Kerry Bartoletti's work is part observation and part science, and her success depends on whether she can tell which homeless cats will get pregnant next.

``It is such an overwhelming prospect,'' said Bartoletti, who helps tend more than 150 homeless felines through her organization, Friends of Feral Cheshire Cats.

Her goal is to keep the Cheshire cat population under control with neutering or spaying. But controlling the feral population is an uphill struggle: For every animal sterilized, another could be having kittens.

But Bartoletti has a new tool in her arsenal: a feline birth control pill.

The inexpensive pill -- actually an off-label hormone medication designed for dogs -- is intended as a stopgap measure, not a permanent answer to cat fertility or a tonic for household pets in heat.

The Westbrook-based Tait's Every Animal Matters, or TEAM, which runs a spay-neuter program, has been distributing the pills for the past 2 1/2 years to people like Bartoletti, who work against time and cats' instincts as they try to curb the feral population. TEAM sells the pills for about $50 per 500.

Its use remains limited, but the cat pill is nothing new.

And the concept -- controlling animal population through contraception -- stretches far beyond cats.

Across the country, contraceptives are being used or developed for animals ranging from deer on New York's Fire Island to elephants in South Africa. The Humane Society of the United States is treating deer with vaccines shot from a dart gun. The Bureau of Land Management has used a contraceptive vaccine on wild horses for years, and a branch of the U.S. Department of Agriculture will soon be seeking approval to sell a similar drug commercially. The New Jersey Department of Environmental Protection has tried a contraceptive vaccine on black bears, and late last year the first oral contraception for geese hit the market.

Animal Politics

But like almost any matter involving four-legged creatures, animal contraception also inhabits the world of human politics.

Animal contraception ``takes things like assisted suicide, abortion, gay marriage, and it pushes them aside as something that gets people's knickers twisted,'' said Jay F. Kirkpatrick, director of the Science and Conservation Center in Billings, Mont., which produces 5,000 units of vaccine a year for animals worldwide.

Armed with a degree in reproductive physiology, Kirkpatrick was in Billings in 1971 when two cowboys from the Bureau of Land Management walked into his office and inadvertently inspired a three-decade career in changing how animals reproduce.

Congress had just passed a law protecting wild horses and burros in designated areas, and Kirkpatrick's visitors, wild-horse managers for the bureau, realized that could lead to overpopulation.

``Can you make horses stop reproducing?'' one of them asked.

``I looked at him and I reasoned that every human contraceptive was going through animal trials first, so of course we can make animals stop reproducing,'' Kirkpatrick said.

It took them 15 years to figure out how. After first experimenting with hormone contraception -- the types humans use -- Kirkpatrick switched his focus to vaccines.

In the years since, the concept has gained traction among researchers.

``It's often said there are no alternatives [to population control] except to kill the animal,'' explained John Hadidian, director of the urban wildlife program for the Humane Society. He and other researchers are intent on disproving that.

They're researching vaccines for deer at sites in Fire Island, Gaithersburg, Md., and Fripp Island, S.C. The animals are shot with darts designed to deliver the vaccine, which causes their immune systems to react against their reproductive hormones and render them temporarily infertile.

Researchers with the National Wildlife Research Center, an arm of the USDA, meanwhile, are working on a similar vaccine, with plans to submit it shortly to the Environmental Protection Agency.

But Kathleen Fagerstone, program manager for product development at the national research center, said the vaccine, called GonaCon, won't solve deer-population problems by itself. It's not a substitute for hunting, she said, but is a way to manage a population before it gets out of hand or after it has been cut down by other means.

``If you've got a huge deer population in an urban-suburban area, contraception is not going to cut that population down very fast,'' she said.

Approval from the EPA would make GonaCon the first such product for deer available outside the bounds of research.

But whether it will gain popular acceptance for use is in question, in part because animal contraception -- like that for humans -- is a political tripwire.

``We could go way, way, way beyond where we are right now,'' Kirkpatrick said. ``Politics are the biggest single hurdle.''

The arguments come from all sides: Science shouldn't be manipulating animals' reproductive cycles; contraception is too slow and therefore a waste of time; the focus should be on hunting or quicker ways to trim the population.

Meanwhile, hunters' groups worry that contraception could threaten their activities. They question its effectiveness and point out that their activities bring in revenue for states and address population problems more quickly.

The contraception concept has had a limited track record in Connecticut.

A three-year experiment in Groton ended in 2000 with state Department of Environmental Protection officials questioning the effectiveness of using tranquilizer guns to deliver contraceptives to deer.

And residents of the Mumford Cove neighborhood where the experiment took place expressed their views in a vote, with a roughly 2-1 ratio in favor of a controlled hunt.

Other communities grappling with deer problems have expressed interest in contraception, but the methods are not yet commercially available, said Howard Kilpatrick, a wildlife biologist for the DEP.

``It will probably be a tool in the future for specific unique circumstances, but it's not going to be something that has widespread use,'' Kilpatrick said.

That's in part because reducing a population through contraception could take 15 to 20 years -- not the time frame people want when deer have already become a problem, he said.

``You may treat a deer with fertility-control agents and make it infertile, but she may live for 10 more years,'' he said. ``Often when people have problems with deer, they're at the point where they're frustrated.''

Too Many Cats

A backyard in Wethersfield is a long way from the politics of animal population. But it's also the front line.

That's where one 70-year-old man tends to the 15 or so feral cats that have come to depend on him. It began with one small kitty in the backyard. When he began caring for it, others came, too.

``They had nowhere to go,'' he said. ``It was cold and nasty.''

Now there's a group that arrives daily for Friskies, wet food and cottage cheese. He has had eight sterilized, but others are hard to catch, and he doesn't have the heart to set traps for the animals he knows by fond nicknames -- Little Small Tail, Big Tail, Little White Paw.

Like many who care for feral-cat colonies, the man feared having his name printed out of concern that his landlord would learn that he harbors that many cats.

He was resigned to his lot, spending about half his grocery money on food for the cats. But then four new cats showed up this month.

``I got nervous,'' he said. ``I thought, well, I'm going to hire somebody to catch all these cats. Whatever it costs.''

But his call to TEAM ended up with a new solution: contraception. He began putting pills into the cats' food about three weeks ago.

The pills are similar to human birth control, using hormones to fool the body into believing it's pregnant. Because it must be administered weekly, TEAM president John Caltabiano considers it strictly a stopgap measure until a cat can be sterilized.

Bartoletti hopes the stopgap will be enough to keep her ahead of the tide in Cheshire. Among the colonies she helps is one on a farm with about 100 cats; people leave kittens there, perhaps figuring it's a good place for them.

Some of those are young enough to socialize and be adopted, but others will remain feral. About half have been sterilized -- an accomplishment, but not enough to stop the kittens from coming.

Still, it's a start, Caltabiano said. ``People will say to me, `We're down to our last cat now.'''

Contact Arielle Levin Becker at alevin-becker@courant.com.