Just Never Cry Wolf
In 1946, Farley Mowat was hired by the Dominion Wildlife Service to prove that wolves were to blame for the declining caribou population in the northern Canadian wilderness. His employers hoped his research would give them a good reason to shoot the wolves, but instead discovered they were eating rodents.
Source: YouTube
Mowat had further proof: he knew that a large predator could survive eating nothing but voles because he’d done it himself. Although much of what Mowat wrote in the 1963 book Never Cry Wolf about his experiences in the Arctic has been more or less discredited as heavily fictionalized, no one disputes that the vole experiment actually happened.