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."

Brandie Smith, a senior zoo curator, said: "We're getting him in shape. We're building up his stamina. . . . I think Tian is in pretty good shape, but, you know what, we're turning him into an Olympic athlete."

"We would love to have them breed naturally," she said of the pair. "And if there's anything we can do to enhance that success, then we're going to try it."

She said she had heard that some institutions have even tried showing inexperienced pandas films of mating - so-called "panda porn."

"Nothing that we've learned indicates that it might help our pandas reproduce," she said. "And so we haven't considered that as an option for our pandas right now. But you know, ask me again in a couple years."

Mei Xiang, who weighs 233 pounds, is also in training, of sorts. Keepers have been teaching her to lie across a large log in the giant panda enclosure to try to improve her positioning. She tends to get flattened by Tian Tian's advances.

"We don't know if it will work," Smith said. "But it's one of those . . . necessity is the mother of invention."

As zoo scientists wait for breeding season, they are also trying to keep noise down in the panda enclosure and limit the amount of light that gets into the compound at night.

These are just some elements of what the zoo said will be a big effort over the next two years to help the pandas breed, or find out why they can't.

And if they can't, the new agreement suggests, they could be replaced by pandas who can.

The goal is to increase the population of what Interior Secretary Ken Salazar said Thursday is one of the most endangered species on earth. There are believed to be only about 1,600 giant pandas surviving in the wild.

"What we've got here is a wonderful blend of scientists, both on the American side and on the Chinese side, who are dealing with endocrinology and reproductive science," zoo director Kelly said.

"But we also have [Chinese] practitioners who've dealt with dozens and dozens of [giant panda] pregnancies that can give us tips and insights on what techniques, what routines, have worked with . . . other matings and pregnancies."

"There's some science and there's some art, and we are getting the best of both in this study," he said. "We're strong on science, they're also strong on science, but they're also strong on the art and practice."

Thursday morning, as Kelly emceed the signing ceremony, he said the zoo and its Chinese counterparts were inking an agreement on "the cooperative research and breeding of the giant panda."

"Mei Xiang," he joked, indicating the panda snacking nearby, "did you hear breeding?"