Why Don't Poisonous Animals
Poison Themselves?
One fine day, when Charles Darwin was still a student at Cambridge, the budding naturalist tore some old bark① off a tree and found two rare beetles underneath. He’d just taken one beetle in each hand when he spotted a third beetle.
① bark /bɑːrk/ n. the tough, protective outer covering of a tree 树皮
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. Darwin’s assailant was the bombardier② beetle. It’s one of thousands of animal species, like frogs, jellyfish, salamanders③, and snakes, that use toxic chemicals to defend themselves – in this case, by spewing poisonous liquid from glands ④ in its abdomen.
② bombardier /ˌbɒmbəˈdɪər/ n. a member of a combat aircraft crew responsible for aiming and releasing bombs 轰炸机机组成员
③ salamanders /ˈsæləˌmændərz/ n. small amphibians with slender bodies, long tails, and moist skin, typically found near water 蠑螈
④ gland /ɡlænd/ n. an organ or group of cells that produces and releases substances used by the body, such as hormones or sweat 腺体
But why doesn’t this caustic⑤ 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.
⑤ caustic /ˈkɔːstɪk/ adj. able to burn or corrode organic tissue by chemical action; sarcastic in a scathing and bitter way 腐蚀性的；尖刻的，刻薄的
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 nematocysts⑥. 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.
⑥ nematocysts /ˈnɛmətəˌsɪsts/ n. small, capsule-like structures within the cells of cnidarians (such as jellyfish and sea anemones) that contain venom and are used for defense or prey capture 刺细胞
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 alkaloids⑦ is the chemical epibatidine⑧, 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.
⑦ alkaloid /ˈælkəˌlɔɪd/ n. organic compounds found in plants, often having potent physiological effects on humans and animals 生物碱
⑧ epibatidine /ˌɛpɪˈbætɪdiːn/ n. a potent alkaloid derived from the skin of a tropical frog, known for its analgesic properties, used in scientific research for its action on nicotinic acetylcholine receptors 外蛙毒素
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.
Poisonous and venomous animals aren’t the only ones that can develop this resistance: their predators and prey can, too. The garter⑨ snake, which dines on neurotoxic salamanders, has evolved resistance to salamander toxins through some of the same genetic changes as the salamanders themselves.
⑨ garter /ˈɡɑːrtər/ n. a band or strap worn around the leg to hold up a stocking or sock; often used in weddings as a decorative garment worn by the bride 袜带
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.
This pattern appears over and over again. Grasshopper mice resist painful venom from scorpion⑩ 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.
⑩ scorpion /ˈskɔːrpiən/ n. a predatory arachnid with a venomous stinger at the end of its tail, known for its pincer-like front claws and segmented tail 蝎子
10个内容
「点击封面购买杂志」
购买或咨询可直接联系客服
长按图中二维码
识别后下滑到底部可找到客服微信
