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The researchers used computer modeling to predict the effects of three different management techniques, Trap-Neuter-Release (TNR), Trap-Vasectomy/Hysterectomy-Release (TVHR), and lethal control (LC). Here’s a summary of what they found:
Management of feral cat colonies by TVHR has not been suggested previously and may be more effective at decreasing population size because cats retain reproductive hormones and normal social behavior is maintained. Vasectomy does not alter a male cat’s sexual drive or social status, so cats maintain their position in the breeding hierarchy, may better prevent immigration of intruding males into the colony, compete for females as before surgery, and continue to copulate but in an unproductive fashion. Coitus initiates a prolonged, nonreceptive 45-day pseudopregnancy period in females, thereby reducing the chance of a fertile mating. After TVHR, female cats continue to attract males and compete with sexually intact females for male courting and breeding time.
Unless > 57% of cats were captured and neutered annually by TNR or removed by lethal control, there was minimal effect on population size. In contrast, with an annual capture rate of ≥ 35%, TVHR caused population size to decrease. An annual capture rate of 57% eliminated the modeled population in 4,000 days by use of TVHR, whereas > 82% was required for both TNR and lethal control. When the effect of fraction of adult cats neutered on kitten and young juvenile survival rate was included in the analysis, TNR performed progressively worse and could be counterproductive, such that population size increased, compared with no intervention at all. [The paper mentions that only 12-33% of kittens in hormonally intact feral cat colonies survive to 6 months of age, but that rate increases when TNR is instituted, probably because of an increased tolerance of neutered cats.]
So, if TNR and LC are often ineffective at best and counterproductive at worst, it sure seems like giving TVHR a try makes sense. The obvious next step would be to try a instituting a TVHR program and monitoring its success (ideally in comparison to a TNR control). Most veterinarians have probably never performed a vasectomy or hysterectomy on a cat, but I’d bet the procedures wouldn’t be too difficult to learn.
The JAVMA article also provides a lot of evidence that supports the need to do something about feral cat colonies. Leaving the animals to fend for themselves is inhumane. The authors refer to a PhD thesis that revealed that in a feral cat colony that was part of a TNR program, the median survival time for intact adult males was only 267 days (less than a year!) and for intact adult females it was just 593 days. Interestingly, the median survival time for neutered males and females was much longer (>730 days), which on the surface looks like a good thing, but this increased survivability is part of the reason why TNR programs often fail to reduce the size of the population over the long run.