Er Shun and the cubs are now under constant observation by giant panda experts from Chengdu Research Base of Giant Panda Breeding in China, as the next several hours and days are critical to their survival.

"We are so proud to be contributing to the ongoing survival of this endangered species," said John Tracogna, Toronto Zoo's chief executive officer, said in the press release on Oct 13.

For the next several months, the cubs will remain in the maternity area of the giant panda house, which is not open to the public. But zoo staff said they will try to provide regular updates on their progress.

At this time, the cubs' sex is not known yet and the zoo has not confirmed which panda is the father. The zoo announced that Er Shun was pregnant with two cubs a couple of weeks ago, saying the father could be any one of three pandas.

Er Shun underwent two artificial insemination procedures in May. One involved sperm from Da Mao, which came to Toronto from China with Er Shun in March 2013. The second involved frozen sperm from two giant pandas in China.

"Our researchers, veterinarians, and wildlife care staff are some of the best in the world, and we are grateful to the People's Republic of China for entrusting these endangered animals to us," said Toronto city councilor Raymond Cho, also the chair of the Toronto Zoo management board.

"We look forward to sharing our learning with scientists around the world in the hope this will help us save this endangered species," Cho added.

The gestation period for pandas is anywhere from 87 to 186 days. Breeding pandas is a difficult process. Female giant pandas are only receptive to breeding once a year for a period of 24 to 72 hours.

Da Mao and Er Shun arrived in Toronto amid much fanfare, and were greeted at the airport by Canadian Prime Minister Stephen Harper and his wife, Laureen.

Currently on loan as part of a long-term conservation breeding program, both giant pandas will be at the Toronto Zoo until 2018, when they'll move to the Calgary Zoo for five years.