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How Many Cats Do We Need to Sterilize to Reach Zero Population Growth?

The ASPCA’s Margaret Slater, DVM, PhD, and Stephen L. Zawistowski, PhD, CAAB, share some interesting results from their recent research on free-roaming cats.

Our new study just published in PLOS ONE has some interesting answers! This study, funded by the ASPCA and organized by the Alliance for Contraception in Cats and Dogs, was unique because it included a lot more realistic details than previous studies had. We also tried not to make any more assumptions about cats and their reproduction and behavior than we had to. And it was an amazing team effort with experts on cat welfare, cat medicine and wildlife conservation and modeling.

Turns out that including abandonment of even a few cats into your population means you will have to work a lot harder and trap more cats than if no abandonment happens. For example, for a TNR program, you could trap 15-20% of intact cats every 6 months if no new cats enter your population. But in reality, nearly all populations of cats have a few new cats come in. Maybe from a breeding female cat, maybe abandoned kittens. And when that happens, as it usually does, you will need to trap and sterilize 30% of the population every 6 months for zero population growth. This means that about 70% of the cats are sterilized at all times.

Of course, as time goes by, more of the remaining cats will already have been sterilized, and so there are fewer remaining cats needing to be trapped and sterilized. And cats being cats, nothing is really very simple. Some of the cats you’ve already altered will accidentally get trapped again! And some of the cats you would like to trap will be playing hard to get! Getting creative with the type of trap you use and with the types of baits (including catnip and sparkly toys) can really help with this.

If the main goal is for an approach to population control that gets to zero growth or below, then trapping and removing cats will always require slightly less effort than trapping and neutering will. That is simply because the cats you remove don’t return to the population. Cats you return live out their lives in the population, making them “count” when you look at numbers of cats. The good news is that removal could be for adoption, too!  Especially for young kittens (typically less than 8 weeks old) and socialized adults. If a TNR group can partner with a group that can do the foster and adoption, reducing the population by removal as well as sterilization is the way to go!

The study also looked at temporary contraceptive options. The more successful option was a 3-year contraceptive (which unfortunately doesn’t exist yet), where each cat who received it could not reproduce for a full 3 years. However, that was not as effective as TNR in the longer term (more than 10 years), even with retrapping and retreating. You would have to trap and treat 50% of untreated cats every 6 months compared to 30% with TNR for similar control of the population size.

This published study has some other very cool results. If our understanding of male cats and breeding is accurate (they don’t have a lot of social hierarchy that limits who can breed with available females), then sterilizing males at rates similar to females doesn’t have much impact on population growth. However, we know that sterilizing males can really decrease problem behaviors like roaming, spraying and fighting, which makes them much easier to have as neighbors. We also found that knowing more about the survival of the adult cats could really help us make more accurate predictions about the numbers of cats who need to be sterilized in a given situation.

The model showed that focusing sterilization on kittens up to 6 months old gives us less bang for the buck than doing adults (over 6 months). That is because of the way the model is set up: There are so many more adults, and they are adults for their whole lives, not just the first 6 months of their lives. And partially due to pretty high rates of death for kittens under 6 months. That still means that the younger the adult females are when we catch them, the better—because we can catch them maybe before their first litter or at least before they have multiple litters! 

So now we have a lot more information than we used to, including the idea that abandonment of cats really makes our job even harder. 

What do you find surprising about these results?


Dr. Margaret Slater is Senior Director, Veterinary Epidemiology, and focuses much of her work on free-roaming cats. Dr. Stephen L. Zawistowski is the ASPCA’s Science Advisor Emeritus.



Related links:
Research: “Simulating Free-Roaming Cat Population Management Options in Open Demographic Environments”
Blog: “What’s New: ‘Is That Cat Feral?’ Research Project
Research: The Saving Power of Fee-Waived Adoptions

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Can you explain the two numbers --- I've heard in past that to reach zero pop growth you need 80% annually while this study refers to 30% every 6 months?  


The answer to the title is still unknown.  Now if we had the answer to the question  "how many feral cats are there" we could actually know how many we have to s/n.  Interesting study but still doesn't give us actionable info.   30% of an unknown number is still an unknown number.


