Cats as Predators: The Damage They Do

Of all the cultural conflicts that tear our nation apart, perhaps the one that receives the least attention is that between bird lovers and cat lovers.  That is a pity because there are significant issues involved that affect all of us.  Peter P. Marra and Chris Santella inform us that there are things that require more of our attention.  They bring us up to date with their book Cat Wars: The Devastating Consequences of a Cuddly Killer.

The authors believe that the domestic cat species that have become popular as pets evolved from species of the wild cat appropriately named Wildcat. 

“Recent genetic studies corroborate the notion that today’s domestic cats evolved from several subspecies of wildcats and suggest that, of the five, the Near Eastern Wildcat is likely the domestic cat’s nearest relative.  This also confirms the hypothesis that domestication of the cat occurred somewhere in the Fertile Crescent.”

It is assumed that just as wolves and humans discovered a mutually beneficial coexistence leading to the domesticated dog, wildcats and humans did as well, resulting in the domesticated cat.  Whereas wolves and humans could share the same food, the wildcat probably learned that mice and rats and other small mammals started hanging around human food stores looking for an easy meal.  Since mice and rats are preferred meals for wildcats, it is easy to see humans concluding that letting wildcats hang around was a good idea. In this relationship, natural selection would enhance the survivability of wildcats that were less hostile to humans and would eventually give rise to versions of the domestic cat.

The attractiveness to humans of the domestic cat and its utility as a mouse and rat eliminator encouraged humans to begin carrying them wherever they travelled.  This meant that cats were introduced into environments in which they were not naturally occurring and would become an “invasive species.”  That term implies the animals would multiply beyond any natural controls and cause damage to the local ecology.  The authors begin their narrative with the experience of David Lyall who took up a position as a lighthouse keeper on Stephens Island off the coast of New Zealand and brought with him a pet cat named Tibbles who would soon deposit a litter of newborns in its new home.  Like many islands that had long been isolated, Stephens had developed unique flora and fauna—and had never experienced anything as voracious as the domestic cat.

“Cats make the perfect pet for an isolated island inhabitant, in part because they can obtain most of their own food from their surroundings.  Lizards, birds, or small mammals provide a sufficient diet.  Cats are carnivores and need to consume primarily protein and some fat to stay healthy.  They are ambush predators, sitting for long periods, motionless and quiet, waiting for the right time to pounce.  They are quick and efficient and excel at what they do—otherwise they die.  Cats have retractable, razor sharp claws that extend from their strong paws to pin down prey.  Once the prey is immobilized, cats inflict the kill bite with two sharp canines, usually to the neck, and quickly begin tearing into scales, fur, or feathers.  Cats can kill animals as large as rabbits and squirrels, but their primary prey consists of smaller rodents like mice and voles as well as birds the size of (and including) sparrows and wrens.”

“Cats do not always kill out of hunger.  They seem to be stimulated by the chase and if not hungry will still kill; cat owners who allow their cat to roam freely may have received a ‘present’ of a bird or mouse, a testament to their pet’s predatory competence.”

Cats reproduce at an astonishing rate, and in an environment where they have no significant threat from predators, their population will grow until limited by lack of food or by disease.

“Cats average three litters a year; the average number of kittens in each litter is four to six.  Kittens can come into estrus as early as four months after being born, so the numbers of cats can multiply very quickly!”

A single pregnant cat, as in the case of Stephens Island, is all that it takes.

“A female cat can produce a litter of as many as eight kittens, sometimes more, and if a male is around, she can be impregnated again within days after giving birth.  If an unrelated adult male is not around, siblings will eventually mate with one another, or offspring will mate with their mother.”

On Stephens Island, David Lyall, a naturalist at heart, had the unique experience of discovering a new species of bird on the island only to realize that within a year the proliferation of cats that he had initiated had rendered the bird extinct.  It would take 26 years of cat killing to rid the island of the predators.

Cats are an invasive species in the United States as well.  The problem with domestic cats is that most of them are feral and must live off the land by eating birds and other small mammals.

“The Loss et al. paper positioned the domestic cat as one of the single greatest human-linked and direct threats to wildlife in the United States, and emphasized that more birds and mammals die at the mouths of cats than from wind turbines, automobile strikes, pesticides and poisons, collisions with skyscrapers and windows, and other so-called direct anthropogenic causes combined.”

The numbers of predator cats and their victims are staggering.  Counting feral cats is not a simple task.

“Rough estimates do exist and include between 20 and 120 million unowned outdoor cats, with 60 to 100 million cats the most frequently cited range.”

But feral cats are not the only problem.  The estimated number of owned cats is about 84 million.  These are cats that are treated as pets and, presumably, fed and vaccinated against disease by their owner.  Unfortunately, many of these owners allow their cats to wander outside where they can hunt and contribute to the killing even though they have no need for food.

“Based on eight different studies, between 40 percent and 70 percent of owned cats were allowed outside; three additional studies suggested that between 50 percent and 80 percent of these animals actually hunted.”

The unowned, full-time hunters were the most prolific killers.

“Loss et al. estimated that each individual unowned cat annually kills 1.9 to 4.7 amphibians, 4.2 to 12.4 reptiles, 30.0 to 47.6 birds, and 177.3 to 299.5 mammals per year.”

Combining the estimates from both owned and unowned cats one arrives at the death toll.

