Kristin Des Marais crouches in the damp and shines a flashlight into the vast crawl space beneath the apartment building. The light sweeps across rocks and mounds of dirt, the glint of an empty tin can, a flash of tail. Eyes glare back, glittering and metallic, then disappear.

A light, drizzling rain muffles sound, but an unmistakable kittenish mewing rises from the depths of the cavelike recess, beyond the light's reach. A pudgy skunk waddles through the beam, headed away from the noise.

Des Marais stands up and clicks off the torch. "There's so many places they can hide," she says. "This isn't going to be easy."

There are nine buildings in the apartment complex, many with exposed crawl spaces. The place has ponds, courtyards, a clubhouse, without a human in sight in all this rain. But there are cats slipping in and out of the basement-level covered parking, cats huddled next to dumpsters and still-warm car engines, cats on porches and stairs. They slink back silently when Des Marais approaches.

It's impossible to even attempt an inventory on a wet night like this. Some tenants think there could be fifty cats around. Others say it's closer to a hundred, which would make this complex in southeast Denver one of the larger feral cat colonies in the area. It could be even more. From the uncovered garbage cans to the cavernous crawl spaces, all the elements are in place — food, water, shelter, horny cats — for a major population explosion.

Animal-welfare activists have requested that the address be withheld; they say that revealing the location of a colony encourages people to dump unwanted cats there. In neighborhoods that have feral cat problems, the common lament is, "There were just two a while ago." But a female cat can have two or three litters a year for life. Her female kittens can get pregnant at five months. Before long the brood is hooking up with unfixed domestic cats, and a Malthusian nightmare has begun.

Estimates of the number of feral cats in the metro area range from tens of thousands to 125,000 (a Denver Dumb Friends League figure) to an improbable quarter-million (based on a formula that assigns one stray cat to every six humans in an urban setting). The Rocky Mountain Alley Cat Alliance, the lead group spearheading efforts to deal with ferals locally, has identified dozens of colonies and more than 2,000 cats in one survey, showing a heavy concentration along the Colfax and Federal corridors. But that figure is considered just the tip of the tabby.

"We know what's happening in colonies that we've touched, but there are places all around that no one knows about," says RMACA executive director Amy Angelilli.

Thirty thousand cats are euthanized every year in Colorado, double the rate of dogs. Many are homeless cats considered unsuitable for adoption. Working with RMACA, Des Marais and other volunteers are trying to reduce the kill rate by trapping ferals, spaying or neutering them, then returning them — in effect, transforming the entire colony from feral to sterile so that it will eventually die out on its own. But trap-neuter-return, or TNR, is a controversial solution, often unpopular with communities afflicted by the colonies.

"More often than not, people will call and say, 'Come pick up these cats,'" Angelilli explains. "They think we have a special vehicle we drive around and pick up stray cats and bring them to this utopia in the country. If people knew about TNR and why it works — but it needs to start before the population gets out of control. By the time people call us about the problem, they're usually so fed up that they just want the cats gone."

That appears to be the case at this apartment complex. Des Marais tracks down the tenant who invited her here, a man named Santos. He says things are getting worse by the day. He caught a neighbor shooting at the cats with a BB gun and asked him to stop.

"He said, 'Those aren't your cats,'" Santos recalls. "He told me to mind my own business."

Another tenant saw a coyote eating a cat. Cats have been found with their entrails hanging out. One was decapitated. Santos doesn't think all the mutilations are the work of coyotes. "People can be cruel," he says.

Des Marais nods sympathetically. The TNR job at this colony is beginning to look overwhelming, but she's willing to try. She volunteered for the no-kill MaxFund shelter for years, running its mobile adoption program, and eventually started her own nonprofit, Four Paws Pet Center of Colorado, which works with small shelters around the state to arrange low-cost or free spay/neuter clinics. Since last year's economic meltdown, calls to her group have tripled. Four Paws now subsidizes 1,200 surgeries a month — many of them out of Des Marais's own pocket.

