Su Lin was one of the pandas I was trained on. From 2007 to 2010 I watched her grow from a newly weaned cub to a full grown, independent panda before being sent to China. All captive pandas are owned by China, on loan as part of a research agreement. Part of the agreement requires all pandas born to loaned research pandas to be returned to China when they turn 3. Su Lin's 3rd birthday, however, was only a matter of months after the Sichuan earthquake of 2008 which leveled much of the Wolong National Nature Reserve and the Panda Research Center where she was supposed to be sent. So, Su Lin stayed with us at the San Diego Zoo until she was 5 and traveled to China along side her sister Zhen Zhen shortly after her 3rd birthday.
Turns out twins might run in the family. This was actually Su Lin's second set of twins. Her first had one healthy cub and a still born. Her older half-sister, Hua Mei, has given birth to three sets of twins and Bai Yun, although none came to term, has been suspected to have been developing twins when vets did ultrasounds during pregnancy. In the wild, having twins isn't advantageous. The amount of energy it takes to raise two cubs far exceeds what mom is capable of providing so the panda mother is forced to pick the stronger cub to raise. Lucky for these ladies, they are part of breeding facilities equipped with all the "energy" needed to provide for both cubs.
It is so wonderful to see all the success of the San Diego Zoo's Panda Research Facility's breeding program. Not only have six cubs been born in the program but they have given birth to 14 cubs of their own! So many sets of twins really help them up their numbers.