Zookeepers said Cookie had developed cancer and had to be put down. In 2008, the lioness had a serious heart condition called cardiac tamponade, in which fluid builds up in a sac around her heart. She underwent a successful surgery, giving her several more healthy years.
Lions' life expectancy in the wild is 15 to 18 years. They tend to live longer in zoos because they don't have to compete for food like they do in the wild.
Cookie has been a favorite of many zoo visitors since arriving there with her companion, Lionel, in 1997. Lionel passed away in April 2011 at the age of 23.