July 2024 

By Graham Brayshaw, DVM 
Chief Medical Officer, Animal Humane Society 

Trap-Neuter-Return (TNR) — managing feral and free-roaming cat populations by proactively trapping, neutering, and returning free-roaming feral cats — is a controversial practice that is often misunderstood. 

Comments on any article about TNR will include impassioned pleas from supporters and detractors ranging from cat colony caretakers to birding enthusiasts concerned about cats’ impact on wildlife to individuals frustrated by cat nuisance behaviors. These arguments often lack important context about population dynamics and the goals of TNR programs. 

TNR is often derided as ineffective because it does not significantly reduce the population of free-roaming cats, but that is not its primary goal.  

The goal of TNR is to reduce unnecessary cat suffering, death, and euthanasia and to reduce undesirable behaviors associated with unsterilized, unowned cats. TNR is more humane and beneficial than trapping and euthanizing cats, relocating them, or doing nothing, but it will not significantly decrease the overall population numbers.  

Let’s first look at population dynamics. Free-roaming populations in nature – cats, deer, raccoons, and other species – will grow until they deplete food and other available resources. Animals are designed to produce more offspring than will survive, and when you have more resources, more will survive. To decrease a cat population using TNR, you must sterilize and return at least 70% of the cats – and even then, even small populations are rarely eradicated altogether. In the Twin Cities, we would need to perform over 200,000 TNR surgeries every year to see our feral population decrease. (AHS, Minnesota’s largest TNR provider, has performed roughly 6,000 TNR surgeries in the last decade.) 

So, why invest in TNR if we can’t decrease the free-roaming cat population? TNR was developed as an alternative to trapping and euthanizing feral cats, and this is where it shines.   

Animal shelters and impounds once euthanized thousands of free-roaming cats every year — and it made no impact on cat population size. If you simply remove and euthanize cats, new cats will move into the area to use the available resources — a process known as the vacuum effect.  If you sterilize and return a cat instead of euthanizing it, that cat will continue to use resources where it is returned – meaning fewer new cats will enter that population.   

TNR has many other benefits. Sterilized cats are also vaccinated for diseases like Panleukopenia, which is fatal to cats, and Rabies, which also affects humans. They are less likely to engage in undesirable behaviors like spraying and fighting and less likely to transmit other fatal cat viruses like FIV and FeLV.  A colony of sterilized cats will be more stable, and the cats in that colony will have a better quality of life.   

TNR is only part of the solution. To successfully manage free-roaming cat populations, we must also address the issue of feeding outdoor cats. Every bit of food put outside for cats is an extra resource that helps create more free-roaming cats, increasing the overall cat population. Unfortunately, we can’t just ban outside feeding. Abruptly removing all this food would lead to cats starving and fighting over the remaining resources. Gradually decreasing it will lead to a gradual decrease in the unowned cat population.   

Finally, there are valid concerns about outdoor cats' impact on wildlife that should be considered independent of TNR. Cats are great hunters, and they kill and eat large numbers of birds and small mammals each year. Unless you eliminate the free-roaming cat population altogether (an impossible task), wildlife predation will continue. Since TNR doesn’t impact the overall size of the free-roaming cat population, it does not change the impact cats have on wildlife. 

TNR has many benefits, but it is not a panacea — and it is just one of the many services AHS provides to help more than 100,000 animals and people in our communities each year. AHS has supported TNR for more than a decade, and the recent changes we’ve made to our TNR program – sharing the cost of this service with our TNR partners – ensure that we can continue to help free-roaming cats and prevent unnecessary euthanasia for decades to come.  

Longcore, T., Rich, C. & Sullivan, L.M. (2009) Critical Assessment of Claims Regarding Management of Feral Cats by Trap-Neuter-Return Conservation Biology, 23(4):887-894). 

Castillo, D. and Clarke, A.L. (2003). Trap/Neuter/Release methods ineffective in controlling domestic cat “colonies” in public lands. Natural Areas Journal 23: 247-253