"I was in Rocky Ford last weekend," she says. "We had people at 5:30 in the morning lining up and waiting to get their animals fixed for free, including some feral cats. One woman had fifteen around her place. You'd think farm people could care less, but there was definitely a need. They just can't afford it."

Des Marais is prepared to spend two grand to get the apartment complex's cat colony fixed at RMACA's high-volume clinic. But the plan soon hits a snag. Hours after her site visit, Des Marais learns she's not welcome on the property. The company that manages the complex has arranged for a pest-control service to remove the skunks and cats. Ironically, trapping skunks requires a wildlife permit and relocation of the animals, but cats can simply be turned over to the Denver Municipal Animal Shelter — which, since ferals are not adoptable, will end up euthanizing them.

Des Marais is stunned. "They told me my money is better spent elsewhere," she says. "I'm not used to this kind of thing. I'm trying to help."

The management company just wants the cats removed, but it's possible that the pest company might turn the animals over to rescuers — just as long as they don't bring them back to the complex. Des Marais and other volunteers begin working on a relocation plan for the entire colony.

It's a formidable task. Not only is there no magical place in the country for them to go, but feral cats are highly territorial. Relocating them requires keeping them confined for weeks until they get comfortable in new surroundings. Contrary to the myths about cats being able to fend for themselves — the same myths that have helped create the feral cat problem in the first place — a feral released in the country will try to find its way back to its colony and probably perish in the attempt.

"Cats are about twenty years behind the dog movement," notes Angelilli. "In Denver, the dog is treated like a kid. People still think of cats as independent. They think they can fare well on their own. But they're not going to fare well in your alley."

Moving with the brisk efficiency of a field medic, Holly Aubart prepares a seemingly endless procession of cats for surgery. She kneels with a syringe over a plastic carrier, and soon the caterwauling inside the box stops. She places the anesthesized cat on a counter, at the end of a chain of similarly comatose kitties, and shaves its belly.

In the adjacent room, veterinarian Jason Cordeiro is cutting, snipping and sewing. After thousands of identical operations, he has the procedure down to an unvarying series of steps that can be timed with a stopwatch. Spaying a female takes about seven minutes. And neutering a male?

"Thirty seconds," Cordeiro says.

He demonstrates on a gray tomcat, snoozing obliviously in a device that spreads its legs akimbo. In moments the testes are stripped and removed; balanced on the index finger of Cordeiro's glove, they look like the tiniest of seed pearls. He peels off the glove and flicks it and the tom's future generations into a trash receptacle. Aubart moves the patient to the recovery room and brings in another.

Cordeiro and Aubart are the surgical team driving the Feline Fix, the high-volume clinic launched by the Rocky Mountain Alley Cat Alliance. Located a few blocks east of Federal in the Valverde neighborhood, the clinic has been open since July. But today is its official "grand opening," complete with a ribbon-cutting ceremony, a visit from city councilwoman Judy Montero — and 47 speedy surgeries.

Most of the patients are domestic cats, brought in by owners taking advantage of the clinic's low prices ($35 for neutering a tame cat, $20 for ferals). But one row of traps in the pre-op room is covered with blankets, some of them shaking with the displeasure of a wild thing caged. These are ferals. The tip of the left ear of each feral will be removed after surgery, signaling to trappers and colony managers that these have already been fixed.

RMACA doesn't provide fostering, shelter, adoption or relocation services; it's chosen to focus on neutering as the best solution to the overpopulation problem. Last year the group fixed 3,400 cats. With the opening of the clinic and a grant from PetSmart Charities, RMACA plans to double that number in the coming year.

As the program expands, Angelilli expects that an increasing number of the clinic's clients will be feral cats. But at present, the group has no reliable process for getting them to the operating table. If people call to report a feral cat, RMACA will loan them a trap and show them how to use it. The organization also trains volunteer trappers who coordinate raids on colonies, bringing the ferals in waves to the clinic to be fixed, then returning them to the colonies. Another nonprofit, Divine Feline, runs a mobile clinic that can visit a particular colony and fix the population on site. Yet there are far more colonies than volunteers, and residents who call for help usually expect someone else to do all the work and cover the expense.

