Zoo officials said they euthanized Keo Monday because advanced heart disease had "compromised his quality of life," the Chicago Tribune reported.
Keo arrived in Chicago in 1959, sired 11 offspring and entertained zoo visitors ever since.
At 55, he exceeded the life expectancy of 31.7 years for captive chimps, zoo data said.
Steve Ross, director of the zoo's Lester E. Fisher Center for the Study and Conservation of Apes who worked with Keo for more than 10 years, said the chimp had a distinct personality and curious nature.
"He really was a personable chimp," Ross told the Tribune. "He understood how to be a chimpanzee, but he also knew how to be around people. He made people feel good."
Lincoln Park Zoo now has eight chimps.
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