A royal era ended at Lincoln Park Zoo Tuesday. .
Zoo keepers euthanized Marta "The Queen," the zoo's black leopard, because it lost of kidney function, the Chicago Tribune reports.
The 20-year-old leopard surpassed the average life expectancy for zoo leopards by four years.
Several physical ailments in later years slowed down the leopard's step.
Marta lost the ability to land easy on her claws, so keepers laid out a soft mat for her. She became allergic to grass, dirt and dust mites, so keepers moved her from an outdoor exhibit to an indoor one. She lost the ability to eat pork and beef, so she switched to a diet of green peas and minced venison.
Marta arrived at Lincoln Park Zoo as a cub in 1992. Her previous owner declawed and spayed her.
After spending 18 years among zoo staff that loved her, it broke their heart to make "the difficult decision to euthanize" their queen, according to a zoo statement.
"Despite being one of the oldest cats at the lion house, Marta always was very engaging with people," said Mark Karnhout, zoological manager.