Thanks for the hot water idea, I'll definitely try that.
--- Why do you recommend against fillers?
BTW, the flooring is 13 years old. I wish I could tell you what kind of wood it is..., all I can say is that it's a very light colored (creamy yellow, like butter). --- One further detail: I can see that the finish has cracked in the areas where the cracks are. Is there some sort of floor wax that I can use to "reseal" these areas? Or does this call for refinishing the whole area (estimated cost $3,000, for all of the hardwood).