“California Mice” It Dine on Toxin-Filled Monarch Butterflies Without Getting Poisoned

Not every animal can prey on monarch butterflies. Monarchs eat milkweed that is filled with toxins, so many predators can’t dine on these poisonous insects.

But some creatures, like mice, are able to easily eat the toxic butterflies. The black-eared mouse (Peromyscus melanotis) has been known to eat monarchs that fall to the ground in Mexico.

Recently, researchers observed that the western harvest mouse (Reithrodontomys megalotis) also dines on the insects at overwintering sites in California. But because the butterfly population is threatened, so is the mice butterfly buffet.

The study was led by biologists from the University of Utah.

“Our research group studies how animals feed on toxic diets and as part of that work, I have been studying how giant poisonous African rats use sequestered cardenolides for defense,” Sara Weinstein, the postdoctoral researcher who led the study, tells Treehugger.

We knew that mice in Mexico fed on cardenolide defended monarchs, and set up this project to see if the same behavior occurred at California monarch aggregations.”

With insect populations declining, it’s important to document feeding behaviors, the researchers say.

“We are in an insect apocalypse right now. There are estimates that 40% of studied invertebrate species are threatened and that over 70% of flying insect biomass is already gone,” Weinstein says.

“This is devastating on its own and is also going to have enormous impacts on the other organisms that feed on insects.”

Originally, the researchers trapped mice at Pismo State Beach Monarch Butterfly Grove, then released them after getting samples of their feces. They screened the samples for monarch DNA, which they found in one sample.

This first survey happened in February 2020, in late winter as monarchs were starting to leave, so there weren’t many insects there for the mice to eat.

Researchers planned to return in the fall during peak monarch season, but the population crashed later that year after years of decline.

In the past, 100,000 butterflies used to roost there, but in 2020, there were fewer than 200 monarchs.

“When the monarch populations crashed in the fall of 2020 we changed tactics,” Weinstein says. “To test whether wild mice fed on monarchs, we placed dried, lab-reared butterflies in the monarch grove and monitored them using motion-activated cameras.”

She placed the monarch bodies near camera traps and recorded wild harvest mice eating the butterflies. She also caught six mice and offered them monarchs to eat.

The mice usually preferred the abdomen and thorax, which are high in calories but with fewer toxins.

“Many rodent species are likely to have some resistance to cardenolides in monarchs, due to genetic changes at the site where these toxins bind,” says Weinstein.

“The Pismo Grove is one of hundreds of western monarch aggregation sites, and it seems likely that, at least in the past, rodents throughout the western monarch range may have supplemented their winter diets with monarchs. If you can handle the cardenolides in a monarch, their bodies are full of fat and offer a pretty good meal.”

Ref: science.utah, animalia.bio, heartspm, faculty, treehugger, wikipedia
Pic: treehugger, wikipedia, phys.org, Anurag Agrawal, beproactivepestcontrol, science.utah, journeynorth, heartspm, esajournals.onlinelibrary.wiley, eurekalert, ucanr, coasttocactus, pinterest, inaturalist, faculty

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