How brain parasites change their host's behavior - Jaap de Roode

寄生蟲是如何改變生物體的行為 - Jaap de Roode

譯者: 毓翔 王 審譯者: Anny Chung

Which of these entities has evolved the ability

這些物種其中有幾位演化出

to manipulate an animal many times its size?

控制比牠們體型大數倍 的生物的能力呢?

The answer is all of them.

答案是以上均是

These are all parasites,

牠們全是寄生蟲

organisms that live on or inside another host organism,

能在寄宿於另一個生物體內或外表

which they harm and sometimes even kill.

並能夠傷害甚至使宿主死亡

Parasite survival depends on transmitting from one host to the next,

寄生蟲的生存取決於從 這個宿主傳送到另一個宿主的過程

sometimes through an intermediate species.

有時會通過不同種的宿主

Our parasites elegantly achieve this by manipulating their host's behavior,

寄生蟲透過操縱宿主的行為 優雅地做到這一點

sometimes through direct brain hijacking.

有時會直接挾持大腦

For example, this is the Gordian worm.

例如，這個蠕蟲病毒

One of its hosts, this cricket.

寄生在這隻蟋蟀上

The Gordian worm needs water to mate, but the cricket prefers dry land.

蠕蟲需要水交配 但蟋蟀喜歡乾燥的土地

So once it's big enough to reproduce,

所以，一旦它大到足以繁殖

the worm produces proteins that garble the cricket's navigational system.

蠕蟲生產蛋白質 以干擾蟋蟀的導航系統

The confused cricket jumps around erratically,

使蟋蟀亂跳

moves closer to water,

來接近水源

and eventually leaps in, often drowning in the process.

蟋蟀往往死於水中

The worm then wriggles out to mate

隨後蠕蟲於水中繁殖

and its eggs get eaten by little water insects

卵被水中昆蟲吞噬

that mature,

然後隨著宿主成熟

colonize land,

上陸

and are, in turn, eaten by new crickets.

並被蟋蟀獵食

And thus, the Gordian worm lives on.

這就是蠕蟲的一生

And here's the rabies virus, another mind-altering parasite.

狂犬病毒 是另一個改變宿主神智的例子

This virus infects mammals, often dogs,

這個病毒寄生於哺乳動物 狗是常見的例子

and travels up the animal's nerves to its brain

牠們由動物的神經進入大腦

where it causes inflammation that eventually kills the host.

最後導致發炎並使宿主死亡

But before it does, it often increases its host's aggressiveness

但在那之前，狂犬病毒 往往會增加宿主的侵略性

and ramps up the production of rabies-transmitting saliva,

並增加宿主的口水產量

while making it hard to swallow.

同時使宿主難以下嚥

These factors make the host more likely to bite another animal

這些因素使宿主 更容易咬其他生物

and more likely to pass the virus on when it does.

更容易由口水傳染病毒

And now, meet Ophiocordyceps, also known as the zombie fungus.

再來看看蛇形草菌 (又稱冬蟲夏草)

Its host of choice is tropical ants that normally live in treetops.

牠的宿主是熱帶螞蟻 通常生活在樹梢上

After Ophiocordyceps spores pierce the ant's exoskeleton,

冬蟲夏草的的孢子 刺破螞蟻的外骨骼後

they set off convulsions that make the ant fall from the tree.

會引起痙攣使樹上的螞蟻下跌

The fungus changes the ant's behavior, compelling it to wander mindlessly

這種真菌改變了螞蟻的行為 使牠盲目地徘徊

until it stumbles onto a plant leaf with the perfect fungal breeding conditions,

直到螞蟻走到一片 擁有完美的真菌繁殖的葉子

which it latches onto.

並緊緊抓住

The ant then dies,

然後螞蟻死亡

and the fungus parasitizes its body to build a tall, thin stalk from its neck.

真菌從牠的身體 建立一個高高的莖

Within several weeks, the stalk shoots off spores,

幾星期內牠會散播孢子

which turn more ants into six-legged leaf-seeking zombies.

讓其他螞蟻也變成 尋找葉子的殭屍

One of humanity's most deadly assailants is a behavior-altering parasite,

人類最致命的殺手之一 也是一個改變宿主行為的寄生蟲

though if it's any consolation,

稍微令人安慰的是

it's not our brains that are being hijacked.

牠不會直接挾持我們的大腦

I'm talking about Plasmodium, which causes malaria.

我說的是瘧原蟲，牠會導致瘧疾

This parasite needs mosquitoes to shuttle it between hosts,

這種寄生蟲需要蚊子的幫助

so it makes them bite more frequently and for longer.

所以牠會使蚊子 咬得更頻繁、更久

There's also evidence that humans infected with malaria

也有證據顯示 已染有瘧疾的人

are more attractive to mosquitoes,

會更加吸引蚊子

which will bite them and transfer the parasite further.

這將進一步傳送寄生蟲

This multi-species system is so effective,

這種方式非常有效

that there are hundreds of millions of malaria cases every year.

每年都有無數瘧疾的病例

And finally, there are cats.

最後還有貓

Don't worry, there probably aren't any cats living in your body

不用擔心，不是說有貓住在你的身體裡

and controlling your thoughts.

並控制你的想法

I mean, probably.

我的意思是........可能

But there is a microorganism called Toxoplasma

有一種寄生蟲叫弓形蟲

that needs both cats and rodents to complete its life cycle.

牠需要貓和老鼠來繁殖

When a rat gets infected by eating cat feces,

當老鼠吃貓屎被感染

the parasite changes chemical levels in the rat's brain,

這種寄生蟲會 改變鼠腦的化學平衡

making it less cautious around the hungry felines,

使牠們對於飢餓的貓 更不知危險

maybe even attracted to them.

甚至使老鼠想接近貓

This makes them easy prey,

這個輕鬆取得的獵物

so these infected rodents get eaten and pass the parasite on.

使感染了寄生蟲的老鼠被吃掉 並繼續傳染至下一位宿主

Mind control successful.

洗腦成功

There's even evidence that the parasite affects human behavior.

甚至有證據顯示 寄生蟲也會影響人的行為

In most cases, we don't completely understand

在大多數情況下 我們並不完全理解

how these parasites manage their feats of behavior modification.

這些寄生蟲是如何做到的

But from what we do know,

但是我們知道

we can tell that they have a pretty diverse toolbox.

牠們有多元的工具

Gordian worms seem to affect crickets' brains directly.

蠕蟲似乎直接影響蟋蟀的大腦

The malaria parasite, on the other hand,

瘧原蟲則是

blocks an enzyme that helps the mosquitoes feed,

阻擋有助於蚊子吸血的酵素

forcing them to bite over and over and over again.

使牠們一直咬，不停地咬

The rabies virus may cause that snarling, slobbering behavior

狂犬病毒會導致動物 狂吠及不停地流口水

by putting the immune system into overdrive.

是取決於牠使免疫系統癱瘓

But whatever the method,

但是無論何種方法

when you think about how effectively

在你思考這些寄生蟲

these parasites control the behavior of their hosts,

多麼有效地控制宿主行為時

you may wonder how much of human behavior is actually parasites doing the talking.

你可能會懷疑不知多少人的行為 都是寄生蟲在說話

Since more than half of the species on Earth are parasites,

地球上一半以上的物種 可都是寄生蟲

it could be more than we think.

寄生蟲控制人的行為 可能超乎我們想像