The potential harm to the environment that domestic cats cause when roaming outside of the house has become a polarizing issue.
Credit: Cristian Baitg / Getty Images

Despite the internet evidence that a domestic cat's natural behavior is jamming on a Casio – there are 29.6 million YouTube views for "Keyboard Cat," and counting – anyone who's ever observed their pet feline gazing intently out the window at the bird feeder knows that no matter how many hours a day are spent lounging on the sofa, deep down little Simbas fancy themselves as ferocious as the MGM lion. If not for the screen door, it seems, they'd be king of the jungle, hunting down the first robin of spring across the suburban savanna.

Still, it was an eye-opener this past January when a new report by scientists from the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service proclaimed that a shocking amount of carnage results when cats are allowed to roam freely outdoors, estimating that free-ranging domestic cats kill 1.4 to 3.7 billion birds and 6.9 to 20.7 billion mammals annually on the American mainland. Moreover, free-ranging cats that lived on islands were responsible for 14 percent of all modern bird, mammal, and reptile extinctions. "This magnitude of mortality is far greater than previous estimates of cat predation on wildlife," the report concluded, "and may exceed all other sources of anthropogenic mortality" – i.e., that caused by human influence on nature, including collisions with windows, vehicles, and pesticide poisonings. Given such bloodshed, perhaps it was no coincidence that the trendiest male cat name in 2012 was Dexter.

That even the cuddliest kitties might also hunt, and even kill, should come as no surprise; it certainly isn't news to those who study animal behavior. Recent genetic and archaeological discoveries have determined that the cat was domesticated from the Middle Eastern wildcat around 10,000 years ago in the Fertile Crescent, just as man was transitioning from nomadic hunter-gatherer to a sedentary agricultural existence. These permanent settlements required grain storage (which attracted mice) and generated trash; both food sources proved appealing to cats, who are solitary hunters and carnivores. "We think what happened is that cats sort of domesticated themselves," said Dr. Carlos A. Driscoll, who in 2007 published the first DNA-based family tree of Felis silvestris, the species to which the domestic cat belongs. "They most likely chose to live among humans because of opportunities they found for themselves."

Cats' ability to hunt pests like mice and snakes may have helped endear them to people in the first place, and despite centuries of coexistence and even cultish adoration – the ancient Egyptians worshipped the feline goddess Bastet – they've never lost their hunting and scavenging skills. "They have such an ingrained prey drive," said Mieshelle Nagelschneider, who runs the Cat Behavior Clinic in Portland, Oregon, and is the author of the new book 'The Cat Whisperer,' "that they could have just eaten a bowl of food inside their home, and if they go out and see a bird flying around, they'll kill it. They don't have to eat it; they just can't help themselves."

But while hunting behavior is a crucial component of a cat's natural personality, are they really the ruthless killing machines that the SCBI report portrays? The Humane Society of the United States painted the report as "radical" and its mortality estimates as "informed guesswork." The report termed its own numerical conclusions as admittedly "speculative," using existing studies and hypothetically projecting the numbers observed in small samples over larger populations. Five studies are cited as evidence for the claim that individual feral or stray cats can be responsible for the deaths of more than 200 mammals a year, but four of those studies are more than 60 years old. "It's absolutely unsupportable and ridiculous science," says Jackson Galaxy, host of Animal Planet's 'My Cat From Hell' and author of 'Cat Daddy.' "Cats are just not that good at capturing and killing. They spend most of their day sleeping or engaging in hunting behavior, but that doesn't mean they catch. It's an inflated number, and it's unsubstantiated."

Whether the SCBI study is truly "the first for cats to be based on rigorous data-driven methods," as it asserts, is open for debate; what seems clearer is that the report is just the latest salvo in an age-old conflict between supporters of two opposing primordial forces: Call them Sylvester and Tweety Bird. Forget cats and dogs – a feline's true natural enemy is the birdwatcher. All three authors of the critical SCBI report turn out to be migratory-bird experts, and much of the news coverage of the report's publication was also accompanied by quotes from a spokesman for the American Bird Conservancy, whose "Cats Indoors" education campaign aims to keep all pets inside the house.

Even more extreme is the idea proposed by New Zealand bird booster Gareth Morgan, whose "Cats to Go" movement seeks to eliminate the animals from the country altogether. In the name of New Zealand's exotic native fauna, Morgan's plan is to gradually turn the island nation kitty-free by urging Kiwi pet owners to "make this cat your last."

While Morgan's approach is unlikely to gain much momentum, the issue of the ever-expanding number of unowned cats and their ability to survive on their own – potentially at the cost of wildlife – is a real one in the conservation community. Indeed, when the conversation slides to unowned cats – the SCBI report ascribes about 69 percent of the bird mortality and 89 percent of the mammal mortality to this group, including barn cats, strays, and ferals – the issue becomes every bit as fractious as gun control or abortion. Central to the controversy is the practice of Trap, Neuter, and Return (TNR), which cat advocates espouse as a more humane approach to reducing stray and feral populations (estimated to be about 50 million cats in the U.S.) than rounding up and euthanizing cats, while detractors insist that those methods are ineffective. Whether the forum is an online comments board or a community meeting, the TNR debate is passionate and polarizing, with no resolution in sight.

To clear through all this noise, it's necessary to go to the videotape evidence. Thankfully, that's just what was provided last year by the Kitty Cams project: A research team at the University of Georgia partnered with National Geographic Crittercams, outfitting 60 owned but outdoor-roaming cats with tiny video cameras to document their actions when their owners are out of sight. Dr. Kerrie Anne Loyd, who worked on the project as part of her doctoral dissertation, studied more than 2,000 hours of video where cats were caught on camera eating their fair share of reptiles, but more were seen causing potential harm to themselves: dodging traffic, tangling with opossums or dogs, jumping in storm drains, and slurping up dangerous chemicals like antifreeze. (Four of the cats even "cheated" on their owners by secretly spending quality time with another family.) "I'm a wildlife biologist," says Loyd, now a lecturer at Arizona State, "and I really expected to see most of those cats hunting and doing damage, so I was surprised – pleasantly surprised – to see only 44 percent of those cats hunting, and of those, a smaller percentage actually capturing things."

The one thing that all sides agree on is that responsible owners should either keep their cats indoors or limit their exposure to the outside world through an outdoor enclosure like a backyard "catio." "I don't recommend letting your cat run amok outside, and it's not really for the birds' sake," said Nagelschneider. "There's lots of things out there that can harm your cat. Never in a million years would I let my cats run around and possibly never come back home."