The Calgary Zoo's 16-year-old lion, Okonko, died Friday afternoon from heart failure following an intricate surgery to remove its cancerous spleen.

"Over the past year, we could see that Okonko's age was gradually catching up to him," said Jamie Dorgan, a manager at the zoo. "His appetite and activity levels had been slowing down and we were monitoring him daily.

"Over the past week, we saw a more dramatic decrease in his appetite and the decision was made to take him to the animal health centre for examination and surgery if necessary."

A large mass on his spleen was found by veterinarians after an examination.

Since the cancer was isolated to the spleen, staff were hopeful for Okonko's recovery for a brief period but his heart failed post-surgery.

Okonko was born Dec. 8, 1995, in Aalborg Zoo in Denmark and arrived in Calgary April 23, 1997, along with three female siblings. One of the females died Feb. 19, 2011, of age-related health issues but Okonko's two remaining siblings continue to reside at the Calgary Zoo.

The average life expectancy for a lion in captivity is 13 years, according to Walker's Mammals of the World.