The Los Angeles Zoo announced Thursday the deaths of African lion pair Hubert and Kalisa.

The Zoo said in a press release that they made the difficult decision to euthanize the 21-year-old partners "due to their declining health and age-related illnesses that diminished their quality of life."

"Hubert and Kalisa lived far past the expected lifespan of African lions, and they inevitably began to suffer from age-related issues. Of course, we did not want them to suffer physically and wanted to do what was best for them," Beth Schaefer, the Zoo's Director of Animal Programs, said.

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"With very heavy hearts that also contain gratitude for having known them, animal care and veterinary health staff came to the consensus that humane euthanasia was in the best interest of their welfare as their quality of life had diminished."

Kalisa was born in 1998 and Hubert in 1999. They met at the Woodland Park Zoo and were moved to the LA Zoo 6 years ago. Hubert fathered 10 cubs throughout his life, and though he and Kalisa never had cubs together, the LA Zoo said in a news release the two were life partners and loyal companions.

"Kalisa and Hubert were often seen sleeping together and grooming one another," Schaefer explained, noting that visitors often commented on the obvious bond between the two lions.

"You cannot think of Hubert without thinking of his companion, Kalisa; theyve been an inseparable couple for years," Alisa Behar, the Zoo's Curator of Mammals added.

The average life expectancy for African lions that live in the wild is in the mid-teens, while it is about 17 years for those in Zoos. CEO & Zoo Director of the LA Zoo, Denise Verret noted that the longevity of Huberts and Kalisas lives is a testament to the expert care of the Zoos veterinary and animal care teams.

"Hubert and Kalisa are an iconic part of the LA Zoo experience, and our staff and guests have been touched by their loyal companionship," Verret said.

The African lion, which is native to the savannas, arid woodlands, and semi-desert regions from south of the Sahara Desert to South Africa, is categorized by the IUCN Red List as vulnerable due to human-wildlife conflict, prey depletion, the illegal trade of lion body parts for traditional medicine, trophy hunting, and disease.

Read more:
LA Zoo Mourns the Loss of Beloved African Lion Pair - NBC Southern California