"We also showed sexually mature pandas videos of their peers having sex, which they could learn in the wild but not in captivity," Zhang said.

Another problem they faced was how to keep pandas active.

Wild pandas generally stay active for hours each day.

To emulate their natural environment, researchers tried putting biscuits rich in vitamins and trace elements in places the pandas could not find easily, aiming to get them to move around.

Many newborn panda cubs used to die because of a quirk bred by nature.

While 50 percent of newborns are twins, a mother typically chooses to care for only one.

In the wild, a mother panda first tries to care for both babies. But several hours later, she realizes she can't. If she tries to support both, both would die. So the mothers desert one of their cubs, even if it cries for her, Zhang said.

Researchers settled on a course that was part philanthropy and part trickery.

They would take away the deserted baby and feed it milk. Then they would switch it with the favored cub from time to time. In this way, the mother unwittingly supported both.

With the three primary obstacles hindering the breeding of captive pandas overcome, a self-sustaining and growing panda population can be developed, Zhang said.

huangzhiling@chinadaily.com.cn