Colonies of feral cats can grow at a rapid pace, leaving them to compete in miserable conditions and causing the death of millions of birds, small mammals, reptiles, amphibians and fish. In Australia, New Zealand and Hawaii, such cats have been blamed for the extinction or severe reduction of populations of imperiled species. The U.S. Navy and other agencies spent $3 million over several years to rid San Nicholas Island of wild cats that preyed on native animals there.
In urban areas, wild-born cat colonies can create health hazards to humans and other species, and quality of life issues that include foul odors, loud fighting and flea infestations. In the U.S., the population of wild-born cats exceeds that of cats living with humans, according to the American Veterinary Medical Association, which published the study Thursday in its monthly journal. Truly feral cats are difficult to tame and adopt.
Estimates vary, but animal advocates report 1 million to 3 million feral or stray cats in Los Angeles, including those that are fed by residents but don't reside in a home.
Traditional methods to control feral cat populations involve trapping them and removing the ovaries, uterus and fallopian tubes of females, and removing the testes of the males. The method also tamps down hormone production, reducing behaviors such as territorial fighting and spraying.
But veterinarians from Tufts University's Cummings School of Veterinary Medicine propose that performing vasectomies and hysterectomies instead would preserve hormone-driven behavior that can help reduce a colony's reproduction rates.
Sex drive and social status would remain intact, allowing males to protect turf from other feral males, ward off new strays, and compete for females, which go into a prolonged state of "nonreceptive" pseudo-pregnancy after sexual activity. Females likewise continue to compete with sexually functional females. The result: fewer successful matings, the researchers conclude.
Using data on cat populations and behavior, the Tufts researchers created a program that tracked a virtual feral cat population over 6,000 days, under different population-control regimes and with many nuances that allow for such factors as age, reproductive cycle and ovulation probabilities. The academic farm game showed that the less destructive surgical technique was 10-90 percent more effective, depending on variables.
For spay-neutering methods to have any long-term effect on feral cat colonies, they would have to be applied to at least 57 percent of the population each year, the study found. For the vasectomy-hysterectomy method, the threshold was lower: 35 percent. And if the less destructive method had a capture rate of more than 57 percent, it would eliminate the population entirely in roughly a decade, according to the study. Neither method would work if less than 19 percent of the feral population was altered per year, the study concluded.
"The next step is to gather evidence on how it actually works in the field," Tufts University veterinarian Robert J. McCarthy, lead author of the study, said in a written statement.
The Humane Society of the United States advocates spaying and neutering, a position that generally gets more public support. But the society remains open to other methods.
"We would agree that it needs to be tried somewhere," said John Hadidi, senior scientist for wildlife at the society. The study, he said, "exemplifies what we would hope to see in this very controversial area: new approaches, new strategies and bringing new science aboard, and better science."
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