She's about 80 years old and the last of her kind. He's 100. They aren't the most likely candidates for parents, but they are their species' last hope.

In this photo released by Wildlife Conservation Society, a female soft-shell turtle rests near a pool inside a zoo in Suzhou, China, May 9, 2008. Breathless scientists watched as they successfully mated. But the attempt to breed an endangered turtle's last known female with China's last known male has failed because the eggs didn't hatch, disappointed conservationists say. [China Daily via Agencies]

When the two mated last spring, scientists were hopeful that their attempts to keep the rare Yangtze giant soft-shell turtles from fading into extinction would be a success. But conservationists announced this week that the turtles' eggs didn't hatch and the breeding had failed.

As the world's most threatened turtle species, the news is disappointing, but scientists say the elderly pair can try again next year.

Just four of the turtles are left, and three are male. The only female was found in a Chinese zoo just last year after a desperate search.

The turtles' situation, conservationists say, couldn't be more critical.

Conservationists were thrilled this spring when the female and male were introduced, nudged each other curiously and mated. Artificial insemination had been deemed too risky.

Within weeks, two clutches of eggs, dozens altogether, were found in the sandy nesting area at the Suzhou Zoo, about an hour's drive west of Shanghai. Conservationists predicted possible hatchlings by early August.

"Unfortunately, none of the eggs successfully hatched this time," Stephen C. Sautner, a communications official with the US-based Wildlife Conservation Society, said in an e-mail Saturday.

In this photo released by Wildlife Conservation Society, scientists transfer a female soft-shell turtles for mate at a zoo in Suzhou, China, May 6, 2008. Breathless scientists watched as they successfully mated. But the attempt to breed an endangered turtle's last known female with China's last known male has failed because the eggs didn't hatch, disappointed conservationists say. [China Daily via Agencies]

While more than half of the eggs seemed fertile, the embryos died early, a statement released Wednesday by the US-based Turtle Survival Alliance said.

"A number of the eggs had very thin or cracked eggshells, suggesting that the diet of the animals prior to breeding was not optimal," the statement said.

Once found the only female left, the turtle was quickly placed under close watch at Changsha Zoo with a surveillance camera, a guard and bulletproof glass.

A team of experts from the US and China then prepared her for the potentially stressful move to Suzhou Zoo, about 600 miles (965 kilometers) away.

"I hate to call this a desperation move, but it really was," Rick Hudson, co-chair of the turtle alliance, said at the time.

Now, settled together in Suzhou, the two turtles are preparing for another mating attempt next year with a high-calcium diet of whole fish, whole crayfish and chicken necks, meant to result in eggs with stronger shells.

In this photo released by Wildlife Conservation Society, Dr. Paul Calle, WCS Director of Zoological Health, center, provides a ultrasound scan on the female turtle at a zoo in Suzhou, China, May 5, 2008. Breathless scientists watched as they successfully mated. But the attempt to breed an endangered turtle's last known female with China's last known male has failed because the eggs didn't hatch, disappointed conservationists say. [China Daily via Agencies]

During a visit to the zoo Saturday, the turtles were nowhere to be seen. Their watery home was split by a metal gate to keep the male and female separate until the next breeding season. The male can be too aggressive otherwise, workers explained.

"We've worked very hard on this," said Liu Jinde, the director of the organization that manages the zoo. "Wait until next year. We ought to succeed. The turtles are very healthy."

He said one reason breeding failed this year was the rush to get the turtles mating.

Though it isn't clear how long the turtles can live, the female's fertility is the key. Conservationists had been relieved when the newly discovered female at 80 years old or more was still producing eggs, if unfertilized ones.

Now the turtles' supporters must wait until spring, when the female should be ready for the next attempt.

The other known Yangtze turtles are two males in Vietnam.

(China Daily via Agencies October 6, 2008)