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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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