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Science meets the great cat debate

Among the Pigeons: Why Our Cats Belong Indoors

John L. Read
Wakefield Press
2019
364 pp.
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Cats in Australia: Companion and Killer

John C.Z. Woinarski, Sarah M. Legge, and Chris R. Dickman
CSIRO Publishing
2019
343 pp.
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With their shared passion for animals, bird watchers and cat lovers should be allies. Instead, they’re often at each other’s throats. The reason is simple: We love cats—there are more cats in the United States than dogs and a loosely estimated 600 million Felis catus worldwide. The problem is that many cats are outside some or all of the time, killing birds, rodents, insects, and just about every other type of small creature. Plus, outdoor cats can spread human diseases, most notably toxoplasmosis.

As a result, many conservationists and public health workers would like to see house cats kept indoors and stray populations greatly reduced. Unfortunately, that is easier said than done.

The debate over the extent to which cats are a problem and, if so, what solutions to pursue has become increasingly nasty. In part, this vitriol has been the result of inflammatory statements, exaggerations, distortions, quotes taken out of context, and good old-fashioned mudslinging.

But underlying this unpleasantness has been a lack of enough good information. An estimate that cats annually kill 1 billion to 4 billion birds in the United States (1)—by necessity, an extrapolation from small-scale studies—has been highly controversial, but the more important question concerns which species are getting killed. Billions of pigeons might not be a matter of much concern; a lot of endangered species would be another story.

And it matters, too, how this death toll affects bird populations. Are these the “doomed surplus” that would have died in short order because of some other cause, or is cat predation driving down bird populations and endangering species? The answer is that in the United States, the epicenter of the cat debate, we mostly just don’t know.

But there is a place where scientists do know, where research on cats and their ecological impact has been deep and broad, where cutting-edge experiments are exploring cat impacts and possible solutions. That place is Australia, and two new books provide insightful analysis of what we have learned and what can be done.

Cats in Australia is an authoritative and detailed synthesis that focuses primarily (though not exclusively) on the tremendous amount of research conducted on the ecology and behavior of felines down under. Less monographic and full of personal vignettes, Among the Pigeons (2) is written in an engaging and casual style. Broader in scope, the book has an Australian emphasis but devotes more attention to cats and cat research beyond the Antipodes.

Despite their different approaches and audiences, the two books come to very much the same conclusions. Before getting to these conclusions, though, we must consider why so much cat research has been conducted in Oz.

Australia combines an unfortunate trio of circumstances that make cats a huge problem. First, the island continent is renowned for its evolutionary isolation, producing a fauna unlike that found anywhere else and thus of exceptional conservation value. Second, Australian small animals have no evolutionary experience with a predator like a cat. Cuddly cuteness notwithstanding, cats are superbly adapted killing machines; the combination of cats and prey with no evolved defenses against them—sometimes not even recognizing a cat as a threat—is a recipe for slaughter. And third, Australia has lost its larger native predators. Around the world, big predators regulate the populations of smaller ones, often by killing them. With thylacines extinct, Tasmanian devils banished from the mainland, and other large predators eliminated, cats nearly have free rein.

Synthesizing an extraordinarily large number of studies, scientists estimate that the more than 2 million feral cats in Australia annually kill 400 million birds and more than a half billion of both reptiles and native mammals. A wide variety of data—from historical accounts, correlations of cat abundance and prey population declines, and increased abundance of prey species both on offshore islands lacking cats and in areas where cats have been removed—all confirm that native populations are sharply and negatively affected by cats. Moreover, controlled field experiments in fenced enclosures covering many square miles—some with cats, some without—conclusively demonstrate the feline effect: Cats have an enormously negative impact on many native species.

Painstaking research, detailed in both books, unravels more specifics: Cat impacts are often greatest during lean times, when cats switch from no-longer-abundant rabbits to other prey. In some cases, individual “catastrophic” cats hone in on a particular type of prey and can have a devastating effect. Read notes how a single cat once wiped out an entire wallaby colony, for example. Overall, cats have played a major role in the extinction of 27 Australian species since European settlement and are currently threatening the existence of scores more.

The evidence that cats are having a massive effect on Australia’s indigenous wildlife is incontrovertible, but what to do about it? Keeping pet cats indoors is one important step, but the feral cat population is a more intractable problem.

“Trap-neuter-return” (TNR) programs are the solution favored by most animal welfare organizations. Both books examine the data available on such programs and conclude that TNR is unlikely to reduce feral cat populations in most cases; the level of effort, consistently maintained year after year, is just too great. Feral cat advocates will not like this conclusion, but the review of the literature appears thorough and unbiased. In the end, both books reluctantly agree that killing feral cats is necessary.

What other solutions might there be? More effective means of killing cats are in development, but that’s hardly cause for rejoicing. From an animal welfare perspective, the development of gene drive systems that render cats sterile might be more favorable. But concerns about wiping out all cats, including other feline species, probably make this approach untenable. Other ideas are more promising.

Long the scourge of conservationists as an invasive species, the dingo is now emerging as an unexpected hero—conservation in the Anthropocene!—because its presence reduces numbers of introduced foxes and maybe cats, too (dingoes definitely eat cats, but they also eat foxes, which both kill and compete with cats, so the net effect of dingoes on cats is uncertain and the subject of current research).

If cat numbers can be kept relatively low so that some prey individuals survive, perhaps natural selection will favor those individuals with beneficial traits, and the ability to coexist with cats will evolve. Ongoing studies in experimental enclosures are testing just this hypothesis, with promising early results (3).

The scientific data laid out in these books clearly demonstrate that cats are having an overwhelmingly detrimental effect on the fauna of Australia and many smaller islands with predator-naïve species. The big remaining question is whether the same is true on other continents where prey species have evolved in the presence of felids and in theory might be less vulnerable. Unfortunately, there are strong indications that cats are having similarly negative effects on North American biodiversity, although these conclusions are disputed (and the disputers themselves disputed—in one case, likened to climate deniers). What is really needed is much more detailed study of the behavior and ecological impact of cats in North America. Such work is already under way.

Cats in Australia and Among the Pigeons show the value of a solid scientific underpinning for resolving societal disputes. Hopefully, we will soon have a robust enough scientific basis for adjudicating the “cat wars” in North America as well.

REFERENCES AND NOTES
1. S. R. Loss, T. Will, P. P. Marra, Nature Comm. 4, 1396 (2013).
2. The title alludes to a British phrase, “to throw a cat among the pigeons,” meaning “to cause a great disturbance.”
3. A. Braun, Science 364, 421 (2019).

About the author

The reviewer is at the Department of Biology, Washington University, St. Louis, MO 63130, USA, and the author of Improbable Destinies: Fate, Chance, and the Future of Evolution (Riverhead Books, 2017).