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

With monarch populations dwindling, that affects the insect buffet.

Western harvest mouse feeds on a monarch butterfly
Western harvest mouse feeds on a monarch butterfly.

Sara B. Weinstein

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 started thinking about the mice in California because we were looking for a system closer to home where we could also study how animals naturally deal with these types of toxins. 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.”

Monarchs on the Menu

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.”

The results were published in the journal Ecology.

A Domino Effect

Researchers don’t believe that monarch-eating mice are contributing to the butterflies’ population drop.

“We don't think that mice are responsible for monarch declines,” Weinstein says. “Unfortunately, the western monarch populations have been declining for decades, probably due to many factors, including the loss of overwintering and breeding habitat and increased use of pesticides and herbicides."

But they are concerned that the drop in monarch populations is having a domino effect on other species.

“A reduction in monarch populations, and insects in general, will likely have far reaching consequences,” Weinstein says. “For example, impacting both the plants they pollinate and the predators that feed on them.”

Read More:

View Article Sources
  1. Weinstein, Sara B., and M. Denise Dearing. "Harvest Mice ( Reithrodontomys Megalotis) Consume Monarch Butterflies ( Danaus Plexippus )." Ecology, 2021, doi:10.1002/ecy.3607

  2. Sara Weinstein, the postdoctoral researcher at University of Utah who led the study

  3. Pelton, Emma M., et al. "Western Monarch Population Plummets: Status, Probable Causes, and Recommended Conservation Actions." Frontiers in Ecology and Evolution, vol. 7, 2019, doi:10.3389/fevo.2019.00258