sentences:  On April 20, 2009, it was reported that four zebras had died in early March, shortly after two African buffalo were introduced to their enclosure.The zoo did not make the incident public at the time.A spokesperson for the Vancouver Humane Society stated that the zebras were between five and 15 years old, and probably died from exertional myopathy, a muscle disease causing damage to muscle tissues which is caused by physiological changes (often extreme exertion, struggle, or stress).