Feral cat activists, who desperately would like excuses to avoid euthanasia, return time and again to the delusion that outdoor cats reduce rat numbers. The latest report on the variations on the “working cats” theme is a report about the Tree House Humane Society in Chicago, which dumps feral cats outside homes and businesses on the grounds that they will “sustainably deal with Chicago’s rodent problem.”
To be fair, the claim is that the cats will “scare off” rats even if they don’t kill them, but the implication is that feral cats somehow are an effective means of rat control. They aren’t, and we’ve known they aren’t since the early 1900s.
The US Public Health Service used to fumigate ships for rats, because rats do in fact harbor fleas that carry nasty diseases like plague. In the early years of the 1900s quarantine officers would routinely get pushback from captains that their ship need not be fumigated because they had cats aboard that were excellent ratters and that there would be no rats to be found. Such was the case with the captain of the British steamship SS Ethelhilda, arriving in New Orleans from Africa on March 18, 1914. The captain insisted there would be no rats, due to the services of “an exceptionally good cat.” The quarantine officer insisted on fumigation anyway and the captain forgot to remove the cat ahead of time. The following picture is the result.
The SS Ethelhilda had rats in every portion of it. The photograph shows only what was found in the cabin itself: 1 cat, 24 rats that had lived along side it through the journey. The quarantine officer was good to stick to protocol, in the face of nonscientific assertions from the public that are still very familiar today.
In a 1916 review of cats as a means of rat control, ornithologist Edward Forbush documented the well known fact that buildings could be overrun with rats and mice, even while many cats were also kept on the premises. What happens, and this was known over 100 years ago, is that the presence of cats makes the rats more secretive, while not decreasing their abundance. For example, Forbush wrote,
Upon arrival of this cat, the rats soon disappeared and were not seen running about as before. A little careful investigation, however, showed that they were nearly as numerous as ever, but much shyer, keeping out of sight. At the end of the year, notwithstanding the killing done by the cat, the number present had not decreased, as not enough had been killed to dispose of the annual increase. After the cat had been in the barn six months, I set eleven old rusty traps one night and got six rats; two sprung cats and got away. This one night’s work of old and rather ineffective traps equaled six weeks’ work of the cat.Forbush, 1916
In his discussion, Forbush continued:
…[T]he people who asserted they had no rats really had them at the time, although they did not realize it, as there are many more rats than are seen by human eyes. Mr. McMahon, in this canvass, found a village where quantities of fowls were kept and where cats were depended on to exterminate the rats. Everybody there seemed to believe that cats were effective as rat exterminators, and no one seemed to be using traps or poisons. The village was canvassed quite thoroughly, and every place was found infested with rats, while in nearly every place cats were kept. The evidence did not confirm the popular belief in the cat.Forbush, 1916
As research on freeroaming cats and their diets became more quantified over time, the low proportion of rats killed by cats became more evident. From a 1951 study of scat in a Baltimore neighborhood, only 7% contained evidence of consumption of rats. The cumulative predation of cats in the study area only accounted for about 20% of the mortality necessary to keep the population stable. That is, rats were reproducing five times faster than the rate of cat predation.
Subsequent research in Baltimore in 1986 found that the rats killed by cats were juvenile or subadult (<200 g). Adult rats were never attacked by cats and the author observed that “generally these species coexist peacefully in alleys.”
The finding that cats only take juvenile rats and have no effect on overall population levels in an urban context was confirmed by research published in 2009. Again, researchers in Baltimore found that rat populations rebounded quickly from removals of half the population, which cats never remotely approached. They did document cats killing rats up to 500 g, but found the same pattern of most kills being <200 g.
What is happening in Chicago and those other places thinking they are controlling rats by placing cats is the same thing as was happening 100 years ago. The rats keep hidden but maintain their numbers commensurate with the food available for them. It is literally “out of sight, out of mind.” The cats are not, as asserted by the nonprofit moving the cats around, scaring rats away with their “pheromones.” Rather, rats are keeping out of sight more in the presence of cats.
It would be one thing if introducing and maintaining freeroaming cats were simply not useful, but they have their own adverse impacts on people and their environment. This is why responsible public health officials recommend removing, not introducing, stray cats from the environment.
The way to control urban rats is to control harborage and food sources for them. Traps are effective an inexpensive for enclosed spaces. Anticoagulant rodenticides should be avoided because of their adverse impacts on non-target species, both directly and through transmission within the food web. If a city has a rat problem, and Chicago has topped the list of the “rattiest” cities put out by a major pest control operator for the past six years, the solution is not to put out food for or relocate feral cats and imagine that they are helping. Usually food given to feral cats is eaten by rats as well, so feral cats are not part of the solution, they are part of the problem.
Travis Longcore, Ph.D.
May 15, 2021