A 28-year-old Kodiak brown bear who lived nearly her entire life at the Lake Superior Zoo in Duluth was euthanized Tuesday, officials announced.

"Phoebe had been on medication and supplements for arthritis for seven months and was increasingly having difficulty moving," said Dr. Louise Beyea, the zoo's veterinarian. "The staff was concerned about being able to maintain her quality of life at an acceptable level. It is impossible to medicate a bear that's not eating."

Zookeepers also were worried that Phoebe, in her advanced age and state of health, would have been unable to defend herself against Trouble, the younger grizzly bear that shared her exhibit.

The fur has certainly flown over the years between Phoebe and Trouble, the Anchorage zoo transfer whose notorious background validates his name and who is now the Duluth zoo's sole brown bear.

Soon after Trouble's arrival in 2002, he and Phoebe had it out, with the older bear prevailing. A year later, the two battled again. This time, a stronger and wiser Trouble came out on top, and their acceptance of each other finally took root.

Zoo CEO Dawn Mackety said "it's difficult to know if Phoebe's euthanization would have been put off a bit longer in the absence of Trouble, perhaps not. At 28 years old, she lived a normal life span for a bear of her type … in captivity."

Along with managing Trouble, Phoebe also survived the flooding that soaked the zoo in June 2012, when the bear exhibit was in about 3 feet of water.

An unscathed Phoebe was put in temporary quarters while the exhibit was drained and cleaned.

Phoebe was born at the Pittsburgh Zoo and donated to the Lake Superior Zoo in 1988 when she was 15 months old.

It is unclear whether the zoo will replace Phoebe, Mackety said.

"We're sad to lose Phoebe," she said. "We'll be discussing our options as we consult with our staff, the city of Duluth and other zoo experts from around the country."