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Posted on Mon, Sep 2, 2013 : 6 a.m.

New study explores efficacy of vasectomies in reducing feral cat populations

By Lorrie Shaw


Flickr photo by hehaden

For the estimated 80 million domestic cats that are kept as house pets, there are as many roaming free.

Those numbers shouldn't be surprising, considering the rate at which felines are able to reproduce. I've previously written about the overpopulation issue with cats and dogs, and getting these numbers in check has been the focus of many. Spay and neuter programs and protocols are helping to make progress.

First, understand that from a biological standpoint, we are in a battle with pets. Reproductive success drives evolution, pure and simple. It's the strongest biological factor in any species. Biology has a way of taking over, jumping any hurdle that is put in its path and compensating. The pets themselves have no control over their biological drives, and therefore can't curb their behavior when it comes reproducing.

Feral cat colonies are a supreme example of biology's stronghold.

Comprised of a clowder of free-roaming cats that are the descendants of unaltered tame cats somewhere in their ancestral line, the social structure is by no means random: at its core, it has at least one sexually-active dominant male and fertile females who are often well-bonded and who will help care for their respective litters and each other. Colonies are often formed around shelter — be it a wooded area, abandoned house, under a porch area that doesn’t get that much foot traffic or something else — and a food source of some sort.

Because of their unique resiliency, feral cat colonies have posed a special challenge. The structure and reproductive patterns of these groups have piqued the interest of researchers and got them thinking: Could the way that a feline in a feral colony is sterilized impact the overall numbers of new litters that are born?

A new study focusing on one method of sterilizing cats in colonies — trapping them, giving vasectomies or hysterectomies (versus ovariohysterectomy) and releasing them back into the colony (abbreviated TVHR) — offers some insight.

The results of the study, which simulated a cat population of roughly 200, were published in the Aug. 15, 2013 edition of the Journal of the American Veterinary Medical Association.

But why a computer-simulated study?

First, TVHR is not a common way to address feral cat populations. Trap, neuter and release (TNR) on the other hand, is a more widely-accepted approach to controlling feral cat colony populations, and for a lot of reasons.

Because TVHR isn’t put into use as much and because the life span of feral cats is far shorter — an average of three years as opposed to the 15 that their indoor counterparts enjoy — it’s been difficult to extrapolate the long-term data that helps to give some solid numbers that researchers would be looking for. Each computer run simulated the feral cat population over 6,000 days, tracking individual cats on a daily basis, thus predicting effectiveness of TVHR.

New cats were added to the population as they were born and cats that died were removed, creating a “family tree” of sorts.

But before talking about the results of the study, it’s probably a good idea to flesh out the differences between the two methods and the advantages to both.

Same goal, different approaches

Neutering a male cat entails removal of their testicles — thus leaving them not only infertile, but sexually inactive.

Those two things are very advantageous: the cats don't reproduce, and because they no longer produce reproductive hormones, behaviors like fighting, spraying and howling are reduced, addressing the needs of the community-at-large. (Behaviors like those would be troublesome to anyone who lives in close proximity to a feral colony.)

A possible advantage to vasectomy as opposed to neuter procedure is that though the tube that carries semen is cut, the animal retains their testicles and their reproductive hormones. For that reason, upon being returned to the colony, the cat preserves his dominant position and can continue mating with females without producing kittens — and quite possibly protect their turf from other male competitors that are “intact”.

Conversely, a neutered male loses his dominant position in the colony, and the next most dominant male takes his place — and the cycle continues. (It's important to note that when a female cat that has not been sterilized mates with a male that has had a vasectomy, she enters a 45-day pseudo-pregnancy, dipping the chance of fertile mating even further.)

The findings and commentary

Researchers discovered that with an annual capture rate of 35 percent using TVHR, the population would be cut in half and the entire colony would disappear in 11 years. To achieve the same results with TNR, 82 percent of cats would need to be captured and neutered.

