to manipulate an animal many times its size?

可以操控大於自己好幾倍的生物的能力？

The answer is all of them.

答案是全部都有這種能力

These are all parasites,

這些生物都是寄生蟲

organisms that live on or inside another host organism,

牠們寄生在另一個生物體身上或是體內

which they harm and sometimes even kill.

傷害宿主甚至使宿主死亡

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

寄生蟲有時會透過中間物種，從一個宿主傳送到另一個宿主

sometimes through an intermediate species.

而得以存活下來

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

寄生蟲藉由操控宿主的行為優雅地達成這一點

sometimes through direct brain hijacking.

有時會直接劫持大腦

For example, this is the Gordian worm.

舉個例子，這是隻鐵線蟲

One of its hosts, this cricket.

牠會寄生的其中一個宿主是蟋蟀

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

鐵線蟲需要水來進行交配，但是蟋蟀喜歡乾燥的陸地

So once it's big enough to reproduce,

一旦鐵線蟲到了繁殖時期

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

牠會產生一種蛋白質，以竄改蟋蟀的導航系統

The confused cricket jumps around erratically,

受到干擾的蟋蟀就會到處亂跳

moves closer to water,

接近水源

and eventually leaps in, often drowning in the process.

最後跳入水中，通常都會溺斃

The worm then wriggles out to mate

鐵線蟲接著從蟋蟀身體鑽出以在水中繁殖

and its eggs get eaten by little water insects

牠的卵被水中小型昆蟲吞噬

that mature,

接著逐漸成長

colonize land,

登陸

and are, in turn, eaten by new crickets.

接著再被別的蟋蟀獵捕

And thus, the Gordian worm lives on.

如此一來，鐵線蟲就得以一直生存下去

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

這是狂犬病病毒，另一種能改變宿主心智的寄生蟲

This virus infects mammals, often dogs,

這種病毒會感染哺乳類，通常是狗

and travels up the animal's nerves to its brain

牠們經由動物的神經進入大腦

where it causes inflammation that eventually kills the host.

使其發炎，最後使宿主死亡

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

但在這之前，狂犬病毒往往會增加宿主的攻擊性

and ramps up the production of rabies-transmitting saliva,

並使宿主大量產生會傳遞狂犬病的唾液

while making it hard to swallow.

增加唾液的同時也讓宿主難以吞嚥

These factors make the host more likely to bite another animal

綜合以上因素讓宿主更有可能攻擊其他動物

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

與此同時也更容易傳播狂犬病

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

現在來看看偏側蛇蟲草菌，又稱為殭屍真菌

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

牠所選擇的宿主是熱帶螞蟻，通常住在樹梢上

After Ophiocordyceps' spores pierce the ant's exoskeleton,

偏側蛇蟲草菌的孢子刺穿螞蟻的外骨骼後

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

會使螞蟻抽搐並從樹上跌落

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

這種真菌會改變螞蟻的行為舉止，迫使螞蟻盲目地流浪

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

直到螞蟻發現非常適合真菌繁殖的葉子

which it latches onto.

螞蟻就會緊緊抓住葉子

The ant then dies,

接著螞蟻死去

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

真菌會從螞蟻的脖子長出一根細長的莖

Within several weeks, the stalk shoots off spores,

不超過數周的時間，這根莖會散播孢子

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

孢子就會將更多的螞蟻變成尋找葉子的六腳殭屍

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

其中一個對人類最具殺傷力的生物，是一種可以改變宿主行為的寄生蟲

though if it's any consolation,

令人安慰的是

it's not our brains that are being hijacked.

牠控制的不是我們的大腦

I'm talking about Plasmodium, which causes malaria.

我現在說的是瘧原蟲，是造成瘧疾的寄生蟲

This parasite needs mosquitoes to shuttle it between hosts,

這種寄生蟲需要蚊子來讓牠穿梭於宿主之間

so it makes them bite more frequently and for longer.

牠會使蚊子叮咬得更頻繁、更持久

There's also evidence that humans infected with malaria

有證據顯示，感染瘧疾的人類

are more attractive to mosquitoes,

對於蚊子來說更有吸引力

which will bite them and transfer the parasite further.

使得蚊子叮咬他們，並進一步傳播瘧原蟲

This multi-species system is so effective,

這種多物種間傳遞的方法相當有效

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

每年都有數以百萬計的瘧疾病例

And finally, there are cats.

最後一個例子，貓

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

別擔心，應該不會有貓住在你的身體裡

and controlling your thoughts.

或是控制你的思想

I mean, probably.

我的意思是，有可能

But there is a microorganism called Toxoplasma

但有一種稱作弓蟲的微生物

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

需要貓和老鼠來完成牠的生命週期

When a rat gets infected by eating cat feces,

當老鼠吃了貓的排泄物而遭到感染

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

弓蟲會改變老鼠大腦裡的化學物質濃度

making it less cautious around the hungry felines,

讓老鼠對飢餓的貓變得較不警戒

maybe even attracted to them.

甚至使老鼠接近貓

This makes them easy prey,

這就讓老鼠輕而易舉地變成了獵物

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

遭感染的老鼠被吃掉後，寄生蟲就繼續傳染到下個宿主

Mind control successful.

洗腦成功

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

甚至有證據顯示，這種寄生蟲能影響人類行為

In most cases, we don't completely understand

在大多數的案例中，我們並沒有完全了解

how these parasites manage their feats of behavior modification.

這些寄生蟲是如何改變生物的行為舉止

But from what we do know,

但根據我們所知道的

we can tell that they have a pretty diverse toolbox.

我們可以發現寄生蟲有很多樣的工具

Gordian worms seem to affect crickets' brains directly.

鐵線蟲似乎能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,

阻擋了有助於蚊子吸血的酵素

forcing them to bite over and over and over again.

迫使蚊子一而再，再而三的叮咬

The rabies virus may cause that snarling, slobbering behavior

狂犬病病毒藉由讓免疫系統過度反應

by putting the immune system into overdrive.

而使得宿主吠叫和流口水

But whatever the method,

不管方法為何

when you think about how effectively

一想到這些寄生蟲

these parasites control the behavior of their hosts,

是多麼有效地控制著他們的宿主

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

你可能會想著，不知道多少人類的行為，都實際上是寄生蟲在控制

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

由於地球上一半以上的物種都是寄生蟲

it could be more than we think.

牠們所能做的可能超乎我們想像