National Geographic News
Feral cats roam the streets of Baltimore.

Feral cats roam the streets of Baltimore, Maryland.

Photograph by Vincent J. Musi, National Geographic

Christine Dell'Amore

National Geographic

Published August 20, 2013

Getting snipped and tying tubes isn't just for people anymore: Vasectomies and hysterectomies may be a solution for keeping free-roaming domestic cats in the United States in check, a new study says.

Over 80 million pet cats reside in U.S. homes, and as many as 80 million more free-roaming cats survive outside. How to deal with this feline explosion has caused much debate, especially between cat lovers and wildlife advocates who are concerned that cats are regularly killing birds and other animal species.

(Related: "Writer's Call to Kill Feral Cats Sparks Outcry.")

Using a computer model, the researchers found that colonies of feral cats that were trapped, given vasectomies or hysterectomies, and released (TVHR) shrank faster than colonies that were trapped, neutered, and released (TNR), a method of feral cat control promoted by many cat advocates.

Feral cats live in groups that are controlled by a dominant male. A vasectomy cuts the tube that carries sperm without removing a cat's testicles, so a vasectomized cat retains its sexual hormones. Thus, it can also keep its dominant position in the colony, so it's able to mate with females without producing kittens.

On the other hand, neutered or castrated—and thus sexually inactive—cats returned to a colony lose their position to the next most dominant breeding male. (Watch a video about the secret lives of cats.)

What's more, when a non-sterilized female cat mates with a vasectomized male, she undergoes a 45-day pseudo-pregnancy period, further reducing opportunities for reproduction, the study authors found.

The new study "looks like good science—it's kind of provocative," said John Hadidian, senior scientist for wildlife at the Humane Society of the United States.

"It has something more to add to this very controversial issue, which is what we're looking for: new ideas and new strategies."

Virtual Cats

For the study, the authors simulated a feral cat population of about 200 animals, observing them for about 6,000 days, or more than 16 years. That's longer than the typical lifetime of a feral cat—outdoor cats live an average of 3 years, while indoor cats typically reach an average age of 15 years—but the scientists needed a longer time period to observe trends.

As they ran the computer model, the team tracked each cat's behavior, adding and removing individuals as animals were born and died, according to the study, published August 15 in the Journal of the American Veterinary Medical Association.

The results showed that if 35 percent of a cat population underwent TVHR, that population would be reduced by half and would disappear in 11 years. Alternatively, if the cat population underwent TNR, 82 percent of cats would need to be captured and neutered in order to eliminate the colony in 11 years.

"We were surprised at how much better [TVHR] worked," said study co-author Michael Reed, a biologist at Tufts University in Massachusetts.

"The results are promising—now someone needs to go and test it under proper controlled conditions."

That's exactly what Hadidian suggests: Ideally, scientists could begin testing TVHR—still an uncommon practice—in a real cat colony. "You want to see how well your data begin to fit the model you've generated," he said.

Not a Cure-All

However, a potential concern with leaving cats intact is that the males will still howl, fight, and spray to mark their territories—causing complaints by people who live near colonies of free-roaming cats.

Becky Robinson, president and co-founder of the animal-rights group Alley Cat Allies, also noted that reducing these behaviors is a benefit of TNR, which her group has promoted and carried out for two decades.

The virtual-cat-colony study "demonstrates non-lethal population control working," she said, adding that some programs still catch and euthanize feral cats.

"Neutering and spaying is now becoming the norm in the U.S., and Americans support humane and compassionate programs that also work to stabilize cat populations."

Sheilah Robertson of the American Veterinary Medical Association's Animal Welfare Division also noted that "it's unlikely a single tactic will be a cure-all" for the feral-cat problem.

"Instead, a multipronged approach will be required that includes TNVR; programs that use nonsurgical approaches, including immunocontraception and chemical sterilization of male cats; and trap-and-remove. Regardless of the method chosen, it may take 10-15 years of sustained effort to see a positive effect," she said.

In general, a lot of the feral-cat debate is actually a "human conflict" between people with differing visions on how to approach feral cats, added the Humane Society's Hadidian.

But some productive dialogue is occurring between people who care about cats and those who care about wildlife, as both seem to have a common goal: to have fewer cats outdoors, he said.

"Finding new solutions like this will get us to where we want to be."

