The giant panda Ying-Ying, the female of the pair that made Mexico's panda breeding program the most successful outside China, has died of unknown causes - possibly a broken heart - at age 14.

Ying-Ying, who died late Sunday, had been depressed since October, about three months after Pe-Pe, her mate, died of cancer, said Maria Elena Hoyo, director of the Chapultepec Zoo.