Robert J. McCarthy, D.V.M., lead author on the study and clinical associate professor of small animal surgery at the Cummings School of Veterinary Medicine at Tufts University hopes that TVHR can be put to the test in a more broad sense, rather than the controlled environments and small colonies that it has been shown promise in.

“This opens up new conversations,” said McCarthy.

“The computer model indicates that vasectomy and hysterectomy should be much more effective at reducing or eliminating feral cat populations than the traditional approach of neutering. The next step is to gather evidence on how it actually works in the field.”

The topic of feral cat colonies and how to manage them is one that brings up a lot of emotion in many communities, and Washtenaw County is no exception. The feedback on two featured pieces here on — the first in February and a follow up in July — on a TNR program that was piloted in Ypsilanti along with the Humane Society of Huron Valley, was telling.

One letter to the editor submitted by a local resident expanded on one common sticking point that many share: the wild birds that become prey to free-roaming cats.

One thing that is certain is that it’s going to take time in order to see a favorable result, no matter the approach to managing feral colonies.

And, some experts believe that a combination of methods would be advantageous, including Sheilah Robertson of the American Veterinary Medical Association's Animal Welfare Division.

"…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.

Click here to read more on the study from

Lorrie Shaw leads the pets section for and owner of Professional Pet Sitting. Shoot her an email, contact her at 734-904-7279 or follow her adventures on Twitter.


Wystan Stevens

Tue, Sep 3, 2013 : 10:43 p.m.

Okay, here's what we'll do: once we have neutered or otherwise mutilated the feral cat, we return it to its home turf, first explaining to the unfriendly fuzzball the difference between song birds and English sparrows. The cat must be instructed that sparrows are fair game, but song birds are strictly off limits. Washtenaw County used to offer a bounty on dead English sparrows -- perhaps if that were re-instituted, the dull and noxious little birds could be brought to repositories in various locations, from which their tiny corpses could be distributed to feral cat feeding programs, by the Humane Society or other cat rescue agencies.

Laurie Barrett

Tue, Sep 3, 2013 : 11:45 a.m.

There should be cat rescues. Stray cats would go there and if not taken home by farmers who want them to keep rats out of the dairy barns etc, they would live their lives there. (?)

Jim Pryce

Mon, Sep 2, 2013 : 11:41 p.m.

I don't understand releasing them back into the wild to kill songbirds under feeders, & baby rabbits, squirrels, etc.

Jake C

Tue, Sep 3, 2013 : 1:32 a.m.

Feral cats kill rabbits & squirrels? Anyone who owns a bird feeder or a garden would thank you for protecting their songbirds, or their lettuce/tomatoes/spinach gardens.


Tue, Sep 3, 2013 : 1:02 a.m.

We should just make a law against all predating, eh?


Mon, Sep 2, 2013 : 5:47 p.m.

With a feral population of 80 **million** animals, continually replenished by humans and (often) fed by humans, how does any trap/sterilize/release program have even chance of controlling the population? Does anyone seriously think we could trap and treat even a tenth that many animals? Trap-treat-release programs seem like futile gestures to me, sops to the sensitivities of people who won't accept that solving this problem will require killing cats, and that killing them quickly and painlessly is more humane that letting them struggle for a few years outdoors, doing ecological harm and dying young.


Mon, Sep 2, 2013 : 10:33 p.m.

"Does anyone seriously think we could trap and treat even a tenth that many animals?" Well Dr. Robert McCarthy, D.V.M. at the School of Veterinary Medicine at Tufts University is giving it serious thought. I wouldn't dismiss an effort to apply TVHR simply because the task seems monumental when you view it on a national scale. In the end, these remain local problems. While the 80 million figure makes the idea seem daunting, the success of such a program remains at the local level where population control issues can best be identified. Yes, lost cats from time to time could renew a population. But, with an effective control program, the cats would be more apt to mate with sterilized partners. If you can trap them to kill them, you can trap them to sterilize them. Yes, gas or a bullet are cheaper, but you yourself note that society would repel at that as course and inhumane. Here's a possible solution that, with charitable efforts and some governmental funding may serve to curtail the problem. As for keeping cats indoors, there is no such thing as an indoor cat. They all try and usually do escape--often more than once. Only after they age and no longer have the will for it, do they remain. The young and the virile find their way out. All owners should be responsible and ensure their pets are spayed / neutered so if they do get out or are outdoor cats, they do not add to the problem. I don't see requiring cats to remain indoors as being a solution but I do see spay/neutering to be an owner's responsibility--and not just for cats but dogs too. It also helps to keep the home pet more docile and less likely to spray.


