The oldest living male chimpanzee at any North American zoo died Monday.

The Lincoln Park Zoo’s chimpanzee Keo was 55 years old. According to a news release, zoo officials euthanized the animal “after advanced cardiac disease compromised his quality of life.”

In addition to be the oldest chimpanzee in the continent, Keo was the first chimpanzee to live at Lincoln Park Zoo. He arrived in Chicago in 1959 when he was just one year old and sired 11 offspring over the years.

For the final years of his life, Keo lived predominantly behind the scenes at Regenstein Center for African Apes with two female chimpanzees, Vicky, and her daughter, Kibali.

Leave  comments, share memories or express condolences on Keo’s memorial page here: www.lpzoo.org/keo-memorial