And we know that many people live for decades after they go through menopause.

我們知道，許多人在經歷更年期後還能活幾十年。

But for a long time, researchers have wondered why that is.

但長期以來，研究人員一直在思考這是為什麼。

If the whole goal of any organism is to pass along its genes, why live so long after you can no longer reproduce?

如果任何生物體的全部目標都是傳遞其基因，為什麼不能再生育後還要活那麼久？

One of the hypotheses those researchers came up with is called the grandmother hypothesis,

這些研究人員提出的假設之一叫做祖母假說，

which says that the reason that human females live so long is to fill the social role of a grandmother, caring for her grandchildren when she's not able to have any more children of her own.

該假說認為，人類女性之所以長壽，是因為她們要扮演祖母的社會角色，在無法再生育孩子時照顧她自己的孫子輩。

There's just one tiny problem with this hypothesis.

只是這個假設一個小問題。

It turns out humans aren't the only species that experiences menopause.

原來，人類並不是唯一經歷更年期的物種。

Not even close.

差遠了。

The idea that grandmothers can help improve survival rates of their grandchildren has been around since the 1980s.

自 1980 年代以來，祖母可以幫助提高孫子輩存活率的想法就一直存在。

Grandmothers can take on some of the burden of acquiring food for nursing mothers or dependent children, or provide care for those children while mothers are out foraging.

祖母可以承擔一些為哺乳母親或受扶養子女獲取食物的負擔，或是在母親外出覓食時照顧這些孩子。

They can also help teach children the complexities of culture and be repositories of generational knowledge.

她們還可以幫助孩子們了解文化的複雜性，並成為代際知識的寶庫。

This version of the grandmother hypothesis was developed after anthropological field work with the Hadza people of northern Tanzania.

這個版本的祖母假說是在對坦尚尼亞北部的哈札人進行人類學實地工作後提出的。

In this hunter-gatherer group, postmenopausal women spent more time collecting food than women of reproductive age did.

在這個狩獵採集群體中，停經後婦女比育齡婦女花更多的時間收集食物。

Which makes sense.

這就說得通了。

If you don't have kids to take care of, you have more time on your hands.

如果你沒有孩子要照顧，你有更多的時間

These Hadza grandmothers also gathered more tubers in addition to the berries that everybody would gather.

除了每個人都會採集的漿果之外，這些哈扎祖母還採集了更多的塊莖。

Those tubers were harder to harvest, so the researchers hypothesized that older women without children to take care of could devote more energy to that task than younger women could.

這些塊莖更難收穫，因此研究人員假設，沒有孩子需要照顧的老年女性可以比年輕女性投入更多的精力來完成這項任務。

And that meant there'd be more food for these womens' grandchildren, so it would help them survive.

這意味著這些婦女的孫子們可以獲得更多食物，從而幫助他們生存。

In other words, this grandmother hypothesis suggests that being a grandmother and helping to take care of your adult offspring and their offspring is a better way of ensuring your genes get passed on than continuing to reproduce yourself.

換句話說，這個祖母假說表明，作為一個祖母並幫助照顧你的成年後代及其後代，是確保你的基因得以傳承的更好方法，而不是繼續繁衍自己。

Hence, menopause.

這就是更年期。

But why are we just talking about grandmothers?

但為什麼我們只談論祖母呢？

What about my papa?

我的爺爺呢？

Human males can retain their fertility for their whole lives, which would suggest that whatever evolutionary pressures led to grandmothering, didn't apply to grandfathering.

人類男性可以終生保留生育能力，這表明無論進化壓力如何導致祖母式生育，都不適用於祖父式生育。

One potential explanation for this is paternal uncertainty.

對此的一個可能的解釋是父親的不確定性。

Since the only way to be 100% sure that an offspring is yours is to give birth to it, that means the males of a species might not know for sure how many offspring they actually have.

由於百分之百確定後代是你的唯一方法就是生下它，這意味著一個物種的雄性可能不確定他們實際上有多少後代。

Which means that it's in their best interest, evolutionarily speaking to keep being able to have more offspring.

這意味著，從演化的角度來說，保持擁有更多後代的能力最符合它們的利益。

Now, importantly, the data that led to the formation of the grandmother hypothesis was collected in 1989,

現在，重要的是，祖母假說形成的數據是在 1989 年收集的，

and a lot of the research trying to support or disprove the hypothesis has also been done in modern humans.

