The Chicago Rat Hole Was Not Made by a Rat

Science|The Chicago Rat Hole Was Not Made by a Rat

In January 2024, Chicagoans paid their respects to a peculiar attraction: a rodent-shaped hole stamped into a slab of sidewalk on the city’s North Side.

Nicknamed the Chicago Rat Hole or “Splatatouille,” the indent became a social media sensation, attracting locals and tourists alike to glimpse the vestige of a vanished vermin. Many filled the hole with offerings like coins, candles and miniature plastic rats.

Initially, the origin story of the hole seemed straightforward: a brown rat scurried onto a wet layer of concrete and became trapped. There were no signs of escape, so the rat most likely died and was somehow eventually removed, leaving behind a cavity as the concrete dried. The series of events seemed plausible in Chicago, which was named the country’s “rattiest” city for the 10th year in a row in 2024 by the pest control service Orkin. However, there has been little evidence to tie the rat hole to its eponymous rodent.

A team of researchers recently analyzed the anatomical dimensions of the rat hole to identify the critter that left the sidewalk impression. Their findings, published Wednesday in the journal Biology Letters, reveal that another rodent is responsible for the hole. “We can affirmatively conclude that this imprint was not created by a rat,” said Michael Granatosky, an evolutionary biomechanist at the University of Tennessee, Knoxville.

Dr. Granatosky became aware of the site during a stint in Chicago conducting postdoctoral research. He specializes in studying the evolution of animal movements, and the rat hole reminded him of a modern trace fossil, capturing a moment in time, but with a creature creeping onto wet concrete rather than a floodplain or another prehistoric surface.

He thought studying the rat hole may provide insights into interpreting much older traces. However, accessing the physical rat hole proved difficult. The concrete cavity was filled by the city, re-excavated by residents and eventually deposited in City Hall.

So the team sifted through photos that had been posted online, which offered views of the impression from several different angles. The team used the coins and other offerings placed inside the hole like scale bars, which helped them calculate the rat hole’s width, and the length from snout to tail of the animal that made it.

To put the hole’s dimensions in context, the scientists measured mammal specimens in museum collections. These skins belonged to local rodents like mice, rats, chipmunks and muskrats, as well as several species of squirrel.

Statistical analyses revealed that the size and shape of the hole closely aligned with larger-bodied rodents, particularly tree squirrels in the genus Sciurus. The team concluded with nearly a 99 percent likelihood that the rat hole was left by either an eastern grey squirrel or a fox squirrel, both found within Chicago’s city limits.

A squirrel creating the hole is not a complete surprise.

Unlike nocturnal rats, squirrels are active during the day, when construction workers may pour concrete. The lack of footprints around the impression also makes it likely that the animal landed on the sidewalk from above, perhaps after a fall from a tree. Longtime residents of the neighborhood have confirmed that a tree once stood near the impression. Additionally, it would be difficult for a feature as delicate as a squirrel’s bushy tail to leave a mark in concrete.

That the team remains unable to pinpoint the exact species of squirrel that left the mark highlights how difficult it is to connect trace fossils to animals, even when researchers have a creature’s outline to work with, according to Dr. Granatosky.

“In the case of a recent example like the Chicago rat hole, we should have no problem identifying the track maker,” he said. “But we were still only able to get it down to the genus.”

Winslow Dumaine, a local artist and board game designer who helped embed the rat hole into the public mind with a much-circulated post on X, thinks that correctly identifying the critter that made the hole will not change how Chicagoans view the lost landmark.

“The exact dimensions and causality of the Chicago rat hole were not the driving force of the story,” he said. “It doesn’t matter how it was made, but that it brought us together.”

He also notes that many Chicagoans still refer to the city’s tallest skyscraper as the Sears Tower despite its name changing to the Willis Tower in 2009.

“Besides, squirrels are just zhuzhed-up rats,” Mr. Dumaine adds. “Don’t let the tails fool you — they eat garbage too.”