ONE of the Darling Downs Zoo's longest residents is celebrating a milestone birthday after a rocky start to life.

The zoo's oldest lioness Xena turns 20-years-old today, which is the equivalent to 100 human years. 

"Xena was born to Sultan and Sheba back in 1998 and comes from a line of long-lived lions," the zoo said.

"She had a rocky start to life, being born by caesarean section.

"Sheba was taken by us to the vet surgery and proved to be too long for the operating room.

"Eventually the operation was performed on two tables joined together with Sheba's head out one door of the room and her tail protruding out of another."

The zoo said Xena was then hand-raised and was like a family pet for some time.

She went on to have her own four cubs and now lives in quiet retirement at the zoo.

"Her nieces are her next door neighbours and she enjoys being spoiled by our keepers," the zoo spokesman said.

"Old age is starting to catch up with her. She has a touch of rheumatism and there are days when her memory is not the best.

"Apart from that she remains in good health."