One fine day, when Charles Darwin was still a student at Cambridge, the budding naturalist tore some old bark off a tree and found two rare beetles underneath. He’d just taken one beetle in each hand when he spotted a third beetle. Stashing one of the insects in his mouth for safekeeping, he reached for the new specimen – when a sudden spray of hot, bitter fluid scalded his tongue.
這是美好的一天, 查爾斯達爾文還是 劍橋大學的學生, 這位新進的博物學家 從一棵樹上撕下了一些老樹皮, 在下面發現了兩隻稀有的甲蟲。 他一隻手拿著一隻甲蟲, 同時看到了第三隻甲蟲。 他把其中一隻甲蟲 放到口中以妥善保存牠, 接著伸手去抓這隻新樣本—— 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. It’s one of thousands of animal species, like frogs, jellyfish, salamanders, and snakes, that use toxic chemicals to defend themselves – in this case, by spewing poisonous liquid from glands in its abdomen. But why doesn’t this caustic substance, ejected at 100 degrees Celsius, hurt the beetle itself? In fact, how do any toxic animals survive their own secretions? The answer is that they use one of two basic strategies: securely storing these compounds or evolving resistance to them.
攻擊達爾文的,是隻放屁蟲。 有數千種像這樣的動物, 比如青蛙、水母、 蠑螈,和蛇, 都會用有毒化學物來保衛自己—— 在這個例子中, 是從腹部腺體噴出有毒液體。 但為什麼這種腐蝕性物質 以攝氏一百度的溫度噴出, 卻不會傷到甲蟲自己? 應該問,任何有毒動物是如何 不受自己的分泌物傷害? 答案是,牠們會採用 兩種基本策略的其中一種: 將這些化合物安全存放, 或是演化出對抗它們的抗性。
Bombardier beetles use the first approach. They store ingredients for their poison in two separate chambers. When they’re threatened, the valve between the chambers opens and the substances combine in a violent chemical reaction that sends a corrosive spray shooting out of the glands, passing through a hardened chamber that protects the beetle’s internal tissues. Similarly, jellyfish package their venom safely in harpoon-like structures called nematocysts. And venomous snakes store their flesh-eating, blood-clotting compounds in specialized compartments that only have one exit: through the fangs and into their prey or predator.
放屁蟲用的是第一種方法。 牠們會把毒液的兩種成分 存放在兩個分開的腔室中, 當牠們受到威脅時, 兩個腔室間的活門會打開, 這些物質結合起來, 會發生劇烈的化學反應, 導致有腐蝕性的液體從腺體噴出, 噴出過程會通過保護甲蟲 自己內部組織的堅硬腔室。 同樣的,水母也會用 類似魚叉的結構, 叫做刺絲胞,把牠們的毒液 安全地包起來。 毒蛇把其噬肉且會造成 凝血的化合物儲存在 一個特別的隔間中, 這隔間只有一個出口: 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. Rattlesnakes and other types of vipers manufacture special proteins that bind and inactivate venom components in the blood. Meanwhile, poison dart frogs have also evolved resistance to their own toxins, but through a different mechanism. These tiny animals defend themselves using hundreds of bitter-tasting compounds called alkaloids that they accumulate from consuming small arthropods like mites and ants. One of their most potent alkaloids is the chemical epibatidine, which binds to the same receptors in the brain as nicotine but is at least ten times stronger. An amount barely heavier than a grain of sugar would kill you.
蛇也會採用第二種策略: 內建的生化抗性。 響尾蛇和其他類型的毒蛇 會製造特殊的蛋白質, 能和血液中的毒液化合物 結合並解除毒性。 箭毒蛙則是演化出了 對自身毒素的抗性, 但透過的機制是不同的。 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, 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。 牠們最強力的生物鹼之一 是化學地棘蛙素, 它和尼古丁所影響到的 大腦接收器是同樣的, 但強度至少強上十倍。 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? Think of the molecular target of a neurotoxic alkaloid as a lock, and the alkaloid itself as the key. When the toxic key slides into the lock, it sets off a cascade of chemical and electrical signals that can cause paralysis, unconsciousness, and eventually death. But if you change the shape of the lock, the key can’t fit. For poison dart frogs and many other animals with neurotoxic defenses, a few genetic changes alter the structure of the alkaloid-binding site just enough to keep the neurotoxin from exerting its adverse effects.
所以,是什麼讓毒蛙 不會毒死自己的? 把神經毒生物鹼的 分子標靶想成是一個鎖, 生物鹼本身是鑰匙。 當有毒的鑰匙插入鎖中時, 它就會啟動一連串 化學和電子訊號, 可能會造成癱瘓、 失去意識, 最終死亡。 但如果你改變了鎖的形狀, 鑰匙就無法吻合。 對毒箭蛙和許多其他 用神經毒來防衛的動物來說, 一點點的基因改變, 就會轉變結合生物鹼之處的結構, 這轉變剛好足讓神經毒素 不要發生有害的作用。
Poisonous and venomous animals aren’t the only ones that can develop this resistance: their predators and prey can, too. The garter snake, which dines on neurotoxic salamanders, has evolved resistance to salamander toxins through some of the same genetic changes as the salamanders themselves. That means that only the most toxic salamanders can avoid being eaten— and only the most resistant snakes will survive the meal. The result is that the genes providing the highest resistance and toxicity will be passed on in greatest quantities to the next generations. As toxicity ramps up, resistance does too, in an evolutionary arms race that plays out over millions of years.
會發展出這種抗性的, 不只是有毒和分泌毒液的動物: 捕食牠們的動物和牠們的獵物也會。 襪帶蛇會吃有神經毒素的蠑螈, 牠們就演化出對蠑螈毒素的抗性, 演化過程的一些基因改變 是和蠑螈本身一樣的。 那就意味著,只有最毒的 蠑螈才能避免被吃掉—— 且只有抗性最強的蛇 才能在飽餐之後活下來。 這樣的結果就是, 傳給後代最多的基因, 會是能提供最強抗性 和毒性的基因。 隨著毒素增強,抗性也會增強, 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. Grasshopper mice resist painful venom from scorpion prey through genetic changes in their nervous systems. Horned lizards readily consume harvester ants, resisting their envenomed sting with specialized blood plasma. And sea slugs eat jellyfish nematocysts, prevent their activation with compounds in their mucus, and repurpose them for their own defenses.
這種模式一而再再而三地出現。 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。 角蜥能很輕易就吃掉火蟻, 靠著特殊的血漿, 抵抗牠們有毒的叮螫。 海蛞蝓會吃水母刺絲胞, 預防其黏液中的化合物被活化, 並將之轉換為牠們 自己的防衛用途。
The bombardier beetle is no exception: the toads that swallow them can tolerate the caustic spray that Darwin found so distasteful. Most of the beetles are spit up hours later, amazingly alive and well. But how do the toads survive the experience? That is still a mystery.
放屁蟲也不例外: 蟾蜍能吞下牠們, 容忍其腐蝕性的噴液, 也就是讓達爾文非常反感的噴液。 大部分的甲蟲在 幾小時之後會被吐出, 很驚人的是,牠們還是活得好好的。 但蟾蜍在這樣的經歷之後 是如何活下來的? 那仍然是個謎。