Newly discovered mouse-butterfly interaction suggests the decline of the western monarch could disrupt ecosystems in unforeseen ways – sciencedaily


Monarch butterflies possess powerful chemical armor. As caterpillars, they eat plants filled with poisonous cardenolides that build up in their bodies and make them unpleasant to most predators, but not all. In central Mexico, where the largest winter gatherings of monarchs occur, scientists have observed that rodents attack monarchs that fall to the ground. In particular, the black-eared mouse (Peromyscus melanotis) specializes in these bitter tasting insects, which eat up to 40 a night.

In a new study, biologists at the University of Utah have found that mice at California monarch wintering sites can also consume monarch butterflies. Working at one of the largest gatherings of monarchs outside of Mexico, Pismo State Beach Monarch Butterfly Grove, researchers found that the harvest mouse (Reithrodontomys megalotis) also ate the ground monarchs. However, with the precipitous decline of western monarch populations, this butterfly buffet could be threatened.

The authors do not believe that rodents are contributing to the decline of the western monarch, nor that monarchs are the only thing mice can eat. Rather, documenting this new foraging behavior is a reminder of how little we know about interactions that can be lost as insect populations decline.

“We are currently in an insect apocalypse. It is estimated that 40% of the invertebrate species studied are threatened and that more than 70% of the biomass of flying insects has already disappeared. This is devastating in itself and will also have huge impacts on other organisms that feed on insects, ”said Sara Weinstein, the postdoctoral researcher who led the study.

“Western monarchs and other Western butterflies need special conservation attention and part of this awareness highlights the many ways these animals are interconnected with other insects, birds, mammals, as well as with our human communities. This study helps us appreciate more deeply how much less butterflies mean less food for other native animals, ”said Emma Pelton, senior conservation biologist at the Xerces Society.

The study published in the journal Ecology December 12, 2021.

To study mouse-monarch interactions, the researchers first trapped rodents in the grove in February 2020. The rodents were released, but their droppings were saved to search for monarch DNA – which they found in a sample. This first survey took place at the end of winter, as the monarchs left the aggregation and there were few mice left to munch on. Weinstein and his colleagues planned to return the following fall during peak monarch season. However, after years of decline, the western monarch population collapsed.

“At a site where 100,000 butterflies roosted, in 2020 there were less than 200 monarchs. So we had to change tactics,” Weinstein said. “We tested whether rodents fed on butterflies using captive-bred monarchs.”

Weinstein set up laboratory-reared monarch carcasses under camera traps and captured images of wild mouse-eating butterflies. She also caught half a dozen mice and gifted them with monarchs. The mice ate monarchs, usually favoring the abdomen or thorax, parts high in calories and containing 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,” Weinstein said. “Pismo Grove is one of hundreds of Western Monarch aggregation sites, and it seems likely that, at least in the past, rodents across the Western Monarch’s range may have supplemented their winter diet with monarchs. If you can handle the cardenolides in a monarch, their bodies are full of fat and provide a pretty good meal. “

This meal will be much harder to find, as over 90% of Western monarchs have gone extinct in the past 40 years. The missing beauties will surely have an impact on the ecosystem that depends on them for food.

“When the number of monarchs collapsed last year and the project started to go awry, Sara smartly saved it by growing her own monarchs and obtaining frozen monarchs from researchers,” said Denise Dearing, lead author of the study and Emeritus Professor of Biology at U. “His creative solution has advanced our understanding of interactions between mice and monarchs, even during a period of limited monarch numbers. It lays the groundwork for future research in this area.”

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Materials provided by University of Utah. Original written by Lisa Potter. Note: Content can be changed for style and length.


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