(CNN)Hubert and Kalisa were best friends for life.
The African lion duo spent many years together, never having cubs with each other but companions, nonetheless.
But on Thursday, their partnership came to an end. The Los Angeles Zoo made the decision to humanely euthanize the two lions, due to declining health and age-related illnesses.
"Hubert and Kalisa are an iconic part of the L.A. Zoo experience, and our staff and guests have been touched by their loyal companionship," Denise Verret, director of the L.A. Zoo, said in a statement. "Their longevity is truly a testament to the level of expert care our veterinary and animal care teams provide for our elderly animals. These lions will remain a positive part of our history, and they will be greatly missed."
Hubert, born Feb. 7, 1999, and Kalisa, born Dec. 26, 1998 spent six years together at the L.A. Zoo, and even more while at the Woodland Park Zoo in Seattle.
The average life expectancy for African lions in the wild is mid-teens, whereas in captivity is about 17 years, the L.A. Zoo said.
Their population in the wild is estimated between 23,000 to 39,000, but declining, the L.A. Zoo said. The IUCN Red List of Threatened Species lists the animals as "vulnerable" because of "human-wildlife conflict, prey depletion, the illegal trade of lion body parts for traditional medicine, trophy hunting, and disease," the zoo said.