A new study published in the Journal of the American Veterinary Medical Association has found that giving male cats vasectomies, and female cats hysterectomies, is more effective at controlling feral cat populations than trap-neuter-return (TNR), which is the current supported method of control. The Humane Society of the United States, the ASPCA, and Alley Cat Allies all support TNR practices.
According to the study, trap-vasectomy-hysterectomy-return (TVHR) can begin to reduce the size of feral cat populations more quickly than TNR and extermination (though TNR is also more effective than extermination). An annual capture rate of just 35% will start to reduce the population, and a capture rate of 57% eliminated the entire colony in the study in just over 10 years, whereas to achieve the same result with TNR, an annual capture rate of at least 82% is required.
A possible reason behind such results is that a male cat with a vasectomy still produces the hormones that cause territorial behavior. Because of that, he will defend his territory and the females in it from unaltered males. Neutered males, on the other hand, become entirely inactive sexually, and are also more docile, causing them to lose their status as the resident dominant male. The new dominant male can continue to mate with the unaltered females, who will continue to produce kittens.
Male cats that have undergone a vasectomy can still have active sperm in their semen for as many as 49 days after the procedure is performed, hence the need to also perform hysterectomies on the females. Also, while females that have undergone hysterectomies can't become pregnant at all, they, like the males, continue to produce reproductive hormones and thus, go into heat, preserving the mating cycle and helping to keep altered males as dominant.
The AVMA concludes that, if population reduction, or even elimination, is the goal, then TVHR is a more effective, yet still humane, method of doing so than TNR. Extermination is even more difficult than TNR at controlling populations as cats reproduce quickly, and are often hard to trap.