Klondike the polar bear, one of the famous twin polar bear cubs from the Denver Zoo, died September 12 at SeaWorld Orlando at age 18. Although the death was unexpected, it fits within the lifespan of polar bears in the wild, which normally live between 15 and 18 years.

Klondike and his sister, Snow, were born at the Denver Zoo in November, 1994, but rejected by their mother. They were hand raised by the zoo staff, but they needed a larger place to live as they grew older and larger. SeaWorld Orlando took them in in 1995, and they became a popular part of the park’s Wild Arctic exhibit.

Klondike hadn’t been showing any signs of illness, but he didn’t eat his dinner the evening before his death. He went to sleep in his den in Wild Arctic and apparently died peacefully overnight, as he was found in the same position the following morning.

“We have team members who have known Klondike since he arrived in 1995,” SeaWorld Orlando President of Zoological Operations Mike Boos said in a statement issued by the park. “They’re sad and feeling the loss of this amazing animal.”

The park performed an autopsy on Klondike and expects to have the results and know his cause of death within six to eight weeks.

Klondike’s sister, Snow, died a year earlier in Tucson, Arizona, at the Reid Park Zoo. She’d been moved there due to skin allergies in hopes that the drier climate would help her condition.

Wild Arctic is home to other popular animals, including walruses and beluga whales. Guests get into the animal exhibit via a simulated helicopter ride that transports them to the snowy region.