Veterinarians dressed in navy-blue operating-room scrubs huddled around the motionless body of a clouded leopard named Bart. Clouded leopards are among the smaller of the 37 species of big cats. They're twilight stalkers distinguished by short legs, oversize feet and long, bushy tails outstretched for balance as they teeter on branches or run headfirst down tree trunks. They have the longest canine teeth relative to body size of any cat, and they use them to shred flesh.

But for now, Bart's 45-pound body lay anesthetized on a chrome operating table at a research center run by the National Zoo in Front Royal, Va. Over the course of half an hour, the vets collected a vial of Bart's sperm and handed it over to David Wildt. No more than a few dozen people in the world are versed in the delicate art of stealing semen from wild beasts. Wildt's name stands at the head of the list. Within minutes, he was examining Bart's sperm under a microscope.

Bart is one of a few hundred or so clouded leopards left on earth; the species may already be gone from the shrinking jungles of Southeast Asia. "We know absolutely nothing about this species in nature," Wildt said as he peered into the microscope. "They've lived in zoos for hundreds of years, but they rarely breed in captivity. If you put a male and female together, the male almost always walks right over and kills her."

As a result, clouded leopards, like many endangered mammals living in isolated populations, have become inbred -- and therefore woefully susceptible to birth defects, infertility and disease. What Wildt saw under the microscope confirmed what he had suspected: Inbreeding had left Bart with misshapen sperm. Eighty percent had curved tails, which meant they would swim in circles -- with no chance of carrying Bart's genes into the next generation.

David Wildt may be the last, best hope for endangered creatures like the clouded leopard. He stands in the vanguard of a campaign to save endangered animals by freezing their sperm and embryos. Some people think of him as zoology's answer to Oskar Schindler, a high-tech Noah in an era of mass extinction. "It's as if the National Gallery were burning down," says Michael Robinson, director of the National Zoo. "David Wildt is running in and saving as many treasures as he can."

Others are asking: Saving them for whom?

With his beard and khaki safari shirt, Wildt, 49, could pass as Ken Burns's rugged older brother. He operates from a scrupulously neat office perched on a broad, open hill in the zoo's 3,100-acre Conservation and Research Center in Front Royal. Fifty years ago, the Army bred cavalry horses on these sloping pastures overlooking the Shenandoah Valley. Today, in a complex of laboratories built within unassuming red-roofed barns, Wildt is summoning the tools of cryo-preservation -- the storage of living tissue in extreme cold -- to amass a frozen zoo, a 21st-century ark that offers hope of survival to species on the brink of extinction. It is the stuff of science fiction: Hundreds of six-inch glass straws loaded with sperm and embryos are chilled in vaporous freezers to minus 196 degrees centigrade, colder than the lunar night.

The frozen zoo's repositories, samples from some 300 species, are genetic time capsules. As long as the reproductive cells reside in their coolers, the donor species -- from Indian elephants to black-footed ferrets -- cannot go genetically extinct. These samples can lie in a suspended state for hundreds, and perhaps thousands, of years. Someday they could be used to restart a species long after it has disappeared from the face of the earth. In the meantime, Wildt and his colleagues at the National Zoo, along with cryobiologists at half a dozen other U.S. zoos, are drawing on the samples to increase today's animal populations and alleviate inbreeding by injecting genetic "booster shots" into isolated groups.

Wildt and his fellow cryobiologists are in effect racing against time: With animal species vanishing at a rate of some 30,000 a year, they believe it is crucial to freeze as much material as possible now, in hopes of reproducing the animals later. "I can't even begin to tell you the sense of urgency I feel," says Betsy Dresser, director of the Audubon Center for Research of Endangered Species in New Orleans. "It's like we're in the wildlife emergency room."

In his hopes for the frozen zoo, David Wildt is harking back to the long-deferred dream of William T. Hornaday, a Smithsonian taxidermist who founded the precursor to the National Zoo in 1887. While traveling across the American West, Hornaday had seen bison butchered by the millions. In response, he started what he hoped would become an expansive sanctuary where imperiled North American beasts might replenish themselves in the shadows of our national landmarks. He started with a small protective menagerie -- a bear, eagle, badger and several bison -- penned behind the Smithsonian Castle on the Mall. But his plan soon derailed. By the time the National Zoo officially opened in Rock Creek Park a few years later, Hornaday's sanctuary had given way to a gallery of animal curiosities chosen to thrill the crowds, and thereby justify funding from Congress and the District of Columbia. A circus donated elephants; Teddy Roosevelt provided a lion sent to him by the Abyssinian king. Hornaday, his plans for a breeding refuge fallen by the wayside, resigned. (He later joined the Bronx Zoo.)

