Calgary Zoo officials are defending themselves again after a newborn bison was attacked in a paddock over the weekend.

John Preet witnessed the calf, who was only a few hours old, being attacked on Sunday by its sibling.

Preet said he did not see an emergency number anywhere and in a panic called a friend who was a zookeeper but not on shift.

Graham Newton, the zoo's director of corporate services, said the keeper called security from his home but the message was miscommunicated and security was not deployed.

About an hour later, with the attack still happening, security officers came to the paddock during a regular check. They called animal care staff and called the veterinarian who had handled the calf's birth. He checked out the calf and found no injuries, said Newton.

Newton called the attack a case of animal sibling rivalry.

"They are herd animals and certainly sibling rivalry is not unheard of among many species and bison are no different. It fortunately was not a serious incident," he told CBC News on Monday. "The main thing is the calf is safe — there have been no injury. These kinds of things do happen and certainly I don't think it's cause for alarm."

He said the zoo has reviewed the situation and determined that no changes need to be made to procedures.

There was a similar incident last year with a newborn calf, he added.