Two zebras can be seen in their enclosure at the Greater Vancouver Zoo in this undated photo. (CBC)

Four zebras have died at the Greater Vancouver Zoo, prompting an investigation by the British Columbia Society for the Prevention of Cruelty to Animals.

The deaths occurred two months ago after two cape buffalo were placed into the zebras' enclosure, according to the Vancouver Humane Society.

The zoo, located in the Fraser Valley community of Aldergrove, east of Vancouver, did not make public the deaths of the animals. Repeated calls by CBC News on Monday were not returned.

The zebras, who were between five and 15 years old, must have panicked and probably died of stress, Vancouver Humane Society spokesman Peter Fricker said Monday.

The zebras likely suffered from exertional myopathy, a muscle disease characterized by damage to muscle tissues brought about by physiological changes, usually following extreme exertion, struggle and/or stress, Fricker said.

He said cape buffalo are extremely dangerous animals and, although zebras and buffalo co-exist in the wild, keeping them together in an enclosed space was a mistake.

"We feel this is a sign of incompetence on the zoo's part, that either the introduction of these animals into the zebra enclosure should not have been made, or it should've been done with greater care and expertise," Fricker told CBC News in a telephone interview.

Eileen Drever, senior animal protection officer with the B.C. SPCA, said Monday that the organization has opened an investigation into the deaths of the zebras.

Drever said the zoo has not returned calls regarding the animals' deaths, and SPCA representatives were planning on visiting the zoo Tuesday.

History of incidents

Fricker said the zebra deaths represent another black mark on the history of the Greater Vancouver Zoo.

In June 2006, serious questions were raised about the zoo's ability to care for its animals after a baby female giraffe who was born at the zoo died after eight days.

A 4,100-kilogram elephant, Tina, developed health problems in August 2003 while being housed at the zoo. She was transferred to the Elephant Sanctuary in Tennessee for better treatment but died in July 2004.

Also in 2004, two hippopotamuses, Gertrude and Harvey, at the Greater Vancouver Zoo died at the age of 20 — half their life expectancy.