"The ideal situation would be for a partner organization to be involved in the trapping and community outreach, but nobody does that," Angelilli says. "All the other agencies are sheltering and rescuing, that kind of thing. They may have traps to loan out, but they don't have a trapping coordinator, a community-relations person to lead workshops. Right now we don't even have good data to show that our efforts are impacting the community."

Angelilli came to RMACA two years ago, after starting the People Pet Partnership in Philadelphia. Her background is in marketing and public relations, and much of her animal-welfare work has involved breaking down barriers between organizations and getting them to work together toward common goals. As she sees it, the feral cat problem is intimately connected to irresponsible pet owners who refuse to get their own cats fixed, leading to unwanted litters, abandoned animals and feral offspring.

"Even people in the animal-welfare community think tame cats are one issue and feral cats are another, and they're not," she says. "Everything is connected. Tame is one end of the spectrum, feral is the other. But there's all sorts of shades of gray in between."

When colonies do get fixed, RMACA will work with Dumb Friends and other agencies to try to find homes for the kittens. Ferals that are five or six weeks old can usually be domesticated, Angelilli says, but it becomes increasingly difficult as they get older. And at three to four months, forget it; they may socialize with each other, but they will resist confinement and shun human contact.

The average lifespan of a feral cat is about three years, but advocates of the TNR approach say it can be longer if that feral is lucky enough to belong to a stable colony. Ferals are no more prone to disease than house cats, and once they're fixed, much of the behavior that neighbors find objectionable — spraying, fighting, roaming far and wide — is greatly decreased. If there are caretakers willing to "manage" the colony, leaving out food and providing makeshift shelters during cold spells, the colony can die out on its own over time.

"A good, managed colony can lead a decent life," Angelilli says. "To try to put ferals inside isn't going to work. And just to euthanize them — personally, I feel that's us playing God. People started this mess, and we're going to end lives to solve it?"

The effort to establish a grassroots TNR program in the city is enthusiastically endorsed by Doug Kelley, director of Denver's Municipal Animal Shelter. As an "open admission" operation, the shelter takes in every type of stray or homeless pet. Cats labeled as feral will be euthanized — but not before they've already taken up space and expense in feline housing that often operates near capacity. Last year the city shelter put down 1,141 cats; another 389 were adopted by the public, and 820 were transferred to rescue groups.

"We're pretty open with the fact that a feral cat's going to be euthanized if they bring it to us," Kelley says. "If a cat comes in with a notched ear, if it's actually a managed-colony cat, we'll get in touch with RMACA. But animals have to be held five days, and sometimes a healthy, adoptable cat has to be euthanized to make room for an unadoptable one."

Kelley would prefer to see ferals fixed and managed in colonies rather than being hauled to his shelter and certain death. "A managed colony is, to me, planned obsolescence," he says. "It's a way of resolving a neighborhood situation without having to euthanize a bunch of cats."

Yet Angelilli points out that Denver's current animal-control ordinances make the TNR approach tougher. "You have laws that say if you feed a cat, it's yours," she notes, "and there's a five-cat limit."

Many of the volunteers in the TNR movement are violating the city's five-pet limit by virtue of the number of cats and kittens they are in the process of rescuing, fostering or otherwise housing; putting out food for a colony can result in further violations. Consequently, many feral-cat people tend to operate furtively, setting up feeding stations in alleys and hiding their work from the neighbors and animal-control officers. "We need an exemption for ferals," says Angelilli, who's been meeting with city councilmembers over proposed changes in local ordinances.

Kelley would rather not see his office embroiled in neighborhood complaints about feral cats, but they keep coming. "If somebody really pushes, then we have to take some sort of action, and we'll contact Neighborhood Inspection Services," he says of the city agency that handles such matters. "It's a fine line between being a responsible person who helps take care of a colony and being a hoarder. We've certainly been on properties where well-meaning people have a hundred-plus cats. Then it becomes a health and safety issue."

