Preventing Extinction: The Miracle of the Frozen Zoo

On a sunny spring afternoon, the San Diego Zoo is teeming with shorts-clad tourists of all ages. While most visitors gravitate toward the pandas, giraffes, and gorillas, one little boy seems particularly taken with the Javan bantengs, a species of endangered Southeast Asian wild cattle that can grow to be seven feet long and weigh nearly a ton. Asked which one is his favorite, the child sizes up each of the animals before settling on a male with a dark blue-black coat grazing closest to him. It happens to be the spitting image of another banteng that died in 1980, and the resemblance is more than superficial: The four-year-old animal at the zoo is its clone.

The banteng wouldn't be alive if it weren't for a satellite program of the San Diego Zoo located 35 miles north of the city, in Escondido. It too houses an impressive collection of exotic animals, but there are no tourists milling about here. Tucked in a corner room on the first floor of the zoo's Center for Conservation Research, its inhabitants -- not entire animals, but samples of their sperm, eggs, embryos, tissue, and other cells -- are cryogenically preserved. Inside one of its thousands of vials, which are stored on tall racks and kept in huge cylindrical stainless-steel freezers, are cells from the banteng that scientists preserved 27 years ago.

Welcome to the Frozen Zoo, perhaps the world's largest repository of genetic samples from endangered species. For the past three decades, scientists have relied on its collection to carry out a variety of critical conservation and research efforts. And with the number of endangered species on the rise, its leaders hope to one day stockpile samples from virtually every type of animal on Earth.

When geneticist Oliver Ryder and his colleagues began collecting samples for the Frozen Zoo 31 years ago, they had no idea how essential these actions would be to saving endangered species. The project was the brainchild of pathologist Kurt Benirschke, who led the Frozen Zoo until Ryder took over 15 years ago. These days, Ryder sports a neat salt-and-pepper beard, wears a button-down shirt and slacks, and socks with his sandals. His serious demeanor drops occasionally when, without changing his facial expression, he cracks a joke to lighten the mood. But he's all business as he explains that he began freezing cells because of all their practical uses. Zoos often held only one animal of an extremely rare species, he says, and when that creature died, scientists lost the opportunity to study it. Ryder was also interested in studying the chromosomes of various animals to determine how closely related they were to each other (a science that was cutting-edge at the time). So he and his colleagues decided to freeze and stockpile viable cells at every opportunity to save them for later studies.

Over time, advances in technology allowed the preserved cells to be used for a wider variety of research and conservation activities. Today, the Frozen Zoo is a multinational research facility that stores samples from more than 7,200 animals representing some 675 species. Hundreds of scientists across the globe rely on its resources in their work to save endangered and threatened animals, and for a host of other purposes. The facility is run out of the department of Conservation and Research for Endangered Species (CRES), a center funded by the nonprofit Zoological Society of San Diego, along with grants from many other institutions.

Although the cells can be used for cloning, the zoo's purpose isn't to create carbon copies of endangered or extinct animals. In fact, it's quite the opposite. "What we're trying to conserve is the genetic diversity of species," says Ryder, which gives animals the best chance of survival in the wild.