The smallest of three rare Siberian tiger cubs born at the Pittsburgh Zoo and PPG Aquarium died Thursday, zoo officials said.
Respiratory problems likely caused the 5-week-old cub's death, the zoo said. But results of a necropsy performed Thursday won't be available for several weeks.
Zookeepers noticed the cub was lethargic Wednesday when its mother, Toma, allowed her three cubs to leave the den where they were born Aug. 8. Veterinarians tried to treat the cub, but its condition worsened.
The 4-pound female was half the size of her brother and sister, the zoo said.
There are only about 400 Siberian tigers in the wild and about 190 in U.S. zoos. Their mortality rate in the wild is about 30 to 40 percent.