“The final mortality numbers showed that cats killed between 1.3 and 4 billion (median 2.4 billion) birds per year, with unowned cats causing the majority of the mortality (69 percent)….The final estimates for mammal mortality were also alarming; 6.3 to 22.3 billion (median 12.3 billion) mammals were killed every year by outdoor cats.”

Since there are few organizations defending the rights of mice and rats, the focus will be on birds. 

The authors provide data indicating that numerous bird species studied have shown significant declines in number over recent decades.  Cats are not the only contributor to these declines; they compete with environmental disruption caused by the growing human population.  However, the cat problem is the one least justifiable in terms of benefits for humans—or for cats. 

The killing of billions of birds is not the only negative result.  Cats prowling around outside and interacting with animals that are susceptible to plague and rabies can acquire those maladies and infect humans.  Control of wild dogs has been effective, but uncontrolled cats are now the greatest threat for rabies transfer. 

Cats are also the unique source of a parasite called Toxoplasma gondii.  The life cycle of this parasite requires an infected cat to spread the disease to other animals via cysts (oocysts) that are excreted into the environment where other animals can ingest them and allow the cysts to transform into tachyzoites which then multiply rapidly.  These secondary hosts are then eaten by cats and the infectious cycle begins again.

Unfortunately, humans and other valued species can be infected by these cysts that cats distribute.

“Toxoplasmosis, the disease caused by Toxoplasma gondii, is one of the most common parasitic infections in humans.  In fact, it is estimated that approximately 30 to 50 percent of the world’s population and up to 22 percent of the U.S. population (more than 60 million Americans) are infected with Toxoplasma gondii….”

The infection will sometimes produce only mild flu-like symptoms in healthy individuals, but the invasive parasite does not go away; it lodges itself—hopefully in a dormant state— in tissue and will remain there as long as the individual is alive.  This latent stage was long thought to be benign, but recent studies have suggested correlations between infection and long-term changes in physical and mental health.

Infection in a person with a compromised immune system can be dangerous and even fatal.  The unborn fetus has no active immune system making infection via the mother a serious concern.

“Pregnant women and their fetuses have been known since the 1920s to be at serious risk.  If infected with toxoplasmosis in the first trimester, one in ten fetuses will be aborted or become malformed—and this is likely an underreported statistic.  Because of this problem, pregnant women have been warned for decades to avoid changing litter boxes and touching cat feces.  Despite these warnings, congenital transfer of Toxoplasma continues to happen across the world.”

Ardent bird lovers and cat lovers can both agree that there are too many unsupervised cats in the environment, feral cats spread disease, and that life for a cat in the wild is short and brutish.  Cat owners often assume that cats are quite capable of living on their own outdoors, making it easy to abandon them when it becomes convenient.  They may live outdoors, but few of them will thrive, and none will die of old age.

“Unowned cats without veterinary care are prone to disease (including feline leukemia, renal failure, feline panleukopenia, plague, rabies, and toxoplasmosis….).  They are vulnerable to predation by other animals, especially Coyotes and, to a lesser extent, eagles, owls, foxes, and Raccoons.  And they are frequently hit by cars—the most common cause of demise in outside cats.  Such are the hazards if they survive to adulthood, but estimates suggest that 50 to 75 percent of kittens born outdoors do not, dying from exposure, parasites and disease.  If they do reach adulthood, the life expectancy of an outdoor cat without caregivers providing regular feeding, water, and sometimes makeshift shelter is two years.  Outside cats that receive such care have a much longer life span, averaging 10 years.  The average life span of an inside cat is thirteen to seventeen years, depending on the breed.”

It would seem that a cat owner has a moral responsibility to keep her beloved pet inside and safe from harm.

So if everyone can agree that there are too many cats running around outside, what does one do about it?  The place to start is to keep from adding more cats to the environment.  Cat abandonment should be a crime.  There are leash laws for dogs, why not leash laws for cats.  Allowing an owned cat to run free could become a crime at some level.  But what to do with cats already running wild?

The main response from those concerned with protecting cats from harm is to foster trap-neuter-return (TNR) programs.  The assumption was that if a wild cat was trapped, taken to a facility and neutered, and then allowed to return into the environment, this would lead to a decrease in the population of feral cats.  Unfortunately, the data does not support this hypothesis.  Studies of wild cat colonies suggest that one would need to neuter 70-90 percent of the population before the numbers would begin to decline.  This level of TNR efficiency would be extremely difficult to reach.

The authors conclude that there is no alternative to the total elimination of the wild cats.  It can be done—at least locally.  As an example, they describe a program put in place on Ascension Island, a British territory.  Combined poison baiting and trapping (plus euthanasia) efforts began in 2002 as a means of ridding the island of cats.  By 2006 all the cars were gone.  The total cost of the effort was $1.3 million.

“Whether you consider $1.3 million an outrageous sum to pay to save a few birds or a wise investment in biodiversity will depend on your philosophical stance.  But from a purely financial perspective, there is little question that eradication—at least on a local level—will trump endangered species remediation every time.  A breakdown of per species dollars invested in conservation efforts for endangered species from 2004 to 2007 shows that $60.5 million was spent to resuscitate populations of the Southwestern Willow Flycatcher, $67.4 million to protect Red-cockaded Woodpeckers, and nearly $83 million to protect Bald Eagles.”