A deadly meal in the Amazon
Ten snakes faced a harsh situation. Researchers collected them in the Colombian Amazon and kept them several days without food. Then the animals received highly unappealing prey. The scientists offered them three-striped poison dart frogs. These frogs carry toxins like histrionicotoxins and pumiliotoxins. Such substances disrupt vital cell proteins.
Six royal ground snakes refused the toxic meal. Four moved in and attacked. Yet they did not swallow the frogs immediately. They dragged them across the ground first. Biologist Valeria Ramírez Castañeda recognised a behaviour similar to birds that rub poison off their prey. Three of the four snakes survived. Their bodies handled the remaining toxins with surprising efficiency.
The quiet power of poison
Living creatures have used deadly molecules for hundreds of millions of years. Microbes used them first to weaken rivals or invade host cells. Later animals used them to hunt or defend themselves. Plants used toxins to deter herbivores. Many animals evolved ways to resist these substances. Some even store the toxins for their own protection.
Researchers now uncover these defensive strategies. They hope to develop better treatments for human poisonings. They also explore a force that has shaped entire ecological communities. Evolutionary biologist Rebecca Tarvin studies these systems. She helped supervise the snake experiment and described such strategies in a major scientific review from 2023.
“Only milligrams of one compound can shift interactions in an ecosystem,” Tarvin says.
How species become toxic
Animals gain toxins in several ways. Some create them internally. Toads produce cardiac glycosides. These molecules block the sodium–potassium pump. This pump manages cell volume, muscle activity and nerve signals.
Other animals rely on toxin-producing bacteria. Pufferfish belong in this group. Their flesh contains lethal tetrodotoxin. Many species gain toxins from their diet. Poison frogs eat toxic insects and mites. The snakes in the experiment consumed one of these frog species.
Species that become toxic must also avoid poisoning themselves. Their bodies evolve to resist their own chemicals. Animals that eat toxic prey or serve as prey also adapt. The best studied mechanism involves target proteins. These proteins change so the toxins cannot bind. Insects living on milkweed evolve resistant sodium–potassium pumps.
But this adaptation creates new problems, says molecular biologist Susanne Dobler. She studies the large milkweed bug. Strongly resistant pumps work less efficiently. This becomes dangerous in nerve cells where the pump is essential.
Dobler’s 2023 research revealed a solution. The bug produces three versions of the pump. The most effective one lies in the brain. Yet it is also the most sensitive to toxins. The insect must use other defences to protect that tissue. Dobler suspects ABCB transporters play a role. These proteins sit in cell membranes and expel harmful compounds. Some hawk moths use them to remove cardiac glycosides from nerve tissues. Milkweed bugs might use a similar trick.
Dobler also studies the gut membranes of many insects. She believes ABCB transporters may block toxins from entering the body. This could explain why the bright red onion beetle eats toxin-rich lily of the valley without harm. It simply excretes the compounds. The resulting faeces repel ants.
Snake livers as chemical shields
The royal ground snakes seem to rely on their livers. Cell experiments show that snake liver extracts protect against frog toxins. The team suspects specialised enzymes convert deadly molecules into harmless forms. Humans neutralise alcohol and nicotine with similar systems. Snake livers may also contain proteins that bind toxins and prevent them from attaching to their targets. Some poison frogs use such “toxin sponge” proteins in their blood.
California ground squirrels use a comparable method. Their blood contains proteins that block certain components of rattlesnake venom. Rattlesnakes carry similar proteins to protect themselves from accidental exposure to their own venom. Venom varies across regions. Evolutionary biologist Matthew Holding found evidence that ground squirrels fine-tune their defences to match local snakes.
Yet these defences remain imperfect. Rattlesnakes continually evolve new toxins. Even a rattlesnake will die if injected with enough of its own venom.
Avoiding poison first
Animals try to avoid toxins before relying on resistance. This explains why the snakes dragged their frogs across the ground. Some turtles show similar prudence. They eat only the belly skin and organs of toxic newts. They avoid the deadly back skin. Monarch caterpillars resist cardiac glycosides. Still, they cut the veins of milkweed leaves to drain toxic sap before feeding.
Using poison as a tool
Many species store toxins for their own use. The iridescent dogbane beetle collects cardiac glycosides from its host plant. It likely uses ABCB transporters to move these toxins onto its back. When disturbed, the beetle releases tiny droplets on its hardened wings. Predators avoid these secretions.
This process creates long-lasting ecological dependencies. The relationship between the monarch butterfly and the milkweed plant demonstrates this clearly.
A 2021 study found four animals that evolved tolerance to cardiac glycosides. This allows them to prey on monarchs. One is the black-headed grosbeak. It feeds on monarchs in the fir forests of Mexico where the butterflies spend the winter.
Evolutionary biologist Noah Whiteman explains the scale of this chemical journey. A molecule produced by a milkweed plant in Ontario can shape the biology of a bird thousands of miles away. “It is remarkable,” he says. “The path taken by this tiny compound and its influence on evolution.”