I found nothing surprising about these results. The bottom line is you need to get to work on a colony sooner than later. And true, kittens do not have to be a priority BUT they are so easy to trap AND getting them young gives them a chance at adoption. I have trapped many Moms with their litter using my drop trap. Why even talk about a three year contraceptive when it doesn't exist? And the whole oral contraceptive idea is absurd, a waste of money that would be better spent on spay/neuter. And I ask, what was gained with the funds for this study? Those of us working in spay/neuter of street cats could use those funds and get bigger results. A better study would be to find out why people feed cats and don't fix them.


Carmela - I can answer that and save even more money for TNR to be done.

1. Cost/availability

2. Cultural/belief system

3. Doesn't understand the ramifications

4 Can't do it themselves-


Well said. 



Agreed.  A temporary contraceptive is absurd. Repeat trapping is extremely hard.  We face it with sick and injured cats who are already trap shy.  Why base a model on it at all?

As for the kitten issue:  Moms and kittens are ALWAYS the priority. Kittens are getting pregnant at 4 months old.  They have no concept of incest, so we see way too much inbreeding when the kittens start having kittens, which means more illnesses and defects to treat. And the population growth is exponential when a litter of kittens all have kittens of their own.

As a veteran trapper, this study gave no real insight at all.


Hi Emily,

That's a great question.  The answer is that different models provide different estimates of the percentage of cats that need to be sterilized because the models are all a little different.  Many don't include new cats coming into the population (unlike our model) or they use slightly different numbers for survival or birth rates.  Our model also used a 6 month time period because we accounted roughly for seasonality; that is, cats are more likely to be breeding in the warmer weather and less likely (but still happening!) in the cooler seasons.  So all of our outcomes are in 6 month time periods.  Also remember that the 30% number is the number of still intact cats that needs to be trapped and sterilized in each 6 month time period. 

Thanks for your question!!



The results of this study will provide some data for the discussions that compare TNR and trap-remove.  We also know that removing young kittens for adoption can be a really effective way to shrink cat populations.  And having an injectable contraceptive product that doesn't require travel to a clinic or anesthesia could also be helpful. 

One key element to recognize is that we have to sterilize a critical mass of the cat population in a short period of time.  So programs that sterilize a cat here and a cat there are not impacting the size or growth rate of the cat population even though they may be improving that individual cat's life and decreasing that cat's nuisance behaviors.  Another key consideration is in how we define the cat population.  The "population" needs to be small enough so that the available trapping and sterilizing resources can do at least 30% of the cats in a 6 month period.  In some cases, that may be just one colony.  In others, it may be a whole neighborhood.  Determining how many cats are in a colony may not be too hard.  Counting cats in a neighborhood requires a bit more effort and planning. A guide to how to go about counting cats can be found on the Alliance for Contraception in Cats and Dogs' website: under "Counting cats before you herd them".

There are still a lot of important unaswered questions and I'm looking forward to more research to learn the answers!

Margaret Slater


As long as people continue to use automatic traps TNR will be a huge waste of time and energy.Manual traps and full knowledge of the individuals by actually spending time observing and planning is a must.


Thank you for this Dr. Slater.  As someone who is interested in and understands the importance of data, I enjoyed reading this.  What I didn't appreciate were the negative, snarky and even nasty comments.  While I don't have any data to prove my theory, I believe that until people lose their "holier than thou" attitudes, the animal welfare movement as a whole, is not going to progress nearly as quickly as it could.  I wish we could put aside differences, stop being petty about how funds are spent, and work together to help the animals we all care so deeply about.  


just the stats please, thank you and happy holidays.


I am disappointed to see so many negative and downright rude responses to this post. I have found the research that the ASPCA has conducted and shared to be incredibly helpful. Our shelter has made significant positive changes in our housing and adoptions based in part on information that resulted from studies funded by the ASPCA. 

How can our movement expect to move forward and to receive the respect of the community and other sectors that we will need to ultimately resolve our homeless pet problem if we are unwilling to look at the issues we face objectively? Recognizing that trap-and-remove reduces populations more quickly than TNR is not an endorsement of trap-and-remove; it's a statement of scientific findings. We don't need to be afraid of it, we need to understand it so that we can most knowledgeably and skillfully advocate for programs that don't just reduce numbers but that save lives and reduce animal suffering and citizen complaints. And though the three-year contraceptive is not yet available, if we know that it could be relatively soon, why shouldn't we be exploring how and where it might be a useful tool? We know that what we're doing now isn't enough on a national level; we should be open to looking at different ways of approaching problems where our current approaches don't suffice. 

