Oswaldo Rivas/Reuters

Your adorable house cat is also a ruthless predator. A conservation biologist makes the case for keeping cats indoors, or at least on leashes.

What does your cat do when it goes outdoors?

The question may linger in the mind of anyone who has an outdoor cat, and the answer has become quite a bit clearer in the age of YouTube and GoPro cameras. It’s not a pretty picture, though—just take it from this 2013 New York Times headline: “That Cuddly Kitty is Deadlier Than You Think.” An alarming study cited in that piece estimates that cats kill about 2.4 billion birds and 12.3 billion mammals each year in the United States. About 29 percent of those bird deaths and 10 percent of those mammalian deaths could be attributed to house cats in particular, according to the study.  

“Some cats don’t kill anything, some cats kill 20 animals a week,” says Peter Marra, a conservation biologist who co-authored the study as well as the book, Cat Wars: The Devastating Consequences of a Cuddly Killer.

So should you let your pet cat outside, whether it’s to hang out in the backyard or to wander around the neighborhood? CityLab spoke with Marra to find out just how much damage these “cuddly killers” can do, and why he thinks it’s time for outdoor cat owners to do what many outdoor dog owners did about 50 years ago. This conversation has been edited and condensed for clarity.

So this starts as a kind of kitchen table debate: If you own a cat that you normally keep indoors, should you ever let it outside?

The first thing you have to realize is that cats are not a native species anywhere. It’s a domesticated species. Their native ecosystem is laying on a couch or on a rocking chair, or chasing a laser pointer or a feather toy.

If cats go outside, they are an unnatural component to an ecosystem. They’re a predator that normally doesn’t exist in these environments, so other species don’t expect to be preyed upon by a cat. They are often at densities higher than any predator exists in these ecosystems, so their impact is also disproportionately large.

Birds are a sympathetic prey—apparently people love bird-watching! But what about those other animals? I could see someone arguing that alley cats hunting down rodents could be a benefit.

We predict there are about 12 billion mammals killed by cats per year. It’s not surprising [compared to birds] because mammals can’t fly and just have a harder time escaping cats than birds do. Cats will go after anything, they go after insects, reptiles, amphibians. Sure, they go after mice, but they won’t go after the larger Norway rats, which were also introduced as non-native species. The rodents people talk about as pests—whether it’s a deer mouse or it’s a star-nosed mole or a meadow vole—are actually native species, and that can harm an ecosystem. Using cats for pest control is something that no exterminator worth their salt would ever recommend.

I imagine it’s tough to convince people that the neighborhood cat out on a front porch is a threat.

We have a responsibility to protect native species, just like we protect cats if they’re in our care. We have to remember that once a cat goes outside, it is also vulnerable to either getting hit by a car—which is the primary way they get killed—or to being taken by a coyote or an owl or another predator. Letting cats outside is not in the best interests of the cats. We need to get it out of our minds that it’s okay for cats to be outside. It’s just not responsible pet ownership.

It’s a lot like how we don’t let a dog outside to roam around anymore. We used to in the 1960s and early 1970s, but then we realized that rabies was a problem, dogs being hit by cars is a problem, dogs biting people is a problem. Cats are now the number-one domesticated animals to spread rabies to humans. It used to be dogs, but then we got it under control. We just have to change our mindset about how we treat cats.

What would you say about to a neighbor who wants to let their cat out?

Neighbors should educate each other. That doesn’t seem to be asking a lot. I don’t want my neighbor’s cat coming into my yard and impacting things that I value. If they really want to let a cat out on their property, we know that they can build a catio—a cat patio—or put them on a leash to go outside. We’re trying to enrich that cat’s life, I get that. But just opening the door and letting the cat out—there is no excuse for that.

It’s just like using a pesticide or something else that’s toxic to the environment, because cats are just doing what they are meant to do. They are predators. That’s what they do. It’s not the cat’s fault, it’s humans that really are to blame. We’ve seen change in the past, too. We saw the change with dogs, we saw the change with DDT. At first change seems like it’s impossible, but it does happen. It’s possible, and that’s why I remain hopeful about what we’re facing in this biodiversity crisis.

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