ST. LOUIS -- The Saint Louis Zoo announced Friday a nearly 20-year-old Amur tiger named Kalista has died.

The zoo said in a statement Kalista had been under treatment for age-related degenerate joint disease for several years. She was euthanized last weekend when medical treatment was not sufficient to keep her comfortable and pain free, the statement said.

She had been the oldest living female Amur tiger in the Association of Zoos and Aquariums' North American Species Survival Plan Program. The average life span of a female Amur tiger is just more than 14 years.

"Kalista was a very social and energetic tiger. She enjoyed interacting and training with the keepers and greeted everyone with a 'chuff,' which is a tiger-specific vocalization showing affection. We will all miss her," said Steve Bircher, who is the curator of carnivores at the zoo.

Kalista was born in 2001 at the Philadelphia Zoo and moved to the Saint Louis Zoo in 2003. She gave birth in 2008 to an unusually large litter of five cubs. A normal litter size for Amur tigers is two or three.

The Amur tigers are critically endangered. Their natural habitat is eastern Russia, northeastern China and northern regions of North Korea.