"Before hi-tech fighter jets with the capability to avoid detection, there was the "stealth mule" writes Steven McKenzie, Highlands and Islands reporter, on the BBC Scotland website.
He tells us how their voice boxes were removed to keep them silent behind enemy lines during World War II. The operation itself was called the "Hobday's operation", after the veterinary surgeon who developed the technique.
Andrew Trawford, our Director of veterinary services said in the article: "The wartime practice of de-voicing mules was taught as a history lesson at veterinary school. The operation is quite simple and not too traumatic for the animal, but its welfare afterwards can be of concern.
"The animal is not able to communicate and mules are noisy, especially when trying to attract a mate and it is said their call can be heard to up to two miles away.
"It's not an operation I have ever had to perform."
Full story: BBC Scotland website.