In its early years, the zoo saw more deaths than births. It's not surprising, considering the keepers' ignorance of animal husbandry. As late as the 1950s, for example, the zoo veterinarian sought advice from a nearby drugstore pharmacist. No matter: The earth's wild places offered an inexhaustible supply of rhinos, apes and elephants -- or so it seemed. And zoos occupied a place at the margins of science -- until three researchers at the National Zoo dropped a bombshell.

In 1979, they published a paper in the journal Science reporting that zoo-born baby animals were dying at appalling rates -- as high as 41 percent. The reason: Zoos typically housed only a handful of animals of any given species, so captive breeding often meant incest. Incredibly, zoos had never kept detailed records of infant deaths. "We suddenly confronted the harsh reality that our captive populations wouldn't live," Wildt recalls. "We figured we'd better start studying reproductive biology, or we wouldn't have any zoos in 100 years."

Over the last 20 years, zoos have positioned themselves at the leading edge of conservation science. And over the last decade -- ever since in vitro fertilization with frozen sperm produced tiger cubs in 1990 -- cryopreservation has played a major role in keeping them there.

Adherents pitch cryopreservation as the future of conservation, but it might also be described as another step in man's conquest of nature -- reconstituting the "wild" in an array of petri dishes.

For all the reproductive technology involved, the procedure begins with the messy job of collecting semen. In most cases, as in Bart's, this is achieved through electroejaculation, which was originally developed to help paraplegic men father children. An electric probe is inserted in the donor animal's anus and delivers, over half an hour or so, 80 low-voltage charges to stimulate the nerves around the donor's ejaculatory duct. The alerted penis obliges with a pulsing ooze of semen.

Electroejaculation works for most donors, from antelope to zebras. Extracting sperm from a four-ton African elephant bull, however, requires a hydraulic device to hold the bull still, a plastic bag and two trainers to perform what is politely called the "manual stimulation technique." Keepers have it easier with gorillas and chimps; they can be trained to perform the manual technique for themselves and ejaculate into a cup.

Eggs are trickier to collect than sperm, and they're all but impossible to freeze without rupturing their delicate membranes. Consequently, cryobiologists have learned to mix freshly retrieved eggs with thawed sperm, then freeze the resulting test-tube embryos for later use. When there is a shortage of females of a diminishing species, embryos can in certain cases be thawed and implanted into surrogate mothers of a closely related species. A domestic horse has carried a zebra embryo to term. An ordinary house cat has given birth to a rare Indian desert cat. A Holstein cow has delivered a wild ox known as a guar.

Cryobiologists hope to broaden the range of foster species so that a number of threatened or even extinct animals can gestate inside more common ones. In the most ambitious interspecies experiment to date, Betsy Dresser flew to Kenya last year with liquid nitrogen canisters containing 20 frozen embryos extracted from captive bongos, white-striped antelopes now extinct in parts of their native Africa. At a 1,000-acre ranch owned partly by the actress Stephanie Powers, Dresser led a team of biologists who implanted 16 of the week-old embryos into eight wild female elands, a more populous antelope.

It marked the first time frozen embryos had been transported from one continent to another and introduced into animals in the wild. Dresser nicknamed the project "Herd in a Thermos Flask." Although it resulted in no pregnancies -- each eland's uterus had to be at the right stage of its reproductive cycle to accept the embryos -- Dresser is undaunted. "Someday the frozen zoo will be used to move wildlife around the world," she says. "Imagine carrying a microscopic elephant and rhino in your luggage." She is planning to return to Kenya next spring for another try, this time with as many as 80 frozen bongo embryos.

If Dresser's surrogacy scheme works, she will have established a new way of reintroducing endangered animals to their natural habitats. Some animal rights activists and environmentalists are critical of current efforts to release captively bred animals to the wild as wasteful of individual creatures, many of whom, like four lynxes released in Colorado last spring, die before they learn the skills to survive outside their cage. By contrast, Dresser's newborn bongos will presumably learn to forage on the African plains from their eland mothers; they'll also acquire their mothers' immunities. "This is a way for baby animals to be conceived in captivity," Dresser says, "but born in the wild."

