New research documents ongoing northward range expansion of the common Virginia opossum—and one unlucky opossum in particular.
This individual opossum was first spotted in a suburban Grand Forks, North Dakota 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 with the headline “Grand Forks opossum slain; body to go to University of Michigan for research.” Lisa Walsh, a graduate biology student at the University of Michigan, drove to North Dakota during spring break to recover its carcass.
“Very little is preventing their continued dispersal northward, provided they can survive winters.”
“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 says.
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 says. “This is a unique opportunity to study a documented, ongoing range expansion.
Frostbite and a full stomach
The skin, skull, and skeleton of the Grand Forks opossum are now in the collections of the University of Michigan Museum of Zoology and catalogued as UMMZ 178776. A research paper by Walsh, describing her Grand Forks findings, appears in the journal The Prairie Naturalist.
Virginia opossums are generalist omnivores and will eat almost anything. When Walsh dissected the carcass of UMMZ 178776 back in Ann Arbor, she found a full stomach containing sunflower seeds and muscle tissue, hair, and unidentified bone fragments from small mammals. The lower intestine of this male opossum contained grass, leaf matter, and a sunflower seed.
Opossums, which are about the size of a house cat, have long, hairless tails, hairless ears, and a pointed snout. Their fore and hind feet have five toes, including a thumb-like opposable innermost toe on the hind feet.
With so much exposed skin, opossums are susceptible to frostbite, most commonly on their ears and tail. On UMMZ 178776, frostbite led to blackened and crusty hind feet and a nose inflamed with blisters. It weighed 5.3 pounds, which is less than other opossums trapped in January in other northern states.
Hiding from the cold
Opossums do not hibernate in the winter. Previous research showed that they remain in their den, rather than foraging, on winter days when temperatures do not reach above freezing. In Grand Forks, weather records show that between December 1, 2016, and January 22, 2017—the day UMMZ 178776 died—there were 41 days when the temperature did not get above freezing.
“The winter temperatures and the individual’s condition suggest that this opossum spent much of the winter in a den rather than foraging,” Walsh says. “This individual was likely persisting via opportunistic foraging around Grand Forks residents’ homes whenever temperatures reached above freezing.”
In this way, increased access to food in urban and suburban areas and near farms might be increasing winter foraging success and allowing the Virginia opossum to expand into northerly areas that would otherwise be intolerable.
“Although many range expansions are currently being credited to climate change, climate change alone might not be the entire story for the opossum,” Walsh says. “More direct anthropogenic factors may also be important, including resources easily available in agricultural and suburban landscapes.”
The changing landscape
For her master’s thesis in the 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.
“There are more than 100 American opossum species, but the Virginia opossum is the only one to make it into the United States.”
Walsh and Tucker looked at the genetic consequences of two opossum range expansions, analyzing genetic data from opossums in Michigan, Ohio, and Wisconsin. Tissue samples from ears, lips, tails, and hair follicles were collected from roadkill, from animals that died at a wildlife rehabilitation hospital, and from individuals caught by fur trappers and nuisance control operators.
They isolated and amplified DNA from 85 individuals in the three states, and identified two genetically distinct populations, one on the west side of Lake Michigan and one on the east side.
They used US Census Bureau data to examine human population density, housing density, and the density of farms across the study area. County by county climate data—including information on winter temperatures and snow depth—came from the National Climate Data Center for the period 1992 to 2013.
They then constructed computer models to look for correlations among opossum genetic diversity, census data, and climate records. Walsh and Tucker found that measurements of genetic diversity across 15 counties were best explained by days of snow on the ground. Farm density was also found to be an important factor.
“These models suggest opossum expansion may be facilitated by agricultural land development and at the same time may be limited by their inability to forage in snow,” Walsh writes. The presence of two distinct genetic clusters of this nomadic species—one on either side of Lake Michigan—suggests independent colonization of the two regions in the past.
For her doctoral dissertation, Walsh has expanded her study to examine the genetics of Virginia opossums across their entire range, from Central America and Mexico through the United States. Using tissue samples from museum collections, she has collected usable DNA from more than 100 individuals.
She also sought tissue samples from fur trappers and even enlisted her parents to sample roadkill opossums in Vermont. In addition to DNA, Walsh has collected hair samples for isotopic analyses that tell her what the animals ate.
“There are more than 100 American opossum species, but the Virginia opossum is the only one to make it into the United States,” she says. “I want to know more about how they were able to spread north into temperate America and whether their diet shifted as humans changed the landscape.”
Source: University of Michigan