Local animal organizations lack the capacity to do research, and often struggle to see very far beyond our own community's particular challenges. National organizations like the ASPA, organizations that have experts like Drs Slater and Zawistowski, are able to take a broader look at issues our field is facing and then share that information with those who may find it applicable. If you don't find it useful in your own work, fine. But why be so negative about it? Others of us find it helpful! 


Thank you, Karen, for your insightful comments. 

I was surprised at the 30% figure--that actually makes it seem doable, at least on a colony by colony basis. Our small rescue group here on Hawaii Island holds a spay/neuter clinic once a month where we sterilize 80-100 cats. We are dealing with an abandonment culture, though, and need to do even more education, at younger ages. I'm more optimistic now that in the not too distant future we will have a hard time filling the clinics. 


I find that trapping & removing kittens is just not pactical in the south. There are so many kittens clogging our rescues & shelters that are truely adoptable, we don't need to add fearl kittens that need to be socialized.

The climate here is warm to mild all year round, so the survival rate is high. It's good to just Trap/Neuter & Return the kittens & not spend the extra funds & energy for care & socializing. I am a high  volume TNR trapper. I find to focus on spay/neuter & return is the way to go.

No relocating either. It's a waste of time.  Many cats get killed trying to get back to their original colony. Acclimating is required & only 50% stay, if that. ( kittens have agreater rate of staying) 

The caregiver created the problem by feeding without fixing, so I will not relocate just bc they are tired of feeding. I do mediate with the neighbors to explain how TNR will benefit their neighborhood. It helps that we have a law that protects community cats & lets eartipped cats roam and TNRed cats are not considered abandoned. Sometimes the neighbors are happy just to know the cats are fixed, rabies vax & cared for by a caregiver.

With that said, there are exceptions especially when kittens are born in the trap. Also the lone kitten that is thin & full of fleas that would not have a chance needs to be taken in. Kitten foster/adoption programs are a Godsend to find homes.  The nonprofit I volunteer with also has funds set aside for those injured ferals trapped that may need a paw or tail amputated, eye removed or minor medical.

I just find worrying about statistics is a waste of time. If it does not have an eartip, I get it fixed. Eventually I get all in that colony & check back in several months to maintain.

I do agree that the money from studies could be used on solutions like how more shelters can add feral spay/ntr programs open to the public by moving money used for trap & kill services to TNR.

One of our local shelters does a rabies vacination drive once a year at local schools & firehouses with volunteer Vets. The public is charged a small fee. Money from that is used to fund free feral spay/ntr, $10 owned cat sp/ntr & $50 dog sp/ntr. How cool is that? But the same shelter has not put together a TNR program for people or the shelter to go out & TNR. It's all done by volunteeers. Volunteers burn out. Cats reproduce while voluteers are taking a break.... never ending cycle.

I'm not being snarky here. I appreciate everyones concern , help & ideas on the feral situation.

I do see progress on the areas I've TNRed. I wish the shelters saw the value enough to focus on TNR & educating the public. I see a mild shift but it does take time.




    Interesting study. A few of us have abandon the idea of figuring out how many cats there are. It is impossible to know with any certainty and is not really the most important metric. Stray cat admissions and euthanasia at the shelter and complaints about cats in the community are more useful and much easier to measure.


    The results here are that if you TNR one cat for each 250 residents every year you see a 10% drop in shelter cat euthanasia per year. In the last 5 years the shelter cat euthanasia has been reduced by 50%. We focus on trapping entire colonies and I feel this is very important. My thoughts are that in 3 to 4 years the shelter will be euthanizing a very, very small number of cats, less than 1000 per year.


    If we can make this happen we will still have no idea how many cats are in the community. What we will have done is manage the population to the point where the negative impacts on the community are mitigated. Isn't this the real point and much more important that the actual numbers?


Keith Williams


I don't think people were trying to be rude.  It was just so exciting to see the title and think there was a target number that I think people's hopes were raised.  I co-founded a non-profit spay and neuter clinic in 1993.  To date we have spayed and neutered approximately 250K cats and dogs, with the majority being cats and especially ferals.  They are still coming fast and furious.

I also managed several colonies where I worked.  On 500 fenced acres there werre many feral cats and 6 feeding stations.  We trapped all we could (approximately 100), tamed and placed all kittens and a few friendlies, and TNR'd the rest.  Over 13 years, the colonies significantly decreased in number and by the time I retired, there were only 2 feeding stations with approximately 8 cats left.  We fed 365 days a week and had a pretty good sense of any new cats coming and going.  While the 30% rate is interesting, I do not believe it would have gotten us to the zero and negative population grown we were hoping for.  