Although he hopes to one day see TNR lead to a decline in shelter intake, saving tax dollars, Kelley doesn't expect the numbers to change much in the short term. In the meantime, the shelter is acquiring special feral cat boxes that are easier to clean while dealing with a hissing, biting captive. And his staff is receiving training from RMACA in order to better distinguish a true feral, slated for euthanasia, from a cat that might simply be ill-tempered yet salvageable.

"Sometimes it's hard to tell a feral cat from one that's very unsocialized or one that's just nasty," Kelley explains. "What we're calling feral cats aren't always feral cats."

Three years ago, Carol Tudor agreed to help a friend who was trying to corral a feral cat and her kittens in a condominium complex. She learned about TNR and borrowed a trap. The mission was successful, and she decided to get some training and volunteer to help RMACA with other ferals.

An environmental engineer who works for a consulting firm, Tudor has become an accomplished feral trapper. In her spare time, she's tackled colonies with as few as six and as many as thirty cats. Residents are encouraged to do their own trapping, but if the neighbors are too elderly — or apathetic — Tudor helps out.

"Most of the time, people are thrilled to have me there," she says. "We get a lot of calls from elderly ladies. They're home all day; they see the cats come and go. They put out food and more come, and then there's a litter of kittens and they don't know what to do.

"A lot of times, they're skeptical," she adds. "They ask me, 'Are you really going to bring my cats back?' They become quite attached, even though they can't touch them."

Some of the highest concentrations of ferals can be found in mobile-home parks and salvage yards. Tudor recently pulled 22 cats out of a trailer park near the city shelter. Rising foreclosures have aggravated the problem; people clear out and leave the cats behind. If the colony proves to be too enormous for one trapper to handle, the volunteers will combine forces, stacking the traps in SUVs or station wagons and booking blocks of time at the Feline Fix for the surgeries.

Tudor is working with Des Marais on negotiations with the apartment complex that wants to eradicate its colony. She recently sent a letter to the pest-control company, offering to take the cats off the company's hands and fix them. "They're happy to let us take them," Tudor says. "They just want something in writing that says we're not going to bring them back."

Developing a relocation plan for up to a hundred ferals requires persistence and patience. Tudor and Des Marais have found a few rural citizens who wouldn't mind having a couple of barn cats to deal with rodents, but Tudor still hopes to persuade the apartment complex managers to allow at least some fixed cats to stay. "It's better to have forty feral cats than forty fertile cats," she says. "It's going to take a lot of barn homes to accommodate all these cats. I may end up with some in traps in my house this week."

(At press time, the relocation plan was still under review by the property's management; it's also possible the cats could be turned over to another rescue group.)

Trappers often wind up being the principal ambassadors of the TNR movement. They have to get permission to go on private property to set traps, explain to neighbors what they're doing and why they're bringing the cats back, warn people to keep their own cats indoors during the trapping operation — and, if possible, solicit donations to help pay for the whole process. In some cases, their greatest obstacles are not the local cat haters, but the crazy cat lovers, including some of those sweet old ladies who feed the ferals.

Feeders can be the bane of an effective trapping operation. When she's not assisting Cordeiro with surgeries at Feline Fix, Holly Aubart works with another vet in the Divine Feline van, and she's seen how the feeders can get in the way of a total colony fix.

"We have people who are considered caretakers, who are feeding the cats but not fixing them," Aubart says. "They get overwhelmed, and they have to choose between feeding and fixing. They choose feeding, of course. But all that does is make it so that you have to feed more."

In order to lure the cats into traps with tasty bait, trappers ask locals not to feed the ferals just before the operation begins. Some feeders refuse to cooperate. "They won't withhold the food, which makes it more difficult to trap," Aubart says. "Some people will actually hold a couple of cats back because they like having kittens around. That defeats the whole purpose of TNR."

One feeder in Lakewood has gained particular infamy among the fixers. The woman has set up numerous feeding stations, regularly spaced over several blocks. Aubart estimates that she's feeding 180 or more cats. But she refuses to get involved in fixing them.

