Why don’t poisonous animals poison themselves? - Rebecca D. Tarvin

譯者: Lilian Chiu 審譯者: Helen Chang

One fine day,

這是美好的一天，

when Charles Darwin was still a student at Cambridge,

查爾斯達爾文還是 劍橋大學的學生，

the budding naturalist tore some old bark off a tree

這位新進的博物學家 從一棵樹上撕下了一些老樹皮，

and found two rare beetles underneath.

在下面發現了兩隻稀有的甲蟲。

He’d just taken one beetle in each hand when he spotted a third beetle.

他一隻手拿著一隻甲蟲， 同時看到了第三隻甲蟲。

Stashing one of the insects in his mouth for safekeeping,

他把其中一隻甲蟲 放到口中以妥善保存牠，

he reached for the new specimen –

接著伸手去抓這隻新樣本——

when a sudden spray of hot, bitter fluid scalded his tongue.

此時他突然感覺到炙熱的 苦液體噴到他的舌頭上。

Darwin’s assailant was the bombardier beetle.

攻擊達爾文的，是隻放屁蟲。

It’s one of thousands of animal species,

有數千種像這樣的動物，

like frogs,

比如青蛙、水母、

jellyfish,

salamanders,

蠑螈，和蛇，

and snakes,

that use toxic chemicals to defend themselves –

都會用有毒化學物來保衛自己——

in this case, by spewing poisonous liquid from glands in its abdomen.

在這個例子中， 是從腹部腺體噴出有毒液體。

But why doesn’t this caustic substance, ejected at 100 degrees Celsius,

但為什麼這種腐蝕性物質 以攝氏一百度的溫度噴出，

hurt the beetle itself?

卻不會傷到甲蟲自己？

In fact, how do any toxic animals survive their own secretions?

應該問，任何有毒動物是如何 不受自己的分泌物傷害？

The answer is that they use one of two basic strategies:

答案是，牠們會採用 兩種基本策略的其中一種：

securely storing these compounds

將這些化合物安全存放，

or evolving resistance to them.

或是演化出對抗它們的抗性。

Bombardier beetles use the first approach.

放屁蟲用的是第一種方法。

They store ingredients for their poison in two separate chambers.

牠們會把毒液的兩種成分 存放在兩個分開的腔室中，

When they’re threatened, the valve between the chambers opens

當牠們受到威脅時， 兩個腔室間的活門會打開，

and the substances combine in a violent chemical reaction

這些物質結合起來， 會發生劇烈的化學反應，

that sends a corrosive spray shooting out of the glands,

導致有腐蝕性的液體從腺體噴出，

passing through a hardened chamber that protects the beetle’s internal tissues.

噴出過程會通過保護甲蟲 自己內部組織的堅硬腔室。

Similarly, jellyfish package their venom safely

同樣的，水母也會用 類似魚叉的結構，

in harpoon-like structures called nematocysts.

叫做刺絲胞，把牠們的毒液 安全地包起來。

And venomous snakes store their flesh-eating, blood-clotting compounds

毒蛇把其噬肉且會造成 凝血的化合物儲存在

in specialized compartments that only have one exit:

一個特別的隔間中， 這隔間只有一個出口：

through the fangs and into their prey or predator.

透過毒牙，進入獵物 或獵食者的身體。

Snakes also employ the second strategy: built-in biochemical resistance.

蛇也會採用第二種策略： 內建的生化抗性。

Rattlesnakes and other types of vipers manufacture special proteins

響尾蛇和其他類型的毒蛇 會製造特殊的蛋白質，

that bind and inactivate venom components in the blood.

能和血液中的毒液化合物 結合並解除毒性。

Meanwhile, poison dart frogs have also evolved resistance to their own toxins,

箭毒蛙則是演化出了 對自身毒素的抗性，

but through a different mechanism.

但透過的機制是不同的。

These tiny animals defend themselves using hundreds of bitter-tasting compounds

這些小動物防衛自己的方式，

是使用數百種有苦味的 化合物，叫做生物鹼，

called alkaloids

that they accumulate from consuming small arthropods like mites and ants.

