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One fine day, when Charles Darwin was still a student at Cambridge,
在晴朗的某天，查尔斯·达尔文此时仍是一名剑桥大学的学生，
the budding naturalist tore some old bark off a tree and found two rare beetles underneath.
这位崭露头角的博物学家从某棵树上剥下一些树皮，并在树皮下发现了两只稀有的甲虫。
He'd just taken one beetle in each hand when he spotted a third beetle.
他双手各抓一只，此时，他发现了第三只甲虫。
Stashing one of the insects in his mouth for safekeeping, he reached for the new specimen
于是他把其中一只甲虫放在嘴里暂时保存，便伸手去抓第三只，
when a sudden spray of hot, bitter fluid scalded his tongue.
就在此时，他的舌头突然感受到一股滚烫苦涩的液体。
Darwin's assailant was the bombardier beetle.
袭击达尔文的正是放屁虫。
It's one of thousands of animal species, like frogs, jellyfish, salamanders, and snakes,
和放屁虫一样，上千种其他物种例如蛙类、水母、蝾螈、毒蛇，
that use toxic chemicals to defend themselves -- in this case, by spewing poisonous liquid from glands in its abdomen.
都会通过有毒物质来保卫自己，在达尔文这个例子中，放屁虫通过腹部腺体喷出有毒液体。
But why doesn't this caustic substance, ejected at 100 degrees Celsius, hurt the beetle itself?
但是为何这种100摄氏度的腐蚀性物质并不会伤害放屁虫自己呢？
In fact, how do any toxic animals survive their own secretions?
事实上，有毒动物如何能够不受自己分泌的毒素伤害呢？
The answer is that they use one of two basic strategies: securely storing these compounds or evolving resistance to them.
有以下两种方法可以让它们免于中毒：小心地将这些有毒物质存储起来，或者是进化出不受毒害的体质。
Bombardier beetles use the first approach. They store ingredients for their poison in two separate chambers.
放屁虫使用的是第一种方法。它们将生成毒液的物质分别存储在两个腔室里。
When they're threatened, the valve between the chambers opens and the substances combine in a violent chemical reaction
当感受到威胁时，两个腔室之间的阀门便会打开，两种物质进行剧烈的化学反应，
that sends a corrosive spray shooting out of the glands, passing through a hardened chamber that protects the beetle's internal tissues.
生成一种具有腐蚀性的物质，并且从腺体喷射而出，而硬化的腔室可以保护甲虫的内部组织。
Similarly, jellyfish package their venom safely in harpoon-like structures called nematocysts.
水母也采取类似的做法，它将毒液存在鱼叉状的“刺囊”细胞中。
And venomous snakes store their flesh-eating, blood-clotting compounds in specialized compartments that only have one exit:
毒蛇的毒液能腐蚀肌体，使血液凝结，这些毒液存储在特定部位，并且只有一个出口：
through the fangs and into their prey or predator.
那就是通过獠牙注入到猎物或天敌体内。
Snakes also employ the second strategy: built-in biochemical resistance.
蛇类也会使用第二种方法：利用自身的生化抵抗力。
Rattlesnakes and other types of vipers manufacture special proteins that bind and inactivate venom components in the blood.
响尾蛇以及其他种类的蝰蛇能产生特殊蛋白质，这种蛋白质能够结合血液中的有毒化合物，使其失活。
Meanwhile, poison dart frogs have also evolved resistance to their own toxins, but through a different mechanism.
箭毒蛙也进化出了对自身毒素的免疫性，但是其机制有所不同。
These tiny animals defend themselves using hundreds of bitter-tasting compounds called alkaloids
这些小生物使用上百种生物碱，一种苦味化合物，来保护自己。
that they accumulate from consuming small arthropods like mites and ants.
它们通过捕食螨虫、蚂蚁等节肢动物获得这些生物碱。
One of their most potent alkaloids is the chemical epibatidine,
其中一种最强大的生物碱是地棘蛙素，
which binds to the same receptors in the brain as nicotine but is at least ten times stronger.
它能够像尼古丁一样附着在大脑中的感受器上，但效果要比尼古丁强上十倍。
An amount barely heavier than a grain of sugar would kill you.
约0.065克的量就足以杀死一个人。
So what prevents poison frogs from poisoning themselves?
那么是什么让毒蛙自身免受毒害？
Think of the molecular target of a neurotoxic alkaloid as a lock, and the alkaloid itself as the key.
试着把神经毒素生物碱要攻击的分子目标想象成一把锁，生物碱则是钥匙。
When the toxic key slides into the lock, it sets off a cascade of chemical and electrical signals
当有毒的钥匙插入锁中，它会释放大量化学物质和电信号，
that can cause paralysis, unconsciousness, and eventually death.
这能够导致神经瘫痪，失去意识，最终死亡。
But if you change the shape of the lock, the key can't fit.
但如果你改变锁的形状，钥匙就失去了功能。
For poison dart frogs and many other animals with neurotoxic defenses,
对于箭毒蛙和其他能够抵御神经毒素的动物来说，
a few genetic changes alter the structure of the alkaloid-binding site just enough to keep the neurotoxin from exerting its adverse effects.
小的基因突变就能够改变生物碱结合的结构，这就足以保证神经毒素不会对自己产生不利影响。
Poisonous and venomous animals aren't the only ones that can develop this resistance: their predators and prey can, too.
并非只有有毒动物才能够建立起这种抗性：它们的天敌和猎物同样也能。
The garter snake, which dines on neurotoxic salamanders,
束带蛇捕食带有神经毒素的蝾螈，
has evolved resistance to salamander toxins through some of the same genetic changes as the salamanders themselves.
它已经进化出了能够抵抗蝾螈毒素的特性，这是通过进行与蝾螈相同的基因改变而实现的。
That means that only the most toxic salamanders can avoid being eaten -- and only the most resistant snakes will survive the meal.
这意味着只有最为剧毒的蝾螈能不被吃掉，只有最能抵抗毒素的蛇才能活下来。
The result is that the genes providing the highest resistance and toxicity will be passed on in greatest quantities to the next generations.
这样的结果就是基因提供了最强的抗性和毒性，而这些会最大程度地传递给后代。
As toxicity ramps up, resistance does too, in an evolutionary arms race that plays out over millions of years.
随着毒素不断升级，抗性也不断增强，在物种进化的军备竞赛中，这场战争上演了数百万年。
This pattern appears over and over again.
这种模式不断重复出现。
Grasshopper mice resist painful venom from scorpion prey through genetic changes in their nervous systems.
食蝗鼠通过改变神经系统的基因来抵御蝎子蜇伤时的毒液。
Horned lizards readily consume harvester ants, resisting their envenomed sting with specialized blood plasma.
角蜥蜴能够用自身的特殊血浆抵御住农田蚁有毒的叮咬。
And sea slugs eat jellyfish nematocysts, prevent their activation with compounds in their mucus, and repurpose them for their own defenses.
海蛞蝓能够吃掉水母的刺囊，使粘液中的化合物不被激活，并将其重新利用，保护自己。
The bombardier beetle is no exception: the toads that swallow them can tolerate the caustic spray that Darwin found so distasteful.
放屁虫也不例外：吞食掉它们的蟾蜍能够忍受令达尔文反感的腐蚀性液体。
Most of the beetles are spit up hours later, amazingly alive and well.
大多数甲虫都会在数个小时后被蟾蜍吐出来，奇迹般地毫发无伤。
But how do the toads survive the experience? That is still a mystery.
但蟾蜍如何能不受放屁虫毒液的影响？这仍然是一个谜。