許多試圖支持或反駁這個假設的研究也在現代人類身上進行。

The evidence for grandmothers actually improving reproductive outcomes in modern human societies is mixed, but that's not a big deal, right?

在現代人類社會中，祖母實際上改善了生殖結果的證據好壞參半，但這沒什麼大不了的，對吧？

After all, the evolutionary pressures acting on humans now are way different than they would have been when menopause and grandmothering first developed.

畢竟，現在作用在人類身上的演化壓力與更年期和祖母剛出現時的情況有很大不同。

So researchers were able to sort of hand wave away the fuzziness of those results, for a while.

因此，研究人員能夠暫時消除這些結果的模糊性。

And while researchers have noted that there were a few exceptions to the grandmother hypothesis, they tended to be ones that prove the rule.

儘管研究人員注意到祖母假設有一些例外，但它們往往證明了這項規則。

Take killer whales.

就拿虎鯨來說吧。

Killer whales can live more than 20 years after they're no longer able to reproduce, and also just so happen to live in matrilineal pods, with daughters and granddaughters staying in the same pod they were born in.

虎鯨在無法繁殖後還能存活 20 多年，而且它們也恰好生活在母系群體中，女兒和孫女都住在它們出生的同一個群體中。

That means that these grandmothers are using their decades of knowledge to benefit their whole extended family tree, since their grandchildren live right alongside them.

這意味著這些祖母正在利用她們數十年的知識來造福她們的整個大家族，因為她們的孫子女就和她們住在一起。

These wise grandmas know the best places to scope out food, how to avoid predators, and might even know some fun games to play, like smashing boat rudders–

這些聰明的奶奶知道尋找食物的最佳地點，如何躲避掠食者，甚至可能知道一些有趣的遊戲，例如砸碎船舵——

check out our video on the orca uprising for more on that story.

請觀看我們關於逆戟鯨起義的影片，以了解有關該故事的更多資訊。

But for killer whales and humans alike, grandmothering is described as the key to survival, and sets us apart from other species who don't have these matrilineal practices, and are fertile their whole lives.

但對於虎鯨和人類來說，祖母的角色被描述為是生存的關鍵，使我們有別於其他物種，因為其他物種他們沒有這些母系習俗，且一生都能生育。

There's just one problem.

只有一個問題。

It turns out, there are more animals besides just killer whales whose females stop being fertile in their old age.

事實證明，除了虎鯨之外，還有更多的雌性在年老時停止生育的動物。

A lot more.

還有很多。

Oops.

哎呀。

Now, it's true that other mammals don't experience menopause in the way humans do, because menopause for us specifically refers to when menstruation ends, and most other species don't menstruate in the first place.

其他哺乳動物確實不會像人類那樣經歷更年期，因為對我們來說，更年期特別指月經結束的時間，而大多數其他物種一開始就不會月經。

But those other animals do undergo reproductive senescence, or a decline in fertility with aging.

但其他動物確實會經歷生殖衰老，或隨著年齡增長生育能力下降。

And this is where things get tricky.

這就是事情變得棘手的地方。

Because it's really hard to measure menopause or ovulation in species that don't visibly menstruate.

因為對於沒有明顯月經的物種來說，測量更年期或排卵確實很困難。

So scientists have traditionally used demographic data based on what we know about a species's age-associated fertility and survival rates.

所以科學家們傳統上使用的人口數據是基於我們對一個物種與年齡相關的生育率和存活率的瞭解。

But this kind of data is flawed, in part because there are a lot of uncertainties around species' potential lifespans in the wild.

但這種數據是有缺陷的，部分原因是有很多物種在野外潛在壽命的不確定性。

A lot of animals die far before their max possible lifespan, because the natural world is dangerous and scary.

許多動物在其最大可能壽命之前就死亡了，因為自然界是危險和可怕的。

So if we don't know exactly when these animals stop ovulating, and we don't necessarily know how long they can live, we can't actually say that females of most species die while still being fertile.

因此，如果我們不確切知道這些動物何時停止排卵，也不一定知道它們能活多久，我們實際上不能說大多數物種的雌性在仍具有生育能力的情況下死亡。

That's why newer research is turning to the physiological signs of ovulation as a better metric for whether or not a female can still have offspring.

