In Colonial America–and maybe even now–muleskinners made their mules go where they wanted them to go by biting the animals’ ears until they bled. No amount of whipping that tough hide made any difference, but those sensitive ears in contact with human teeth worked wonders.
My old nature is a mule. It may have several different personalities–and that would be all the rage these days–but I could not care less. They all will have their ears bit until they get in line or bleed to death, whichever comes first.