POWELL: The Columbus Zoo and Aquarium says a polar bear gave birth to a pair of cubs, and though only one survived, staff are 'cautiously optimistic' about its health. The zoo in central Ohio said Friday that staff started hand-rearing the week-old female cub after her mother, Aurora, began taking breaks from caring for her. Zoo staff initially had opted not to intervene in caring for the cubs after talking with staff at other polar bear breeding facilities. Polar bears have the lowest reproductive rates of all mammals. The zoo says one in two cubs typically survives. The zoo says the 1.5-pound cub is healthy, feeding regularly and receiving around-the-clock care. Aurora and her 8-year-old twin, Anana, arrived at the zoo in 2010. Both mated with a 28-year-old bear.