BEIJING  Wait until next year.

Scientists trying to save one of the world’s most endangered species of freshwater turtles say waiting is their only recourse after a complicated attempt to mate two elderly turtles during this year’s breeding season ended without producing any offspring.

The fate of the Yangtze giant soft-shell turtle seems especially uncertain because only one female is known to exist  an 80-year-old turtle with a leathery shell that lived without notice for a half century inside a zoo in Changsha, the capital of Hunan Province, in southern China. Only when scientists discovered her existence last year did it become clear that a chance remained to save her species.

In May, scientists drove her more than 600 miles to a zoo in the city of Suzhou. There, a male turtle estimated to be 100 years old awaited her. He had been the last known male of the species, though in recent months scientists discovered two more males in Vietnam.

Gerald Kuchling, a prominent herpetologist helping to oversee the mating program, said the male and female turtles were introduced to each other on May 7.