Mon, Sep 2, 2013 : 8:55 p.m.

I don't have answers to those questions. I doubt that any kind of humane, large-scale cat population control effort is politically feasible. Mass-killing would be too unpopular and probably too expensive, large scale trap-and-release would be less unpopular, but would definitely be too expensive. The Australians are trying to manage their feral cat population at a national level, but even they have conceded that total elimination is not feasible. They are trying to create cat-free zones on islands (something we should do in Michigan) and have ongoing cat control efforts and predator exclusion fences around a few centers of high biodiversity (national parks, etc.). A lot of their techniques are not useful for North America because we have more diverse and larger native predators. I don't have an answer, but I also don't see how trap-treat-release is an answer unless it is done on a massive scale, and there's no political will for that spending that kind of money. Local trap-and-release doesn't fix anything. Fertile cats from outside the local area will continue to migrate in whenever local sterile cats die, and will disperse to other areas if the local sterile population is as dense as it can be. Meanwhile maintaining a dense population of sterile cats doesn't reduce the harm they do to wild prey populations, it's pretty well-established that cats hunt even if they are well-fed. I guess we can help by continuing to promote the idea of keeping cats indoors (better for cat, better for the environment), and by protecting and restoring habitat that wild animals need.

Lorrie Shaw

Mon, Sep 2, 2013 : 6:04 p.m.

Epangar -- I can tell that you understand the plight of these cats. I'm interested in hearing more about your thoughts on moving forth killng feral cats to help get the numbers down. How do you propose that be done? And how well do you think that it would address the overall issue? If a program like that were to be out into place, what hurdles can you think of that might pose a problem in it being successful long-term? Many thanks in advance for the engagement. It's an important topic.


Mon, Sep 2, 2013 : 3:30 p.m.

Thank you Ms. Shaw for an informative and well-presented article about scientific findings and a study that offers a hope for a humane means of addressing the feral cat population with the potential for long term control. Maybe the County and HV Humane Society can make test efforts at applying TVHR or an initial program. There may be shorter term costs but if the cat population can be controlled, then it would reduce the overall long-term costs of any such program.

Lorrie Shaw

Mon, Sep 2, 2013 : 4:25 p.m.

Thanks, DennisP. I appreciate your thoughts on the idea. Positive, long-term results often take lots of dialogue, patience, trial-and-error and cooperation. I know that so many people living in communities everywhere are struggling with how to manage these colonies. Thanks for taking the time, today.


Mon, Sep 2, 2013 : 3:26 p.m.

We need less human beings. After all, how did all of these once domesticated animals get dumped to start feral colonies anyhow? Less people = Less mistreated and abandoned pets. I'm fine with vasectomies for feral cats.....but would like more of them for humans as well.


Mon, Sep 2, 2013 : 9:03 p.m.

It does not take a bunch of people to do this.Have you heard of cat hoarders? One breeder alone can generates hundreds, of not thousands a cats a year. If you consider the fact that each female cat can produced two litters of kittens a year one then see how easy it is for a cat population to explode. One male cat alone can father dozens of kittens. So are you arguing for forced sterilization and vasectomies for human beings, Cash?

Lorrie Shaw

Mon, Sep 2, 2013 : 6:15 p.m.

Cash -- yes, irresponsibility on the part of humans bears some blame, for sure. The attitudes about cats and their ability to fend for themselves certainly feeds that. The rate of free-roaming cats is increased by many other means, too, and some are complicated (but it always comes down to biology!) Quite often, colonies are hi-jacked by a bold, intact male that isn't part of the group and get ahold of an intact female in estrous.

