If Da Mao had been in the mood, it would have been a lot simpler to get Er Shun pregnant.

But the young male panda had no interest in becoming a father, and so the intimate moment between the Toronto Zoo’s two giant pandas was replaced by a special shipment of panda semen airlifted directly from China.

“It’s the first time that the Chinese have ever sent giant-panda semen into North America,” said Maria Franke, the Toronto Zoo’s curator of mammals.

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In a midnight operation last Sunday, Er Shun was artificially inseminated, a Canadian first. It’s unknown who the father might be — two pandas in China contributed the sperm.

Border regulations and the unpredictability of panda biology created a narrow window for the zoo to try to bring another one of these endangered animals into existence.

This was the first time 6-year-old Er Shun, who arrived at the Toronto Zoo last spring, had been in heat.

“She’s a young, unproven female,” said Gabriela Mastromonaco, the Toronto Zoo’s curator of reproductive programs.

Since January, Mastromonaco has been monitoring Er Shun’s hormones to anticipate when she would ovulate — which female pandas do only once a year.

After ovulation, the egg lasts for only about 24 hours. When the signs appeared Sunday morning, 17 veterinarians and zookeepers scrambled to respond.

First they tried to get Er Shun to mate with Da Mao. The 5-year-old male panda is still on the cusp of sexual maturity — Mastromonaco likens him to a teenager. Apparently he hasn’t learned about the bears and the bees, because he ignored Er Shun’s scent and mating bleats, which sound “like a little sheep.”

“You would not expect a bear to make sounds like this,” said veterinarian Simon Hollamby.

After an hour without progress, the keepers intervened. Er Shun was anaesthetized and artificially inseminated twice that evening with the help of Liu Yu Liang, a seasoned Chinese reproductive biologist who had arrived with the semen two days earlier.

Franke said the zoo had anticipated Da Mao might not be mature enough and started seeking sperm from China six months ago.

“I was jumping through hoops to get semen here,” Franke said.

The pandas’ endangered status makes it very difficult to transport body parts — or fluids — across international borders.

Franke had to seek permission from Er Shun and Da Mao’s respective home zoos and the Chinese Association of Zoological Gardens, working with the Canadian embassy in Beijing.

When a flight was finally booked, they faced last-minute problems: first, Franke had to convince Air Canada and Air China to transport the tank holding the liquid nitrogen-chilled semen.

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Then she had to win over the Canadian Food Inspection Agency, which initially refused to allow the semen to enter the country. When Liu touched down at Pearson on Friday night, a CFIA official was waiting to inspect the canister’s safety seal and open it with a pair of bolt cutters.

Now the zoo can only wait. The success rate of artificial insemination in pandas is only 60 per cent, but Liu said he is optimistic.

“I’m confident. I do many, many times. Especially here, good team,” Liu said in careful English.

While the bears’ gestation period is only about one and a half months, the fertilized egg does not immediately begin to develop. It can float in the female panda’s reproductive tract for up to five and a half months.

Newborn pandas are so tiny — only weighing about 200 grams at birth — that Er Shun won’t develop any kind of baby bump. The embryo will only show up on ultrasounds about 18 days before birth.

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