In what could be an important step toward stabilizing the world's population of endangered Asian elephants, German researchers say they've hit on a way to freeze elephant sperm without destroying its viability.

Elephants are notoriously tricky to breed in zoos, with high rates of miscarriages and infant mortality. Artificial insemination (AI) is possible, but fresh sperm is delicate and can be damaged in transport. And in addition to being costly and logistically tricky, bringing elephants from other zoos to do the job can be socially disruptive to the animals and risks spreading disease, particularly a lethal strain of herpesvirus.  A possible solution, experts say: frozen supplies of quality sperm. Suzan Murray, the head veterinarian at the National Zoo in Washington, D.C., says an effective cryopreservation technique "will revolutionize our ability to breed and maintain elephants in zoos."

If zoos have a supply of frozen sperm on hand it will give them much more flexibility in terms of where and when they perform inseminations, upping the chances of success.

The latest research, reported in the journal Animal Reproduction Science, was conducted by a group at the Leibniz Institute of Zoo and Wildlife Research (IWZ) in Berlin, Germany. In 1998 IWZ scientists performed the first successful artificial insemination in an African elephant at the Indianapolis Zoo. They also took part in another zoological coup: the birth last fall of the first test-tube white rhinoceros at the Budapest Zoo in Hungary.

Although scientists have had success cryopreserving the sperm of African elephants, the reproductive cells of their Asian cousins are much harder to freeze. That’s because the African variety have a tougher shell composed of different fatty acids that better resists the ordeal of deep cold.

In the new study, the IWZ team led by Thomas Hidebrandt evaluated several methods of freezing Asian elephant sperm using various combinations of chemicals to protect the reproductive cells, along with centrifugal techniques to separate them from the semen.

Study co-author  Robert Hermes, a veterinarian, says  researchers found that elephant sperm remain most viable when they are spun in test tubes containing a chemical at the bottom called iodixanol  that shields the cells from physical damage. The sperm also do best in rapid, "directional" freezing, a method that employs a frigid metal block, instead of liquid nitrogen vapors used in the conventional freezing process—to encourage orderly formation of ice crystals in the frozen material.

With that protocol, slightly more than half of sperm tested appeared normal under a microscope, nearly 60 percent could swim forward after thawing and nearly 70 percent were considered viable, meaning their membranes were intact.

Prior to the latest work, "no one was really able to freeze Asian elephant sperm so that enough of the cells were alive and moving," Hermes says.

But the real proof will be in the next test: whether the cells can bring a female elephant a visit from the stork. Hermes says he and his colleagues are now planning to perform AI with sperm frozen and thawed through their technique, a test they don't want to rush.

"An elephant AI takes a lot of effort and planning," he says. "You don't play around with this if you're not really sure that you have semen that can do the job."

Behavioral ecologist Michael Hutchins, executive director and CEO of the Wildlife Society, says a better method of preserving elephant semen could potentially boost the genetic variability of both captive and wild populations.

"It's important to be able to have access to a number of different males' reproductive cells to maintain genetic diversity," Hutchins says.  Because elephants are so large, individual zoos cannot handle more than a few of them. As a result, the captive population tends to be geographically far-flung—making traditional mating options narrow. The availability of viable frozen—and therefore shippable—semen could help to eliminate that problem, Hutchins says.

And, Hermes adds, because frozen sperm can be screened for their chromosomal makeup, the approach also could allow zoos to better balance their ratios of males to females through sex-selected fertilization.