Nicholas Urfe

Mon, Sep 2, 2013 : 2:03 p.m.

The state insists that wolf hunts are appropriate. So why no feral cat hunts? They damage private gardens with fecal matter, spray other private property (causing corrosion and stink), and threaten lifestock that is outside (baby chicks and birds).

Nicholas Urfe

Tue, Sep 3, 2013 : 1:09 p.m.

When the wolf legislation was being passed, it was a popular topic on the radio. Reports at that time cited the economic benefits, as touted by Snyder and the GOP.

Jake C

Tue, Sep 3, 2013 : 1:25 a.m.

I have my own issues about the wolf hunt program, but you seriously need to back up your allegation when you say "Snyder said shooting Wolves is great for the Michigan economy"?


Tue, Sep 3, 2013 : 12:58 a.m.

Rhetorical? Seems to me you've had similar comments about feral cats before. This isn't about Snyder or wolves.

Nicholas Urfe

Mon, Sep 2, 2013 : 10:40 p.m.

It was a rhetorical question. If Snyder says shooting Wolves is great for the Michigan economy, where does it end? Why not feral cats?


Mon, Sep 2, 2013 : 8:58 p.m.

Feral cats in appropriate numbers are a positive as they keep down rat populations. It isn't a matter of exterminating feral cats, but rather keeping their numbers in check. The vasectomy approach seems like a good idea. However--and this is where Cash's simplistic moralizing falls short--there are times when extermination is necessary. For example, the Burmese Python was unleashed on the Florida Everglades by people. This snake has managed to decimate deer, rabbit, and other small animal populations.The Burmese Python is a direct and dire threat to the ecology of the Everglades. Unfortunately for the snakes that means physical destruction. Yes, it isn't their fault that they are there and they are only doing what comes natural to them. But that does not mean we permit invasive species to carry on in the destruction of how natural habitats. What we need is stringent laws on what kind of animals people can own because bozos across America are buying and then releasing into the wild animals they cannot take care of. This is having a serious deleterious environmental impact on our habitats.

Donald Martin

Mon, Sep 2, 2013 : 8:12 p.m.

It's "livestock", even though you're misappropriating the word.

Lorrie Shaw

Mon, Sep 2, 2013 : 4:20 p.m.

Cash -- it seems that's the case with a lot of things. Thanks for chiming in!

Lorrie Shaw

Mon, Sep 2, 2013 : 4:18 p.m.

Tex Treeder -- A very thoughtful interjection! Yes, you're right. I thank you for saying that. Working with a cat's biology seems to make a lot more sense. A vasectomized (I do think that's correct, yes!) cat would keep things in check naturally. I appreciate you taking the time to add that. So wise. :)

Lorrie Shaw

Mon, Sep 2, 2013 : 4:13 p.m.

Brad -- yes, and that's precisely why some feral cat colony locations are not disclosed by those maintaining them.

Lorrie Shaw

Mon, Sep 2, 2013 : 4:11 p.m.

Nicholas Urfe -- i hear your concens. Since you proposed the idea of them, I'm curious to understand your thinking on feral cat hunts. How do you think it will ultimately bring down the numbers of or eliminate feral cats? In your estimation, how should a hunt be organized/carried out? Thanks for considering a response -- and for commenting.


Mon, Sep 2, 2013 : 3:36 p.m.

Humanity...gotta love it. 1. Man creates a problem by thoughtless cruelty to a helpless animal. 2. Man's answer to that problem is to kill the animal.

Tex Treeder

Mon, Sep 2, 2013 : 3:10 p.m.

The same modeling of vasectomy vs neutering would apply. A male cat which is killed would be replaced by another male cat, whereas a vasectomized (?) cat would retain its position in the colony.


Mon, Sep 2, 2013 : 2:33 p.m.

Oh, I'm sure there is some idiot somewhere hunting them.