When you feel the weight of the world on your shoulders, and that mountain just ahead seems almost insurmountable, it often helps to turn to a buddy. Especially one that can carry, say, heavy cases of ammunition, anti-tank missiles and gun tubes. It’s why pack animals – like the mules at the Marine Corps Mountain Warfare Training Center in Bridgeport, Calif. – are the wheels of choice for the heavy lifting for much of the world, including armies over the centuries. A group of Hawaii-based Marines with 3rd Battalion, 3rd Marine Regiment, this month took to the Sierra Nevada mountains with…