What did it take for the National Zoo’s Mei Xiang to produce a panda cub? A great sperm sample and perfect timing.

Sisters Naba and Shera, left, and 4-year-old male, Luke, at right. Images courtesy Smithsonian’s National Zoo. Two of the National Zoo’s lions, 4-year-old male Luke and 6-year-old female Naba, recently successfully mated, zoo officials said Tuesday, though the Zoo will not be able to determine whether Naba is pregnant until the beginning of April. The hook-up was engineered by zookeepers who are hoping to breed an entire lion pride at the National Zoo. The…