In the summer of 2017, Michael Parsons found the urban rat haven of his dreams: A Waste Management transfer station—aka a literal trash heap, aka rat paradise—in Brooklyn, New York. For nearly two years, the behavioral ecologist and visiting scholar at Fordham University had been searching for a place to observe the city-dwelling rodents in their natural habitat.

Trouble was, he needed to not only capture the critters and tag them, but then to set them free. Rats are wildly, wildly successful animals, a success that comes at great expense to human health and commerce. They spread disease, gnaw through infrastructure, and demolish foodstores, a cumulative devastation that costs tens of billions of dollars a year. But to stop them, researchers first have to study them. "As the saying goes: Know thy enemy," Parsons says. "And the only way to know a rat is to catch it and release it, so you can observe it."

As it happened, lots of New Yorkers were OK with Parsons and his colleagues catching their rats, but almost nobody was cool with the releasing part. So when he and his colleagues found a waste-processing facility willing to let them do their thing, they were beside themselves. "I'm talking about grown men and women in tears here," Parsons says, "because this research is that important, and it's incredibly difficult to do."

But a few months into their investigation, they discovered with horror that five feral cats had infiltrated the waste-treatment facility and had begun patrolling the entrances to the rats' burrows.

At first, Parsons and his colleagues assumed that the felines posed a mortal threat to their subjects. Then it hit them: They actually had no idea how the rats would respond. Few studies have documented interactions between feral cats and wild rats. To a behaviorist like Parsons, the cats' surprise arrival presented an irresistible opportunity. They decided to let nature take its course.

Their observations, which the researchers recount in the latest issue of Frontiers in Ecology and Evolution, revealed the cats to be miserable predators of rats—a finding that not only contradicts popular perception but also adds to growing evidence that feral cats, which are increasingly deployed by major cities across the US to tamp down on rampant rat populations, may pose a far greater threat to smaller, more vulnerable urban wildlife.

One of the camera traps inside the dust-covered waste-processing facility Michael Parsons

Parson had designed his original experiment to investigate how concentrated rat pheromones attract and repel other rats. That set-up turned out to also be perfect for studying cat–rat interactions. Prior to the cats' arrival, he and his colleagues had caught some 60 rodents, each of which they weighed, measured, and implanted with a microchip before releasing it back into the dusty bowels of the waste-processing center. In a corner of the facility heavily trafficked by rats, the researchers installed two camera traps and a pair of RFID antennas, which they smeared with various rat pheromones. Every time a chipped animal passed an antenna, a data logger recorded its presence. Meanwhile, the cameras captured video footage of the rats’ behavior in the presence of different pheromones.

When the feral cats showed up, the researchers started to monitor instead how the rats behaved in the presence of the cats—and vice versa. "That was the cool part," says Greg Glass, a disease ecologist at the University of Florida and an expert in rat-control strategies who was unaffiliated with the study. "It's one thing to show that cats don't have much of an impact on rat populations, but the microchips and the cameras let these researchers ask, OK, so if the cats aren't killing the rats, then what are they doing, exactly?"