File 2006 / DMN

Boris was one of the zoo's most popular attractions.

DALLAS — Boris, the Dallas Zoo's only lion, died Wednesday, zoo officials said. The majestic 415-pound African lion had shown signs of discomfort over the last week, the zoo said, and X-rays showed multiple growths in his chest cavity.

The veterinary staff sedated the lion and spotted what was believed to be a large cancerous mass pressing against his stomach and lungs. The results of a necropsy – the animal equivalent of an autopsy – will determine the official cause of death.

Zoo officials said that the mass inhibited Boris' breathing and that he was unable to recover from the sedation. He died in the early afternoon.

"Boris was one of the most popular animals at the zoo," Gregg Hudson, the facility's executive director, said in a written statement.

His death comes months after the September death of Hildy, the world's oldest giraffe at the time and another popular animal at the zoo.

Boris was born at the Houston Zoo on June 13, 1990, and was moved to Dallas in 1997.

Lions live an average of 15 years in the wild. At 17, Boris was a senior member of the zoo's Cat Row and a favorite attraction for the last 11 years.

It's a sad time for zoo staff, volunteers and visitors, said Sean Greene, the zoo's director of community relations.

"We're in early discussions about a way that we can have a little memorial for this animal," Greene said.

Although there are no immediate plans to replace Boris, the zoo does want to show lions in the future.

"We can never replace Boris, but we have every intention of bringing lions back to the Dallas Zoo in the future, once we have completed our African savanna exhibit," Hudson said.