Dresser also hopes to use surrogacy to rescue the mountain gorillas of war-torn Rwanda and the Congo (formerly Zaire). "They're being shot for food," she says. "It may be too late if we don't act soon." If local authorities permit her, she'll retrieve sperm and eggs from mountain gorillas in the wild, then implant the resulting embryos in captive lowland gorillas. Dresser says several women have volunteered to carry gorilla embryos. It's an option she won't rule out.

Reproductive cells aren't the only animal parts on ice. Cryobiologists are also filling freezers with pea-size scraps of skin tissue. The DNA in the skin cells may someday be used to clone the animals they came from. A Japanese biologist has already embarked on a quest to clone a woolly mammoth from snatches of hide preserved in Siberia's Ice Age sediment, in the hope of resurrecting a fabled creature that last walked the earth 11,000 years ago. Australian geneticists may try to clone the Tasmanian tiger, a marsupial wolf extinguished in the 1930s, from a cub preserved in a jar.

David Wildt rolls his eyes when he's asked how soon such efforts might succeed. "Cloning provokes a `gee-whiz' in

the newspapers," he says, but he downplays its benefit, because it simply duplicates a single animal's genes. It is the ultimate case of inbreeding.

Cryopreservation, on the other hand, may become an insurance policy against extermination -- but for now, it isn't close.

For one thing, the biology of sex varies considerably from one creature to the next. The formula of freezing and thawing that works for the clouded leopard will not work for, say, a Humboldt penguin or Suni antelope. Conditions differ even among strains of the same species. So biologists like Wildt are obliged to laboriously unlock the cellular and hormonal secrets of each species as they go, consuming years of research and precious funding. "There's only a handful of us," Wildt says, "so we have to be creative."

To appreciate the magnitude of the job, consider this: It took the cattle industry 30 years to perfect artificial insemination for cows. Hundreds of scientists spent millions of dollars on the project -- for just one procedure used with one species. Wildlife biologists understand the basic reproductive biology of fewer than 100 species, with more than 2,000 endangered candidates languishing in the waiting room.

"Until you can freeze and thaw with confidence that you'll end up with viable semen, freezing doesn't do any good," says Lee Simmons, director of Omaha's Henry Doorly Zoo, which has frozen 20,000 samples from some 30 species. "You might as well be freezing carrots. You can freeze a rock, but it won't get anybody pregnant."

Given the numbers, there are murmurs within conservation circles that Wildt and other cryobiologists should scale down their ambitions.

Michael Hutchins, director of conservation and science for the American Zoo and Aquarium Association, believes that Wildt and company should focus on just 10 species. "Cryopreservation is no panacea," he says. "I doubt we can save species by putting them in a frozen can. We don't have a hell of a lot of time, and we've got limited dollars. So far, not a single species has been saved by freezing sperm."

Other, more behavior-minded biologists question whether high-tech intervention is necessary. Endangered animals that mysteriously fail to breed in captivity -- such as rhinos, pandas and cheetahs -- may be more likely to breed on their own as zoos grow more sophisticated about diet, behavior and surroundings, according to Jill Mellen, a research biologist at Disney World's Animal Kingdom.

Captive cheetahs, for example, have long been notoriously difficult to breed -- in part because their keepers housed them together. Nobody realized until recently that females travel solo in the wild, and are more apt to reproduce when they're separated from the hormonal interference of competing females. It also helps to give them a choice of males. "By God, that was it!" Mellen says. "Now we've got a cheetah baby boom on our hands."

Beyond biology, cryopreservation has raised concerns about commerce. One hundred and eighty-four North American zoos participate in the Species Survival Plan, a meticulously detailed dating service maintained by the American Zoo and Aquarium Association -- in effect, a blueprint for maintaining genetic diversity within captive populations. These zoos now trade sperm and embryos freely, but they have not yet resolved the question of who gets custody of the offspring. According to one alarmist scenario, Disney could use its deep pockets to amass a genetic stockpile on behalf of its Animal Kingdom. If so, Disney would effectively "own" the rights to the live reminders of a vanished world.

And beyond biology, beyond commerce, there lies the question of why humans are spending so much effort to preserve species if animal habitats are disappearing from the planet. Is a clouded leopard still a clouded leopard if we consign it to a life in captivity, and it never learns how to stalk its prey by night? "We may be able to reproduce the animals, but we can't reproduce their culture and behavior in a petri dish," says Vicki Croke, who writes a column on animal issues for the Boston Globe. "Maybe we're just playing God."

