Tian Tian, the National Zoo's male giant panda, didn't look that fit Thursday, as the dignitaries sitting nearby signed a new agreement extending his stay in Washington for five years.

The hefty bear sat on a rock, his head buried in a cardboard box, scarfing a fruitsicle. But the days of total leisure for the 264-pound munching machine are over. He is already in training. Working out. Getting ready.

For love.

A major focus of the new agreement between the zoo and Chinese wildlife experts that extended the stay of both pandas is why Tian Tian, 13, and his mate, Mei Xiang, 12, have produced only one cub during their 10 years in Washington.

Giant pandas in China and in other U.S. zoos have done better. Pandas at the San Diego Zoo, for example, have produced five cubs in recent years.

Part of the problem here, zoo scientists have observed, is the Washington pair's imprecise mating techniques.

Their encounters look more like a college wrestling match than a romantic meeting, the zoo says. Tian Tian gets points for the takedown, but nothing else.

The two have never mated naturally with success, the zoo said. Their lone offspring, Tai Shan, was born in 2005 via artificial insemination. Female pandas generally can breed only once a year.

The zoo said indications are that Mei Xiang may be about to go into heat.

So, in consultation with Chinese "practitioners" experienced in panda mating, experts are trying to get Tian Tian in top shape, hoping to improve his moves.

Keepers have been training him to do leg-strengthening exercises by inducing him to stand up more often.

"He's being asked to stand up as tall as he can," zoo director Dennis W. Kelly said, after watching some of Tian Tian's regimen his week. "Our Chinese colleagues think that having his legs as strong as possible might make a difference."