這就是為什麼新的研究正在轉向排卵的生理跡象，作為衡量女性是否仍可以生育後代的更好指標。

Researchers can check if the ovaries have recently released an egg by measuring hormone concentrations in urine, or by looking at the different types of cells that are in an animal's vagina.

研究人員可以透過測量尿液中的荷爾蒙濃度或透過觀察動物陰道中不同類型的細胞來檢查卵巢最近是否釋放了卵子。

While these methods are more reliable than demographics, they can also be pretty… invasive.

雖然這些方法比人口統計數據更可靠，它們也可能很......有侵略性。

And they work better when studying animals in captivity than animals in the wild.

在研究圈養動物時，它們的效果更好比野外的動物還多。

That said, there are ways to study physiological changes in wild animals.

不過，研究野生動物的生理變化還是有辦法的。

You can still get urine samples, for instance, as long as you're quick enough.

例如，仍然可以獲取尿液樣本，只要你速度夠快。

So as our analytical methods have gotten better, researchers wanted to take a look at our closest relatives, chimpanzees.

所以隨著我們的分析方法越來越好，研究人員想看看我們的近親——黑猩猩。

And it turns out that we're not so special after all, because research indicates that chimpanzees stop ovulating too.

事實證明，我們並沒有那麼特別，因為研究表明，黑猩猩也會停止排卵。

A longitudinal study in the Ngogo chimpanzee community in Uganda used both demographic and physiological data and found that none of the chimpanzees gave birth after the age of fifty.

烏干達 Ngogo 黑猩猩群落的一項縱向研究使用了人口統計和生理數據，發現黑猩猩中沒有一隻在 50 歲之後生育。

And their hormone changes looked a lot like what happens when humans go through menopause.

他們的荷爾蒙變化看起來很像當人類進入更年期時。

But those females kept on living, spending a full fifth of their lifespan unable to have more offspring.

但這些雌性仍然繼續活著，在她們生命的五分之一的時間裡無法生育更多的後代。

And there is no way that the grandmother hypothesis could explain any of this.

祖母假說無法解釋這一切。

For one thing, chimpanzee daughters leave the groups that they were born in when they reach adulthood, so older females don't live in the same groups as the offspring they're totally sure they're related to.

一方面，黑猩猩的女兒在成年後會離開她們出生的群體，因此年長的雌性黑猩猩不會與她們完全確定有親屬關係的後代生活在同一個群體中。

And while they do continue to live with their sons, remember, there's that whole parental uncertainty thing.

儘管他們確實繼續與兒子住在一起，但請記住，父母的不確定性是存在的。

Grandma might not know for certain which babies in the group are really her descendants.

奶奶可能不確定這群孩子中哪些孩子真的是她的後代。

And anyway, older chimps don't really babysit for anyone in their groups, even though this isn't uncommon in other primate species.

不管怎樣，年長的黑猩猩並不會真正照顧群體中的任何人，儘管這在其他靈長類動物中並不罕見。

Chimpanzees are just kind of jerks.

黑猩猩就是有點混蛋。

But that's just the chimps.

但這只是黑猩猩。

What if menopause is still really rare in the natural world?

如果更年期在自然界中仍然非常罕見呢？

Surely that would mean our grandmas are special!

當然，這意味著我們的祖母很特別！

Well, no.

嗯，沒有。

It turns out that there are a lot more species that have some form of reproductive senescence, too.

原來還有更多的物種也有某種形式的生殖衰老。

In studies that using physiological and demographic data, it turns out that basically all mammals experience life after ovulation.

使用生理和人口統計數據進行的研究表明，基本上所有哺乳動物都會在排卵後經歷生命。

One study compiled the results of a whole bunch of these physiological tests and demographic surveys, and estimated that across mammalian species, females spend 28% or more of their lifespan physically unable to reproduce.

一項研究匯總了一系列生理測試和人口統計調查的結果，並估計在哺乳動物物種中，雌性在其一生中 28% 或更多的時間在身體上無法繁殖。

And while some had complex social structures, they didn't align with any observations of a particular importance on a grandmother in helping to care for young.

儘管有些動物具有複雜的社會結構，但他們與祖母在幫助照顧孩子方面特別重要的任何觀察結果並不相符。

So once again, we see an end in fertility, but no significant relationship with grandmas.

