The road to parenthood for Ying Ying and Le Le took more than 10 years, and throughout the decade they had to endure numerous false pregnancies and miscarriages.
The panda pair, which arrived in Hong Kong as a gift in 2007, began mating naturally between 2010 and 2011, with zoologists also attempting rounds of artificial insemination.
But none of the attempts have been successful, including one in 2015 when Ying Ying and Le Le were brought to Wolong in Sichuan province as part of a national mating program in which pandas are encouraged to breed naturally or are artificially inseminated.
Ying Ying became pregnant as part of the program, but the fetus was ultimately not viable.
She also experienced fake pregnancies every year thereafter until 2020. During fake pregnancies in pandas, the mother bear experiences changes in behavior and hormonal levels associated with a real pregnancy, without actually carrying the fetus.
In 2020, Ying Ying and Le Le mated again at their Ocean Park home, but this attempt also failed to produce a pregnancy.
How were the Yingying twins conceived?
Twins Ying Ying and Le Le were conceived naturally.
Giant pandas typically bear one cub per pregnancy. The chance of conceiving twins for a giant panda is about 50 percent, and successful births of Ying-Ying cubs are considered very rare.
Is it difficult for pandas to conceive?
Breeding pandas is a challenging task for caretakers and experts, as these gentle bears are known for their short fertility periods. Finding the fetuses is also difficult.
Female pandas can only become pregnant for two to three days a year, with false pregnancies common.
Pregnancy can only be confirmed approximately two weeks before delivery using an ultrasound scan.
The time it takes female pandas to gestate can also vary greatly, from 70 to 324 days, as implantation of the fertilized egg is usually delayed.
How do baby pandas grow?
Unlike their black-and-white-furred parents, panda cubs are born nearly hairless, with pink skin and are blind.
According to the Smithsonian's National Zoo in Washington, D.C., newborn pandas weigh three to five ounces (80 to 100 grams) — about 1/900th the weight of their mothers.
A 2020 National Geographic article states that panda cubs begin growing white fur within 48 hours of birth, with dark fur appearing shortly after. Their fur is fully formed in about two weeks.
The babies begin to open their eyes at six weeks of age, and their ear canals open in the second month. Around three or four months of age, their external genitalia begin to develop, and they can finally defecate and urinate on their own.
The babies will also begin to crawl and grow teeth, some of them will be able to chew bamboo at six months of age and will stop sucking their mother at about a year of age.
What other zoos breed pandas?
Breeding pandas in captivity can be difficult, but it is not uncommon in zoos around the world.
Giant panda Mei Xiang, now 26, lived at the Smithsonian's National Zoo for more than two decades and gave birth to four male cubs who survived to adulthood: Tai Shan in 2005, Bao Bao in 2013, Bei Bei in 2015, and Xiao Qi Ji, which means “little miracle,” in 2020. She returned to China's Sichuan Panda Sanctuary last November.
At Tokyo's Ueno Zoo, giant panda Shin Shin gave birth to a female, Xiang Xiang, in 2017, and twins, Xiao Xiao and Lei Lei, in 2021.
Singapore's panda couple Kai Kai and Jia Jia gave birth to a male panda named Le Le in 2021. Unlike Hong Kong's Le Le, the cub born in Singapore's name is a reference to “Shi Le Po,” the ancient Chinese name for the city-state.
Panda cubs born overseas are sent to the reserve in Sichuan when they are between two and four years old.