When we speak of REMOVAL, removal means exactly what, RELOCATION of some kind? in my experience this has proven to be a failure and only allows for such results as the VACUUM EFFECT to take place with new, unsterilied cats entering a location ; as for new cats entering an area in any case I have not found that to occur in managed colonies with observant caretakers simply because cats are very territorial by nature (especially colony "families") and usuallly do not welcome outsiders; in any case caretakers are quick to identify newcomers; in addition, the reality is that relocation DOES NOT WORK for many cats, regardless of how "ideal" or habitable the new environment may be because many cats do not adapt well to leaving their very familiar neighborhood surroundings; and, sad to say, REMOVAL still means one thing to many regressive shelters and that means that any cat removed (regardless of health and adoptability) will be considered disposable and meet an untimely death.


We specified removal in the model to mean the cat does not return to the trapping location.  This model was looking at population size reduction as the primary goal.  However, we certainly recognize that neuter and return can have potential benefits even though, for the same percentage of cats trapped, the population size reduction is slower than for removal.  In addition, while in some locations removal does mean euthanasia, removal can also mean adoption!  We certainly encourage adoption of young kittens and socialized cats when that is an option--that will provide both the quickest reduction in population size as well as a humane outcome for those cats.



Dr. Margaret Slater


I would like to thank Dr. Slater for her work. Basic knowledge about feral cat population dynamics is sorely lacking. The better we understand the cats the better decisions we can make for managing them. My experience has been that given a choice between trap and remove or trap, neuter, and return the public is fairly evenly divided. What we are doing here is to allow both to be done. In about 1% of the cases there is a conflict between neighbors over what to do about the cats at that location. In those cases we have a mediation process to help them come to an agreement. While this is not the traditional model of picking one or the other method community wide, this mixed arrangement is providing good results for us. 


What I found interesting is that cats might be tempted to brave the trap for catnip or sparkly things.  Haven't tried that yet.  Any other tips for catching the Hard-to-Trap adult females?

I had the same questions about 30 percent of what?  I guess we don't know.  It's like advertisements that say 30 percent off.  My immediate thought is 30 percent off of what?



The method I used to catch a very, very trap-wary female was to set up 3 drop traps at the feeding site. The other cats (12 or so) were mostly willing to go under the traps after a couple of the bold ones started eating the bait. The wary one took several minutes to decide they were safe, but she finally picked one to go under, and I got her. I was lucky to know other people whose drop traps I could borrow, because I had not been able to get her with just one. I wanted to have many of the cats eating at once, because I've learned that if they have to take turns going under the trap, they get edgy with each other, and the ones that stay back may lose interest and decide not to go in at all. You have to get them when they are excited about food, and that's usually when you first arrive at the colony to feed. Good luck to all you TNR practitioners!


I have been maintaining a small neighborhood cat colony for over 10 answer used to be spay them all....if you are short on time...let the males slide for a bit but catch those females....hold the pregos inside until they give birth...hold the kittens until they are big enough to "fix" and socialize them while you are the kits have a chance at adoption....BUT lately I have more faith in ACC drop off not euthenising the kittens and I have dropped off a couple while they were to young to neuter....what a relief that not have to care for them for them so much longer....thanks ACC...for becoming a "NO KILL" as best as possible so far....good luck becoming a totally no kill....that would help us with the TNR Sooooo Muuuuuuch..!!!!



By the way...the way I was able to catch all the cats new and old is I set up a 10 foot kennel and put plywood across the top....I call all the cats for feeding and all the regulars go in and I close the door....then I set up a drop trap for the new beees and catch them for works very well....I also use the kennel for relocated cats that need to get used to the yard...the other cats introduce themselves through the kennel and eventually accept the neutered new bee without the scratched eyes and bitten ears....of course there is comfy seating available for the caged cats and a few deep tupperware litter boxes....



I love opening threads from years back! A lot of people get caught up in creating a solution based upon "how many are there?" and then sterilizing that magic number. A study that I found fascinating is "Estimation of effectiveness of three methods
of feral cat population control by use of a simulation model" published in the JAVMA August 15, 2013. The authors look at the efficacy of different methods based off of trapping probabilities. It seems the two studies are somewhat similar.

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