所以我們再次看到了生育的終結，但與祖母沒有明顯關係。

Which means that grandmothering can't be the evolutionary reason behind menopause.

這意味著祖母不可能是更年期的演變原因。

Or at least, it can't be the only reason.

至少，這不可能是唯一的原因。

The authors of the chimp study suggest that reproductive senescence might be related to competition with younger females for opportunities to reproduce.

黑猩猩研究的作者認為，生殖老化可能與年輕雌性爭奪繁殖機會有關。

But there's another, even simpler hypothesis around menopause that might explain a lot of this, called the extended mothering hypothesis.

但還有另一種更簡單的關於更年期的假說可以解釋很多問題，稱為母愛延伸假說。

The extended mothering hypothesis basically says that as a female gets older, it becomes a better strategy to invest energy into the current offspring she's raising, rather than having more new ones.

母愛延伸假說基本上認為，隨著女性年齡的增長，將精力投入到她正在撫養的當前後代身上，而不是生育更多新後代，就成為更好的策略。

Pregnancy takes a ton of energy, which could be spent rearing the current crop of offspring instead of growing more.

懷孕需要大量的能量，這些能量可以用來培育目前的後代，而不是生育更多的後代。

Plus, dying before all of your children are able to take care of themselves is not great for the survival rates of any offspring who haven't reached independence.

另外，在所有孩子都能夠照顧自己之前死亡對於任何尚未獨立的後代的生存率來說並不好。

Since baby mammals survive on milk, they're totally dependent on their mothers for at least a little while after birth, and have next to zero odds of survival without her around.

由於哺乳動物幼崽靠牛奶生存，它們在出生後至少一段時間內完全依賴母親，如果沒有母親在身邊，它們​​的生存幾率幾乎為零。

So being able to reproduce for the entirety of your lifespan would require a huge resource investment, which would all become moot if you die before your kids are all grown up.

因此，在你的整個生命週期中能夠繁衍後代需要巨大的資源投資，如果你在孩子長大之前就去世了，那麼這一切都將變得毫無意義。

This hypothesis fits with reproductive senescence across all these other species too, not just ours.

這個假設也適用於所有其他物種的生殖衰老，而不僅僅是我們的物種。

Of course, it's also possible that there's no evolutionary reason for menopause at all.

排卵停止可能是因為我們的生物系統會隨著年齡的增長而磨損，就像頭髮變白或皮膚起皺時我們不會死亡一樣。

It could be that ovulation stops because our biological systems just have a tendency to wear down with age, just like how we don't die when our hair turns gray or our skin gets wrinkly.

排卵停止可能是因為我們的生物系統會隨著年齡的增長而磨損，就像頭髮變白或皮膚起皺時我們不會死亡一樣。

So it turns out that this decades-old hypothesis was built on a house of cards, and it may have come crashing down.

事實證明，這個已有數十年歷史的假設是建立在紙牌屋上的，它可能已經崩潰了。

We now know that females no longer being able to reproduce in their twilight years is super common, thanks to these new techniques and studies.

我們現在知道，由於這些新技術和研究，雌性在晚年不再有能力繁殖是非常常見的。

But hey, just because natural selection might not have made all of our grandmas for some special reason, that doesn't make your grandma any less special.

但是，嘿，僅僅因為自然選擇可能不會因為某些特殊原因而造就我們所有的祖母，但這並不會讓你的祖母變得不那麼特別。

If you liked this video, you might want to listen to a few of us at SciShow ramble about some other science topics that tickle our fancy.

如果你喜歡這個影片，你可能想聽聽我們 SciShow 中的一些人閒聊一些其他引起我們興趣的科學話題。

And boy do we have a podcast for you!

我們為你準備了播客！

Some of the fun people involved in SciShow get together for a lightly competitive knowledge showcase.

參與 SciShow 的一些有趣的人齊聚一堂，進行一場輕度競爭的知識展示。

Every episode, they rack up points for teaching the others—and everyone listening at home—the most mind-blowing science facts related to the week's theme.

每一集，他們都會透過向其他人以及每個在家聆聽的人傳授與本週主題相關的最令人興奮的科學事實來獲得積分。

It's called SciShow Tangents, and you can find it anywhere you get your podcasts.

它叫做 SciShow Tangents，在任何地方都能找到它。