At 18, Adelor was considered elderly, according to the zoo. He arrived at Lincoln Park Zoo in 1995 and fathered five cubs over the years.
He was noticeably absent from the Kovler Lion House on Wednesday afternoon as lioness Myra, the only remaining African lion in the exhibit, hid from the crowd.
"[Adelor] would often be seen lying on the top of the large boulders of the exhibit where he could survey his territory and monitor his pride," said Mark Kamhout, zoological manager. "Guests of all ages would flock to the lion exhibit when Adelor began roaring as he announced his territory several times a day. He was a wonderful leader of the pride and was very protective and affectionate of the females under his care."
Suhaib Omari, a native of Jordan, was waiting for Adelor when he heard the news.
"I'm not from here. I live in Jordan, so when I come to America, I come here just to see Adelor," Omari said.
The zoo said it is looking for another mate for Myra, 15.
The median life expectancy for lions is approximately 14 years. The longest African lion lifespan recorded is 26 years.
A memorial page has been set up in Adelor's honor. Adelor fans and friends of the zoo are welcome to leave comments, share memories or express condolences to animal care staff who are especially affected by his loss. Visit the memorial page here.