Well for one since the ground is frozen the monster dogs don't track in mud all over.  So that's one reason.   The other still has to do with frozen ground.  And since it is frozen he stupid fat little girl pictured can't dig under the fence. 

I get home today and notice my invisible fence is beeping, telling me that there is a break in the line.  Sure enough when I walk into the yard I see evidence of an escape.   She was tunneling into my neighbors yard.  He must have caught or stopped her and plugged the hole with a sand bag I had.  But no broken wire here.  On to the back of the yard where she had a huge hole.  Bingo.  Broken wire here.  I fix it in the 32 degree cold and plug the holes with some more rock.  Yes I have a fence and an invisible fence.  I was hoping the shock would deter her from getting close enough, long enough to dig..  Apparently the battery ran out.  The shock collar doesn't even work on Totem.  He just stands there getting shocked, no big deal.  Then again he doesn't dig like the little Houdini.  Damn dogs.