Loonie, a rare baboon, may never be able to breed naturally.

Diabetes prevents Loonie, one of only 17 African drills in captivity in North America, from living with females in a wild-animal park.To Barbara Durrant, a reproductive physiologist at the San Diego Zoo's Center for Reproduction of Endangered Species, a sample of his semen offers hope for saving another one of the more than 1,000 species of mammals, birds and reptiles now threatened with extinction.

After being treated with a protective agent, Loonie's semen will join the center's "frozen zoo," or "20th century ark." This zoo within a zoo is housed in a pair of 39-inch-square metal tanks, where more than 2,500 samples of sperm, ova and embryos of endangered animals are chilled by liquid nitrogen to minus 196 degrees Celsius (minus 385 degrees Fahrenheit).

In theory, the cryopreserved sperm and eggs may be used to ensure genetic diversity in small captive populations by breeding species from different regions of the world or from widely separated generations. Frozen embryos also may be implanted in surrogate mothers of a closely related species, or stored until a recipient mother is found.

But before such practices become commonplace, scientists at the San Diego Zoo and a half-dozen similar facilities face more painstakingly slow research. While artificial insemination and embryo transfer have long been used successfully in domestic cattle, comparatively little is known about the reproductive physiology of wild animals.

"Until we can understand the basic physiology of reproduction (of wild animals), we can't manipulate it," Durrant told National Geographic. As she places Loonie's sample inside the freezer, a smoky nitrogenous cloud escapes.

Although there have been a few successes, "we don't know enough yet to make artificial reproduction a routine part of captive-breeding efforts," Durrant says. She and colleagues at other zoos hope to develop artificial breeding programs that could be widely used as part of the broader effort to save certain endangered species by breeding them in captivity.

Durrant's work focuses on semen-freezing techniques, using a small group of animals that includes cheetahs, Asian elephants, koalas, clouded leopards, Chinese Monal pheasants, margays and tapirs. Unusual behavior often dictates which species receives her attention.

"We are working to artificially inseminate a female clouded leopard from southeastern Asia, because 50 percent of the time the male will kill the female he's breeding with," she explains.

Such artificial-breeding techniques are not needed when animals can breed naturally, says veterinarian Werner Heuschele, the center's research director. In these cases, zoo officials concentrate on building up captive populations and then reintroduce the animals into preserves in their native habitats.

"Things have changed drastically from 30 years ago, when zoos had postage-stamp collections of only one or two animals from a single species," Heuschele says. "Now zoos are becoming sources of animals for reintroduction."

But captive breeding and reintroduction programs have risks. Heuschele recently returned from Saudi Arabia, where a reintroduced population of Arabian oryx is threatened by disease. On the brink of worldwide extinction in 1960, the buff-colored antelopes were successfully bred in several zoos, including San Diego's, over the following two decades, then transferred to preserves in the Middle East.

Their numbers have continued to increase. But in 1986, 15 Arabian oryxes in Saudi Arabia died from tuberculosis. With the help of Heuschele and others, the disease has been controlled.

"We must be sure that animals being reintroduced into their native habitats are free of disease, as well as animals that already exist there, so that they don't infect the reintroduced population," says Heuschele. "Otherwise, it can negate the whole success of captive-breeding programs."

At the Cincinnati Zoo's Center for Reproduction of Endangered Wildlife, which focuses on artificial breeding, director Betsy Dresser has pioneered the use of embryo transfer and in vitro fertilization in zoo animals. She has discovered that many of the techniques used for humans can be applied to animals.

Since the beginning of her program in 1981, Dresser has achieved a number of successes. In 1984, a surrogate eland antelope gave birth to a bongo antelope as a result of embryo transfer. Soon after, an eland calf became the first exotic animal born from a frozen embryo. More recently, the zoo successfully delivered a rare Indian desert cat from a surrogate domestic cat.

"Now we're attempting to determine the relationship between domestic and exotic cats, to see if we can develop the domestic as a universal surrogate for embryos from small exotic cats," Dresser says. "If we can use this technology to prevent one or two species from going extinct, it has served its purpose."