Boris, the 11-year-old polar bear at Reid Park Zoo in Tucson, died Friday evening following a routine medical procedure earlier that day.

An initial necropsy did not reveal a cause of death, though the zoo veterinarian suspects a reaction to the anesthesia. The final results of detailed tests will be available in several days, a zoo release said.

Born at Denver Zoo, this male polar bear had been a resident at Reid Park Zoo since 2001. Boris had a history of low thyroid levels and dermatitis. Over the past several weeks, Boris exhibited unusual behavior including loss of appetite, periods of lethargy, foot sensitivity and agitation. He was tranquilized Friday by the zoo’s veterinary staff for a complete physical. Zoo vets were assisted by a dermatologist to check for allergies and a polar bear reproductive specialist from Cincinnati Zoo. While under anesthesia, the bear responded appropriately with good vital signs. When he woke from the procedure, he immediately began exhibiting symptoms of an anesthetic reaction.

“We were all shocked and saddened by his death,” said Susan Basford, a zoo administrator. “Unlike the death of an elderly animal you are preparing to lose, the death of an animal like this is devastating.”

Plans to replace the bear have not yet been considered. Polar bears are solitary. The zoo’s staff will work closely with other scientists to make the best decisions for the remaining female and the entire population of captive polar bears, the release said, and the Reid Park Zoo remains committed to polar bears through its partnerships with other institutions accredited by the Association of Zoos and Aquariums and its Species Survival Plan.

Kobe, the remaining female polar bear, is doing well.

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