University of Michigan graduate student Lisa Walsh holding a 2-month-old opossum named Joey. Image credit: Beth Lenoble

The headline reads like something from the satirical newspaper The Onion: "Grand Forks opossum slain; body to go to University of Michigan for research."

But for U-M EEB graduate student Lisa Walsh, this was no laughing matter.

Walsh's doctoral dissertation is focused on the ongoing northward range expansion of the common Virginia opossum, and she'd been expecting the nomadic marsupials to show up in North Dakota's Red River Valley eventually.

This individual opossum was first spotted in a suburban Grand Forks neighborhood in January 2017, eating sunflower seeds at the base of a birdfeeder. Someone later killed the animal with bow and arrow, as the Grand Forks Herald reported, and Walsh drove to North Dakota during spring break to recover its carcass.

Walsh's doctoral dissertation will examine the ongoing northward range expansion of the common Virginia opossum, the only marsupial found in North America north of Mexico. "It was so exciting because it was further confirmation that this range expansion is still happening and that these animals are probably going to keep moving even further northward as the climate warms and as humans continue to alter the landscape with more agriculture and urbanization," Walsh said.

The Grand Forks specimen marked a 137-mile northward range extension for the Virginia opossum, the only marsupial found in North America north of Mexico. The opossum's ancestors evolved in South America, and the creature invaded this continent about 800,000 years ago, following the formation of the Isthmus of Panama.

Opossums have been steadily marching northward since that time, becoming common in southern Michigan by the 1920s and now found as far north as southeastern Ontario. These animals are built for the tropics but keep pushing north with help from humans.

"Very little is preventing their continued dispersal northward, provided they can survive winters," Walsh said. "This is a unique opportunity to study a documented, ongoing range expansion.

The skin, skull and skeleton of the Grand Forks opossum were deposited in the collections of the U-M Museum of Zoology and catalogued as UMMZ 178776. A research paper by Walsh, describing her Grand Forks findings, was published in the latest edition of the journal The Prairie Naturalist.

For her master's thesis in the U-M Department of Ecology and Evolutionary Biology, Walsh tested the idea that both warming winter temperatures and increased urbanization are helping opossums expand northward in the Great Lakes region. That work, conducted with Professor Priscilla Tucker, Walsh's adviser, was published last August in the Canadian Journal of Zoology.

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