Aelian

The Beaver is an amphibious creature: by day it lives hidden in rivers, but at night it roams the land, feeding itself with anything that it can find. Now it understands the reason why hunters come after it with such eagerness and impetuosity, and it puts down its head and with its teeth cuts off its testicles and throws them in their path, as a prudent man who, falling into the hands of robbers, sacrifices all that he is carrying, to save his life, and forfeits his possessions by way of ransom. If however it has already saved its life by self-castration and is again pursued, then it stands up and reveals that it offers no ground for their eager pursuit, and releases the hunters from all further exertions, for they esteem its flesh less. Often however Beavers with testicles intact, after escaping as far away as possible, have drawn in the coveted part, and with great skill and ingenuity tricked their pursuers, pretending that they no longer possessed what they were keeping in concealment.