The Louisville Zoo says its oldest polar bear has died. The Zoo says Arki, a female, was 28 years old and died from unknown causes on Wednesday. Arki came to Louisville in 2011 from the Brookfield Zoo in Illinois. A news release from the Louisville Zoo says polar bears usually live into their mid-20s. Arki was considered one of the most prolific polar bears with five successful litters. One of the two remaining polar bears at the Louisville Zoo, 3-year-old Siku, is Arki's grandson. The Zoo says a necropsy will be performed on Arki to determine the cause of death.