How Can Predators Eat Poison Butterflies?

Monarch butterflies are filled with poisonous milkweed toxins yet some animals are still able to eat them easily. Researchers recently uncovered how certain predators are able to safely dine on these poisonous insects.

In high concentrations, milkweed is highly toxic and can kill sheep, cattle, and horses. Monarchs have evolved certain mutations in their cells so that they can eat the plant. Now, researchers have found that some of the butterfly’s predators have adapted in the same way.

They found similar mutations in four types of monarch predators: a mouse, a worm, a bird, and a parasitic wasp.

“It’s remarkable that concurrent evolution occurred at the molecular level in all these animals,” said study lead Simon “Niels” Groen, an evolutionary biologist at the University of California, Riverside. “Plant toxins caused evolutionary changes across at least three levels of the food chain!”

A decade ago, Groen and his colleagues discovered changes in the DNA that is the blueprint for the main part of the sodium pump in the monarch and other insects that dine on milkweed. The sodium pump is critical for important body processes like nerve firing and heartbeats. When most animals eat milkweed, the pump stops functioning.

They found DNA changes in three spots on the pump that allowed monarchs to not only eat milkweed but also accumulate the milkweed toxins—called cardiac glycosides—in their bodies. Having the stored toxin helps protect them from predator attacks.

Groen and his team introduced the same changes in fruit flies using gene-editing technology and found that they became just as invulnerable to milkweeds as monarchs.

"Monarch butterflies even evolved the capacity to store plant-derived cardiac glycosides in their own bodies so that they become toxic to many animals that might attack the butterflies. Cardiac glycoside sequestration could thus protect monarch butterflies from attack by predators and parasites,” Groen says.

“However, there are several animals such as the black-headed grosbeak that can successfully feed on monarch butterflies. We wondered if these predators and parasites of monarchs could also have evolved changes in their sodium pumps that might confer a level of insensitivity to the plant-derived cardiac glycosides stored in the butterflies' bodies.”

For their study, the researchers studied DNA sequence information for many birds, wasps, and worms that are monarch predators. They looked to see if any had evolved the same changes in their sodium pumps that would allow them to survive milkweed toxins. One of the animals that had the adaptation was the black-headed grosbeak, which eats up to 60% of monarchs in many colonies each year.

The results were published in the journal Current Biology.

Milkweed Poison

Milkweed toxins contain cardenolides (cardiac glycosides). In very low doses, they are used as heart medicines.

“Starting at even just slightly higher doses, however, cardiac glycosides become very toxic to animals and quickly become lethal,” Groen explains. “When animals ingest too much of these toxins their heart may start beating irregularly or stop, their muscles stop working properly, and their brains slow down. Throwing up before too much toxin reaches the blood can save animals from the worst effects.”

Researchers believe the results can help with education as well as conservation plans.

“Our study's findings teach us about how evolution may work, in particular when animals are confronted with toxic chemicals in their environments or diets. In addition to the natural toxins made by plants that plant-feeding animals or their predators and parasites might ingest, this scenario also occurs in the case of man-made pesticides that animals might encounter,” Groen says.

“Understanding likely evolutionary trajectories might help us with plans for conserving biodiversity in nature and managing pests in agricultural settings.”