生物鹼的累積是來自牠們 吃進的小型節肢動物，

如蟎和螞蟻。

One of their most potent alkaloids is the chemical epibatidine,

牠們最強力的生物鹼之一 是化學地棘蛙素，

which binds to the same receptors in the brain as nicotine

它和尼古丁所影響到的 大腦接收器是同樣的，

but is at least ten times stronger.

但強度至少強上十倍。

An amount barely heavier than a grain of sugar would kill you.

只要比一小顆糖還要重的量， 就足以致你於死地。

So what prevents poison frogs from poisoning themselves?

所以，是什麼讓毒蛙 不會毒死自己的？

Think of the molecular target of a neurotoxic alkaloid as a lock,

把神經毒生物鹼的 分子標靶想成是一個鎖，

and the alkaloid itself as the key.

生物鹼本身是鑰匙。

When the toxic key slides into the lock,

當有毒的鑰匙插入鎖中時，

it sets off a cascade of chemical and electrical signals

它就會啟動一連串 化學和電子訊號，

that can cause paralysis,

可能會造成癱瘓、

unconsciousness,

失去意識，

and eventually death.

最終死亡。

But if you change the shape of the lock, the key can’t fit.

但如果你改變了鎖的形狀， 鑰匙就無法吻合。

For poison dart frogs and many other animals with neurotoxic defenses,

對毒箭蛙和許多其他 用神經毒來防衛的動物來說，

a few genetic changes alter the structure of the alkaloid-binding site

一點點的基因改變， 就會轉變結合生物鹼之處的結構，

just enough to keep the neurotoxin from exerting its adverse effects.

這轉變剛好足讓神經毒素

不要發生有害的作用。

Poisonous and venomous animals

會發展出這種抗性的，

aren’t the only ones that can develop this resistance:

不只是有毒和分泌毒液的動物：

their predators and prey can, too.

捕食牠們的動物和牠們的獵物也會。

The garter snake, which dines on neurotoxic salamanders,

襪帶蛇會吃有神經毒素的蠑螈，

has evolved resistance to salamander toxins

牠們就演化出對蠑螈毒素的抗性，

through some of the same genetic changes as the salamanders themselves.

演化過程的一些基因改變 是和蠑螈本身一樣的。

That means that only the most toxic salamanders can avoid being eaten—

那就意味著，只有最毒的 蠑螈才能避免被吃掉——

and only the most resistant snakes will survive the meal.

且只有抗性最強的蛇 才能在飽餐之後活下來。

The result is that the genes providing the highest resistance and toxicity

這樣的結果就是， 傳給後代最多的基因，

will be passed on in greatest quantities to the next generations.

會是能提供最強抗性 和毒性的基因。

As toxicity ramps up, resistance does too,

隨著毒素增強，抗性也會增強，

in an evolutionary arms race that plays out over millions of years.

這場演化的軍備競賽 已經持續了數百萬年。

This pattern appears over and over again.

這種模式一而再再而三地出現。

Grasshopper mice resist painful venom from scorpion prey

透過神經系統中的基因改變，

草蜢小鼠能抵抗 來自蠍子的痛苦毒液。

through genetic changes in their nervous systems.

Horned lizards readily consume harvester ants,

角蜥能很輕易就吃掉火蟻，

resisting their envenomed sting with specialized blood plasma.

靠著特殊的血漿， 抵抗牠們有毒的叮螫。

And sea slugs eat jellyfish nematocysts,

海蛞蝓會吃水母刺絲胞，

prevent their activation with compounds in their mucus,

預防其黏液中的化合物被活化，

and repurpose them for their own defenses.

並將之轉換為牠們 自己的防衛用途。

The bombardier beetle is no exception:

放屁蟲也不例外：

the toads that swallow them

蟾蜍能吞下牠們， 容忍其腐蝕性的噴液，

can tolerate the caustic spray that Darwin found so distasteful.

也就是讓達爾文非常反感的噴液。

Most of the beetles are spit up hours later,

大部分的甲蟲在 幾小時之後會被吐出，

amazingly alive and well.

很驚人的是，牠們還是活得好好的。

But how do the toads survive the experience?

但蟾蜍在這樣的經歷之後 是如何活下來的？

That is still a mystery.

那仍然是個謎。