Louisville Zoo officials say the zoo's oldest polar bear passed away today from unknown causes.Arki was 28 years old, and came to Louisville in 2011 from the Brookfield Zoo in Illinois. According to Polar Bears International, the bears live an average to 15 to 18 years in the wild, but in captivity can live into their mid- to late 30s. The oldest known polar bear in captivity--Debby, who lived in a Canadian zoo--was 42 years old.From the zoo: “Arki was a great ambassador for her species,’ General Animal Curator Steve Wing commented. “She’s been a great addition to Glacier Run where she flourished in her last few years. Arki and the Glacier Run staff had a special bond and this is going to be a tough loss.”There are now two polar bears in the zoo's Glacier Run exhibit: 2-year-old Qannik, who was born in the wild, and 3-year-old Siku. Arki was Siku's grandmother.Zoo officials say it's too soon to make plans for whether the zoo will try to get another polar bear for Glacier Run.