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. 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. 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. 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. 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 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. 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. 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.
