A mule was built to do one thing, plow. No matter the time of day, the type of weather, or the temperature, it was built to bear a load, and it’s sole purpose on earth is to transport weight that is to ultimately too great for a single man to carry. A mule is obedient and steadfast, and respects that load that is placed on its back. It will go until it no longer has to, or it will go until death.
The mule skull represents going until the air in your lungs no longer exists, the beat of your heart no longer thumps, or your inner will outlasts the overall well being of your physical body.
You will work until death and the only thing that matters is how much weight did you bear during your time in the field and in this case your life.