A rat-shaped indentation in a Chicago sidewalk went viral early last year, with people flocking to the hole with coins and other offerings. But new research suggests that the so-called “Chicago rat holes” weren’t actually made by rats, but are probably squirrel tracks.
The researchers compared the characteristics of the hole with information about local rodent populations and determined that a squirrel most likely left the mark. The study was published Wednesday (October 15) in the journal Biology Letters.
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The hole, with its well-preserved remains of a dead rodent in wet concrete, existed for decades before going viral. It became a tourist attraction in 2024 after Chicago-based artist and comedian Winslow Dumaine posted about its existence on social platform X.
Had to make a pilgrimage to the Chicago rat hole pic.twitter.com/g4P44nvJ1f January 6, 2024
“Squirrels are much better suited for television than rats,” Dumaine told Live Science. “But I think the fact that it was called the Mouse Hole was an important part of why it became popular.”
When researchers saw photos of the hole on social media, some thought it looked “very unrat-like,” lead study author Edwin Dickinson, an archaeologist at the University of Calgary, told Live Science. It also seemed unlikely that a rat could make such a cast on wet concrete without leaving footprints. These suspicions prompted researchers to look deeper into the hole.
The study authors used iNaturalist, a citizen science app that catalogs local fauna, to identify the animals that may have burrowed. They knew from its hollows that it must be an animal with four legs, five clawed feet, and a tail.
After that, it was all a matter of proportion. Although the study authors did not have a chance to measure the hole itself, city officials removed it in April 2024 due to the disturbance the crowds caused to the Roscoe Village neighborhood. However, thanks to the many photos people have posted of their engravings with coins and other items, we were able to get accurate measurements.
The researchers compared their measurements to specimens from the American Museum of Natural History. Statistical analysis of this data showed that there was a 98.7% chance that the holes were made by gray squirrels (Sciurus carolinensis) or fox squirrels (Sciurus niger).
In light of this discovery, the scientists proposed renaming the rathole “Windy City Sidewalk Squirrel” to better describe its origins.
The authors hope this study will help people understand the fun of science.
“While a rat’s nest could be the vehicle, the idea that we want to perpetuate with this study… is that there is nothing inherently too complicated or scary about following the scientific method,” Dickinson said. “It’s something we can all use in our lives.”
