When I first read your Post where you said, "... dried out and spread apart", I though you meant to say, "dried out and shrunk". I'm sure the boards dried out and shrunk, and NOW I know that you mean that the boards look like they SPREAD apart as a result of drying out. WHEW!!! I'm glad I'm past that one. For a few seconds there, my head was spinning. Anyways ...
First off, about the boards shrinking, it's normal for wood to shrink AND expand as well. This usually means that by adding some humidity to the house, you can get the boards to expand a bit. You didn't say what's UNDER the boards in terms of subfloor, and if you have a slab or basement or crawlspace or what. Depending, you may not have many options.
In short, I wouldn't recommend you fill the gaps with anything. (But if you do, sometimes a hemp-type rope is 'stuffed' inbetween.) The reason I don't recommend this is because if the 'environmental conditions' are ripe for EXPANSION, then either whatever you 'stuff' between the boards will be forced out, or, the boards themselves will buckle on you. And next thing you know, you'll be asking how to fix that.
See about 'upping' the humidity. Clean the gaps REALLY well by getting down on your hands and knees and brushing a little with a stiff brush. It is possible that fine bits of sand/stone may be premanently in the groves, thus preventing the boards from expanding. 'UP' the humidity 5% at a time maybe over the course of a week to see how they do. (All this assuming you can introduce humidity.)