"She feeds all the time," Aubart sighs. "Other volunteers have tried to help her, and they've given up."

Crazy cat ladies aside, Aubart may have a deeper acquaintance with feral cats than anyone else in the state. She works at the Feline Fix four days a week and with Divine Feline two days, assisting with dozens of surgeries. Then she comes home to her own feral cat colony.

A few years ago, a woman in Swansea decided to leave her house to the RMACA in her will, on one condition: The organization had to feed and protect the feral cats that frequented the property until they died a natural death. Aubart agreed to become the caretaker of the colony.

When Aubart moved into the house in 2007, the place was almost uninhabitable. Feral cats roamed indoors and outdoors. There were no litter boxes. Aubart scoured, repainted, refinished. She turned a carport into a heated shed, decked out with shelves and cubbyholes and numerous exits, where she could feed ferals. And she set about trapping and fixing the cats, determined to transform the property into a model managed colony.

These days the house is presentable, if still a work in progress. Late in the afternoon, Aubart gets ready for the evening feeding, bustling from kitchen to living room, surrounded by three domestic cats and two dogs, including Little Man Tate, a small, vocal scamp of indeterminate breed, rescued months earlier in Mexico. In a pen are three feral kittens somebody asked her to foster, assuring her they were only four weeks old. They're not.

"These were dumped on me Saturday," she says. "They're eight weeks old. At four weeks, they're easy to socialize. But eight weeks is a little old. You need someone who's going to be holding them a lot, not someone who works twelve-hour days."

She picks up one of the kittens. It hisses, but tolerates her caress. "It sucks to be me," Aubart mutters.

As the sun sinks, the ferals begin gathering in the back yard. Four, five, six — more lurking in the shadows. Some have matted fur and resemble tufted rugs that have been left out in the elements too long. Others are sleek, black or gray, but clearly not in their prime. Aubart says she has about a dozen regulars, most of them ten years or older, well beyond the life expectancy of the typical feral.

Sparrow, a one-eyed snaggletooth, is at the front of the line. When Aubart first arrived, the other eyeball was still attached but dangling out of the socket. She has names for all of them, knows their individual quirks and temperaments: Carrot. Rainy. Pettie. Mattie. Lucas. "I love these cats as much as I love my own," she says. "They're not domesticated cats, but they're not wildlife. They're their own category."

Not all of the neighbors love the ferals. Many longtime residents are used to the cats that wander the alleys, but others have complained about unwanted visitors and deposits in their yards. Aubart plans to install mesh barriers above the privacy fence in her back yard to keep the cats contained there. But first she has to rip up the yard, which now is a sea of weeds and stickers, and reinforce the fence at ground level to keep the cats from digging out.

Aubart opens cans in the shed, dividing the contents among several plates. Sparrow and the others surround her. The crowd seems to double magically, as cats slip into the shed from two directions. A few still hold back, wary of the two-legged visitors. Some ferals can become accustomed to a single caretaker, enough to tolerate contact with that person — but no others.

Some want nothing to do with any human. Four ferals live in Aubart's house and never go outside. They are all failed experiments in socialization. Three of them the former owner tried to domesticate. "The other one is my fault," Aubart says. "You can only wait three weeks to get them back to their colony. I waited too long. Now I have a cat I only see every few weeks. He loves the other cats, but he hides from me."

Colonies such as this one are the only alternative to mass extermination of ferals, says Angelilli. She points to another successful colony in her own Five Points neighborhood, where one couple has trapped fifty cats over a period of three years and returned them. The neutered veterans now stay close to their feeding stations. "This is a community problem, and it needs a community solution," she says. "You can't depend on one small agency to manage the entire feral cat population of the metro area. It's unrealistic."

After all, cats are inherently social creatures, and over time, colonies can spill into each other. Managers have to be vigilant and keep an eye out for newcomers, especially if the left ear isn't notched. Aubart knows her shed is a beacon for wandering cats of all kinds, even if they have a home somewhere.

"I have nine regulars that I see every day," she says. "But I've also had seven new ones since February. Four of them are tame."