California mice can feed on poisonous monarch butterflies

New research led by a team of biologists from the University of Utah has found that California mice eat monarch butterflies that fall to the ground. This is despite the fact that these butterflies possess a potent “chemical armor” from eating milkweed with toxic cardenolides. The milkweed toxins build up in their bodies and make monarchs unpalatable to most (but not all) predators.

Scientists have long observed that in Central Mexico, the place with the largest winter aggregations of monarchs, the black-eared mouse (Peromyscus melanotis) eats up to 40 of these butterflies each night. More recently, researchers found that the western harvest mouse (Reithrodontomys megalotis) also consume grounded monarchs at the Pismo State Beach Monarch Butterfly Grove in California.

To study mice-monarch interactions, the researchers first trapped rodents in the grove in February 2020 and screened their feces for monarch DNA, which they found in one sample. The scientists intended to return the next year, but unfortunately, after years of gradual decline, the monarch population finally crashed.

“At a site where 100,000 butterflies used to roost, in 2020 there where were fewer than 200 monarchs. So, we had to change tactics,” said study lead author Sara Weinstein, a postdoctoral researcher at the University of Utah. “We tested whether rodents would feed on the butterflies using captive-reared monarchs.”

Weinstein and her colleagues found that rodents that were offered monarchs ate them, usually favoring high-calorie parts with fewer toxins such as the abdomen or thorax. “Many rodent species are likely to have some resistance to cardenolides in monarchs, due to genetic changes at the site where these toxins bind,” explained Dr. 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 massive decline of monarch populations is only one example of an ongoing “insect apocalypse.” According to Dr. Weinstein, about 40 percent of invertebrate species are currently threatened and over 70 percent of flying insect biomass may already be gone. “This is devastating on its own and is also going to have enormous impacts on the other organisms that feed on insects,” Dr. Weinstein said.

“Western monarchs and other western butterflies need conservation attention and part of that awareness-raising is illuminating the many ways these animals are interconnected to other insects, birds, mammals, as well as our human communities. This study helps us appreciate more deeply how fewer butterflies means less food for other native animals,” added Emma Pelton, a senior conservation biologist at the Xerces Society for Invertebrate Conservation. 

The study is published in the journal Ecology.

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By Andrei Ionescu, Earth.com Staff Writer