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You floor is continually expanding and contracting. It will contract the most when the relative humidity is at its lowest and expand the most when humidity is at its highest levels. The problem is that if you fill the cracks, when your floor expands again, it will be very likely to buckle. Wide plank flooring normally shows fewer cracks than do narrow planks.
The advice of flooring professionals is not to fill these ever expanding and contracting cracks in order to allow the floor room to naturally expand and contract.