I use netting, not wire. Azelgin can give you an answer from experience, but that is the purpose of having the lower wire 6" off the ground. That will stop most things from crawling under.
It's such a joy when you hear that stray dog yelp and leave, never to return. it's not hurt but it is gone.
Our soil (loose use of the term) Is as hard as concrete. Should something decide to try and dig under (they better bring a backhoe), they will contact the lower hotwire. Most predators use scent, to locate their food. All the dogs, that have been zapped by the fence, have sniffed the wire as they were checking out the tempting chicken smells.. I would guess the coyotes have done the same. My reasoning behind the wires placed at 18" and 36", is that should something decide to climb the run fencing, there's no way to avoid getting past the upper wires, without contacting both the hotwire, and grounded run wire.
I have personally seen a bobcat checking out the chicken run. Cats locate prey by sight and sound. A cat might not hit the low wire, as it would not necessarily be sniffing, and would have it's target locked in it's visual senses. I know this cat must have experienced the shock of the electric wire. Whether it was from the low wire, or, it decided to climb the run and found the upper wires, I'll never know, as it never got in and the run wire was not damaged.
Skunks get nailed regularly. Easy to tell when a skunk has meet something that it doesn't like.
We have skunks, foxes, raccoons, coatimundi, ringtail cats, coyotes, bears and mountain lions around here. I've seen all of them within 200 yards of my home.