Tanya Loos
Tanya Loos is an ecologist and science writer based in regional Victoria, Australia.
Feral cats fail to fulfil their role as rat controllers in New York City, new research reveals.
Cats (Felis catus) are often released in New York in the belief that they will make a dent on the city’s centuries-old rat problem. However, a study of the two species in the mean streets of Gotham reviewed 306 videos of cat-rat interactions over a 79-day period and found that just two – yes, two – rodents lost their lives.
In contrast, earlier research from the US suggests that feral cats are likely the single greatest source of anthropogenic mortality for US birds and mammals. In Australia, a 2017 review on cat predation on birds estimated that they consume up to a million native birds per day.
The latest study, from a team led by Michael Parsons of Fordham University in the US, acknowledges that cats are clearly efficient killers, but concludes that the low rat kill rate over nearly three months suggests they are avoiding them and opting for less challenging prey.
New York rats are brown rats (Rattus norvegicus) and famed for their size and fearlessness. Online videos abound of subway rats crawling on sleeping commuters, or stealing slices of pizza.
“New Yorkers often boast their rats ‘aren’t afraid of anything’ and are the ‘size of a cat’,” says Parsons. “Yet cats are commonly released to control this relatively large, defensive and potentially dangerous prey.”
Parsons’ team – which included US and Australian biologists, as well as a professional pest exterminator – suggests that the practice of releasing cats to control rats has a deleterious effect on smaller creatures of the urban ecosystem. Brown rats weigh on average 330 grams, in contrast to a typical 15-gram bird or 30-gram mouse.
“Until now, no one has provided good data on the number of city rats killed by cats,” says co-author Michael Deutsch, from the Arrow Exterminating Company. “But the data have been very clear as to the effect of cats on native wildlife.”
The team already had an ongoing study monitoring the behaviour of a colony of some 100 microchipped rats in an abandoned waste recycling facility. When a small feral cat colony moved into the rats’ territory, a study of cat and rat interactions became possible, providing data not only on predation rates, but also the behaviour of the rats in the presence of the cats.
“We wanted to know whether the number of cats present would influence the number of rats observed, and vice versa,” explains Parsons. “We were also interested whether the presence of cats had any effect on eight common rat behaviours or their direction of movement.”
The findings provide a clue to why people believe cats are efficient rat control agents. The video footage revealed that the presence of cats changed the rats’ behaviour, making them likely to hide and thus stay out of sight. For every additional cat sighting, a rat is 1.19 times more likely to move in the direction of shelter.
“The presence of cats resulted in fewer rat sightings on the same or following day, while the presence of humans did not affect rat sightings,” says Parsons. “People see fewer rats and assume it’s because the cats have killed them – whereas it’s actually due to the rats changing their behaviour.”
The research is published in the journal Frontiers in Ecology and Environment.
Related reading: How did the spread of feral cats begin?