Maybe so. But a freezer full of animals is still better than no animals at all, Wildt responds. If cryopreservation buys some time, then so much the better. "We're just trying to maximize our options," Wildt says, "and minimize our regrets."

After inspecting Bart's freshly extracted semen, Wildt got in his car with a burly veterinary endocrinologist named Steve Monfort. They drove up the hill to inspect their herd of 34 Eld's deer, a skittish, reddish-brown cousin of our white-tailed deer. The Eld's once bounded among the forested hills of Cambodia, Laos, Vietnam and Burma. Over the last 10 years, the wild population has dropped by 40 percent, to 2,000. Wildt slid a barn door open to reveal a gang of Eld's deer warming themselves under heat lamps. In one hay-strewn corner crouched a trembling fawn, one of 17 Monfort has produced by artificially inseminating females with frozen sperm.

The Eld's deer is a cryopreservation success story. After four years of painstaking study, Monfort unlocked the mysteries of the deer's sex life -- how to read cycles from urine, how to trigger ovulation with hormones, how to gauge sperm production from spring antler growth. He now conducts artificial insemination with a 40 percent success rate -- about the same as human fertility clinics -- and thus helps protect the genetic diversity of the six herds of Eld's in the United States. "It's no longer about simply producing more offspring," Monfort says. "The goal is to manage the deer's gene pool. Genetic diversity is our gold standard."

The Eld's program has been so prolific that Wildt is starting to wonder where to house the progeny. It's no small problem. Overcrowded and underfunded, U.S. zoos collectively have room to keep no more than 1,000 animals for breeding purposes. Monfort and Wildt recalled a curator at a Canadian zoo who was blackballed for euthanizing superfluous tigers. In the odd logic of conservation these days, keepers want to save animals -- but not too many of them.

The frozen zoo allows keepers to maintain the all-important genetic diversity without having to care for live animals. "If you can produce offspring half the time you do artificial insemination," Monfort says, "do you really need to have a herd around anymore? Once you have an animal that has performed his duty genetically, what do you do with him?"

Faced with declining habitats and the disappearance of species and low levels of funding, Dave Wildt is pressing on, assuming the conservation equivalent of Sophie's choice: which species to save, and which to abandon. In truth, it may be the public that decides.

Zoos are the public face of natural history, but they're also entertainment venues. More than 120 million people visit U.S. zoos every year -- more than attend all professional sporting events. The crowds come to gawk at marquee mammals -- lumbering elephants, snarling tigers and cuddly black-and-white pandas sweetly nibbling on bamboo shoots. The fuzzy and picturesque creatures (known within the field as "charismatic" species) have guaranteed places in the frozen zoo. There will be no room on this ark for critters that don't spin the turnstiles, like Puerto Rican crested toads and Partula snails.

Where will zoos procure their crowd-pleasers when their habitats have all been plowed under or developed? The answer: They'll produce their own. Already 90 percent of all zoo mammals are born in captivity. Whatever its shortcomings, cryopreservation is a potent tool for zoos facing a future in which they'll have to be self-sufficient.

Wildt has already made five trips to China to help local biologists freeze sperm and embryos from the highly endangered giant panda, the cutest and most coveted of all zoo attractions. Less than 1,000 pandas remain scattered in 32 isolated reserves. "It's a two-way street," Wildt says. "The Chinese need our biomedical expertise, we need their sperm. Every zoo in North America wants a giant panda. Eventually, there'll be plenty of giant pandas for everybody."

In his book Ethics on the Ark, Dale Jamieson, a professor and specialist in global change at Carleton College, discusses our "moral schizophrenia" that "drives species to the brink of extinction and then romanticizes the remnants . . . We should have the honesty to recognize that zoos are for us rather than for the animals." In other words, why save the panda -- or any animal -- if we can't save its habitat? What would we be saving it for, except our own pleasure?

Wildt considered these questions as he drove his car past a Przewalski's horse and a scimitar-horned oryx, impossibly exotic beasts grazing incongruously in their Virginia paddocks. Wildt is generally reluctant to step outside the scientist's role, and he paused to compose his response. "It's a mission of the heart as well as the mind," a colleague answered for him from the back seat. "We can't contemplate a world without tigers and elephants and rhinos." Wildt finally nodded his agreement, squinting into the dusk overtaking the valley.

Michael Cannell, a freelance writer who lives in New York, last wrote for the Magazine about a multibillion-dollar campaign to build federal courthouses.