SAN DIEGO -- San Diego Zoo officials hope the floodgates of passion will be opened along with the barrier that now separates the giant pandas Gao Gao and Bai Yun.

Researchers believe 12-year-old Bai Yun is in estrus, her brief yearly fertile period, and Thursday will open the gate that has separated her from her would-be mate, new zoo resident Gao Gao.

The pair won't be on view yet, though 3-year-old Hua Mei will be on display in an adjoining exhibit.

The gate between Bai Yun and the roughly 11-year-old Gao Gao has so far offered "protective contact," and given researchers hope of a natural mating, said the zoo's Yadira Galindo.

"What our researchers have been seeing is interest between both the male and the female," Galindo said. "They're coming to the gate, smelling each other, just continually going back there making sure the other is still there."

Any first contact Thursday would be a trial run, as researchers don't think Bai Yun is ovulating yet.

The pandas will get to know each other and possibly "practice" mating before Bai Yun actually begins ovulating, Galindo said.

They surely can't do any worse than Gao Gao's predecessor Shi Shi, an older panda who met Bai Yun's advances with a growl. Since Shi Shi was uninterested, Hua Mei, born in August 1999, was the product of artificial insemination.

The gate on the otherwise solid border between the pandas' enclosures was added after Shi Shi returned to China in January, while Gao Gao was in mandatory quarantine.

Gao Gao emerged from that quarantine late last month, in plenty of time for Bai Yun's rather regular estrus cycle. Signs of estrus include more scent marking, vocalizations and interest in the male; Bai Yun's urine is also being monitored.

The pandas are here as part of a 12-year research loan. Researchers want to learn more about the mating habits of the solitary animals.

Their low reproductive rate is one of the reasons the giant panda is endangered, with only about 1,000 living now. Other factors are habitat destruction, bamboo shortages and being caught in traps meant for other animals.

Hua Mei is scheduled to be returned to China in June.