Azar Attura
Azar Attura

Computer simulations are not Real Life On The Streets for feral cats .
No ferals live to be 16?? Guess what? Some DO!!!!!

Peace Seeker
Peace Seeker

The story of Winky, a feral cat, and the problem of over population of cats. Volunteers from a cat rescue organization and a veterinarian explain how trap-neuter-return, or "TNR" can humanely reduce the population of homeless cats and improve the lives of stray and feral felines. Winky and her kittens face the challenges of life in the wild. Some cats are adopted and thrive, others are less fortunate.

Walter Lamb
Walter Lamb

It is always important to keep an open mind and explore new ways to address this issue.  However, these researchers have demonstrated a troubling  "looseness" in their application of data that raises concerns about their research.  In an article they published in the magazine "Tufts Now", these authors not only cited a study that has since been debunked in a peer-reviewed paper, but they did so in a way that indicates they never bothered to understand the study they were citing.

The study they cite from a 2005 issue of Ecological Economics contained a single paragraph relating to feral cats.  In that paragraph was a single sentence claiming that each individual bird death costs the economy $30.  That $30 was then multiplied by the 240 million birds that were estimated to be killed annually by feral cats, and somehow came up with a $17 billion result even though 240 x 30 = 7.2 billion, not 17 billion.  It seems as though some scientists today view math as an annoyance which should be dispensed with if it gets in the way of a desired result. In addition to faulty multiplication, the cited study treats fines as synonymous with economic value, ignores basic principles such as supply and demand, ignores population dynamics, makes inaccurate factual claims, and more.  It is truly dismal science, but the Tufts researchers saw value in the headline.  

The Tufts Now article cited another study in the previous paragraph which concluded (absurdly) that cats might kill up to 3.7 billion birds annually.  That would set the annual economic damage from cats preying on birds at a whopping (and laughable) $111 billion.  To put that in context, the federal budget sequester that received extensive media coverage was only $85 billion.

This kind of disregard for math and common sense doesn't instill a great deal of confidence in these researcher's work.

Barry Harrison
Barry Harrison

Brilliant, brilliant... why would anyone want howling, fighting, urinating, sexually intact but infertile cats in their backyards... Guess what, when a female cat is spayed she never comes in heat again, ever.  And when a tom is neutered, the urine does not foul the front door like a sexually intact, infertile male.  Don't over-analyze when there are perfectly great programs in effect.

Todd Brown
Todd Brown

This is a good example of do something even if it is wrong rather than doing nothing.  I never thought about vasectomies over neutering, but this plan seems to make sense.  Regardless of how it is done as long as done humanely the feral cat population needs to be eliminated or greatly reduced.

Chris Harris
Chris Harris

The McCarthy model TVHR they propose does not offer the benefits that I or others want in our neighborhoods, which helps get more volunteers helping to carry out Trap-Neuter-Return to get the population of unowned community cats under control. TNR includes adopting out kittens. TNR groups can also help return lost cats to their homes. 

Even people that don't necessarily like cats can see the advantages of not having cats spray, fight, yowl, or engage in noisy mating behavior, and that would still happen with TVHR. 

Because TNR reduces fighting, it also helps reduce incidence of disease which TVHR is less likely to do. Less disease means less risk to owned cats too. Cats who are spayed/neutered don't spend as much time roaming, spend more time grooming, are healthier and need less food; these factors haven't been mentioned in the article or by the HSUS.

There are several factors that make well-run TNR programs successful, and it's unlikely that the majority of groups that give grants to help out would support TVHS.  Best Friends Animal Society is one of the groups who are not in support of TVHR.  

Erin LaPorte
Erin LaPorte

Thank you for this most interesting article. We have TNVR here in my town - and it works very well - even with traditional spay/neuter.

Julie Van Ness
Julie Van Ness

@Chris Harris I agree that TVHR would not work in most cat communities.   The ones I know about would never tolerate leaving the 'toms' to howl all night.   Apart from the obvious reason for the program which is to reduce the population of outdoor cats in a humane way, the whole point of the exercise is to keep the noisy males quiet so everybody else can live in peace.  That goes for both people and cats.   I don't understand why anyone would want to fool with TNR.   It seems to me there are so many other problems that have not been addressed at all, like good shelters for outdoor cats in severe weather, which are really hard to figure out in some places, why waste time and money on a study that is basically almost useless?


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