Li Li the giant panda stood up on his hind legs to try and gobble up the bamboo that was hovering on the end of the pole above his head.

As the food was moved this way and that, the panda would stagger after it, like a human with two left feet.

It was all part of giant panda copulation training--the aim being to strengthen the hind legs so the panda doesn't fall over during one of its rare bouts of lovemaking.

In Li Li's case, it seemed to work because he is the father of Ueno Zoo's latest star, panda cub Xiang Xiang.

It was a first for the zoo in that Xiang Xiang was born of natural breeding without direct human intervention.

“The success of the natural breeding at a zoo is a major achievement,” said Yutaka Fukuda, director of Ueno Zoo.

In fact, the successful breeding without recourse to artificial insemination came only at the cost of such unsung training efforts.

Li Li started the muscle-building exercise in January 2012. Shin Shin, Xiang Xiang's mother, also participated in the same program. Continued training made both strong enough to maintain their footing during copulation.

It is anything but easy to breed pandas. The estrus, or period of sexual receptiveness, in the females lasts only between half a day and two days in an entire year so zookeepers need to get the timing right for when the pandas are up for a passion session.

Zoo officials also changed the diet for the pandas as well, making sure that 20-30 kilograms of bamboo are served in five to six small portions to prevent spikes in blood sugar levels and inhibit sleepiness. This keeps the pandas awake and in motion longer, which makes them physically stronger. That is much more preferable than allowing the animals to just eat and sleep.

Zoo officials also prepared for artificial insemination just in case, but that proved unnecessary in the end.

“Anesthesia has its own risks, and it would also have lowered the possibility of breeding,” Fukuda said. “We hope to extract the potential abilities of the animals and sharpen our own powers of observation so we will be seeing a second and third panda cub.”