他群居昆蟲，如蜜蜂及黃蜂身上也會看到此現象。

Since our own sex is determined by genes, and we do know of these other animals that have their sex determined by genes,

既然我們知道人類的性別取決於基因，也知道有其它動物一樣。

it's easy to assume that for all animals the sex of their babies still must be determined by genetics.

我們很容易預設所有動物的性別都是由基因決定。

However, for some animals, the question of whether it will be a boy or a girl has nothing to do with genes at all,

但對某些動物而言，性別與基因一點關係都沒有，

and it can depend on something like the weather.

反是由其它因素決定，比如天氣。

These are animals like alligators and most turtles.

像鱷魚和大部份烏龜就是。

In these animals, the sex of an embryo in a developing egg is determined by the temperature.

這些動物蛋內的胚胎性別取決於溫度。

In these species, the sex of the baby is not yet determined when the egg is laid,

下蛋時，蛋內幼體性別還尚未決定，

and it remains undetermined until sometime in the middle of the overall development period, when a critical time is reached.

且會維持未定狀態，直到大概胚胎發育中期的關鍵時刻。

And during this time, the sex is completely determined by temperature in the nest.

這時，性別完全取決於當下巢內的溫度。

In painted turtles, for example, warm temperatures above the critical temperature will produce females within the eggs,

舉錦龜為例，如果溫度超過關鍵溫度就會孵出雌性幼體，

and cool temperatures will produce a male.

溫度較低則會孵出雄性。

I'm not really sure who came up with this mnemonic, but you can remember that when it comes to painted turtles, they are all hot chicks and cool dudes.

我不知道誰發明了這個助記法，但你可以這麼記：錦龜都是火熱辣妹及冰山酷哥。

For some tropical fish, the question of will it be a boy or will it be a girl isn't settled until even later in life.

對某些熱帶魚而言，是男是女要到生命後期才能揭曉。

You see, clownfish all start out their lives as males.

小丑魚在一開始都是雄性。

However, as they mature, they become female.

但成熟後就會變成雌性。

They also spend their lives in small groups with a strict dominance hierarchy where only the most dominant male and female reproduce.

牠們一輩子活在小群體內，嚴格遵守階級制，只有為首的雄魚及雌魚能繁殖下一代。

And amazingly, if the dominant female in the group dies, the largest and most dominant male will then quickly become female and take her place,

神奇的是，如果為首的雌性死了，最大、最具優勢的雄魚就會很快地變成雌性代替她，

and all of the other males will move up one rank in the hierarchy.

然後其他雄性地位都會向上提升一格。

In another very different ocean animal, the Green Spoonworm, the sex of the babies is determined by a completely different aspect of the environment.

另一種非比尋常的海洋動物，綠海洋蠕蟲則完全不同，其幼蟲的性別由環境決定。

For this species, it is simply a matter of where a larva happens to randomly fall on the sea floor.

這種生物，只看幼蟲掉在海底哪裡。

If a larva lands on the open sea floor, then it will become a female.

如果幼蟲直接掉在海底就變成雌性。

But if it lands on top of a female, then it will become a male.

如果幼蟲掉在雌性上面，就變成雄性。

So for some species, the question of boy or girl is answered by genetics.

所以對某些物種而言，男女由遺傳決定。

For others, it's answered by the environment.

對其他物種則由環境決定。

And for others still, they don't even bother with the question at all.

還有一些則根本不在乎這個問題。

Take whiptail lizards, for example.

舉鞭尾蜥為例。

For those desert lizards, the answer is easy.

對這些沙漠蜥蜴而言，答案很簡單。

It's a girl. It's always a girl.

永遠都是女生。

They are a nearly all-female species, and although they still lay eggs, these eggs hatch out female clones of themselves.

這個物種幾乎全是雌性，牠們也下蛋，但孵出來的也只是牠們自己的複製體而已。

So will it be a girl or will it be a boy?

所以是男是女呢？

Throughout the entire animal kingdom, it does really all depend on the system of sex determination.

整個動物界，從上到下的確都有套性別決定系統。

For humans, that system is a genetic XY system.

對人類而言，這套系統是遺傳 XY 系統。

And for me and my wife, we found out it's gonna be a baby boy.

而我和我太太的孩子？我們後來得知會是個小男孩。