The fourth death of a western lowland gorilla in the last year at the Calgary Zoo is proof great apes should not be kept at the facility, animal protection groups say.

The zoo said Monday it was forced to destroy Donge, a 22-year-old female, late last week because she was rapidly losing weight and growing sicker despite four surgeries earlier this summer.

Two other females died earlier this year from unrelated illnesses and a newborn was lost last August after the dominant female in the group claimed the baby as her own and wouldn't let her go.

The numerous deaths in such a short period of time are a "red flag" that something's wrong, said Rob Laidlaw, executive director at Zoocheck Canada.

"I certainly hope it's a sign that the Calgary Zoo will recognize and that they'll get out of the business of keeping great apes," Laidlaw said from Toronto.

"Obviously, if they're dying in those numbers, there's not even a shred of conservation value there … there's not any reason for keeping these animals."

The zoo said an autopsy showed the gorilla had developed another severe intestinal abscess and had a large blood clot in her heart.

"These decisions are heartbreaking but sometimes necessary," Cathy Gaviller, the zoo's director of animal care and research, said in a release.

"There's a limit to how long we should allow an animal we care for so much to endure pain and diminishing health when there is no hope for recovery."

Donge had been battling complications from chronic diverticular disease for the last four years, and had one surgery each in 2005 and 2006.

Donge's death leaves Calgary Zoo with just three gorillas, down from nine a year ago.