>> reporter: a rare success in the long fight to preserve the giant panda. >> pandas are notoriously difficult to breed. >> reporter: ari thompson tends to bao bao and says there are many reasons. pandas are naturally solitary and usually don't mate well in captivity. artificial inaccept natiinsemin difficult and many don't make it. the panda's american journey, however, has been key to the species' survival. >> this is the crate that ling ling was shipped in in 1972. >> the first panda. >> yes. >> reporter: u.s. scientists have helped the panda along ever since the first bears were sent here from their native china following a visit by president nixon. researchers in the national zoo are now among the top authorities in the world for breeding them. yet births like this remain rare. >> every year you kind of hope for it.