Hi, Dave. Just like metals expand and contract with changes in temperature, wood expands and contracts with changes in humidity. Wood expands or contracts mostly across it's width. It does move somewhat in length but percentagewise a lot less than the width. It moves in thickness too but the dimension is so small to begin with that the change is barely noticeable.
In the summer when humidity is high, the boards are at their widest. In winter, they are normally at their narrowest. That's why you are noticing them now. The problem with putting a hard material between them now is that when they expand again in the summer, the wood tries to expand again only it has nowhere to go since the filler is harder than it is. This means that the wood fibers get crushed and do not resume their shape in the winter so you end up again with gaps between the boards.
A humidifier running in your house in the winter is a help to solve this problem. It is not just your wood floors that react this way but solid wood furniture also benefits from constant humidity.
One solution that can be used if the gaps are really wide and stay that way, indicating that the floor was poorly laid originally, is oakum caulking like is used in a ship. This never gets hard but is very noticeable and does not clean well so it's not an option that many would like to try unless they have an historically important floor.
Investigate adding a humidifier to your furnace set-up. They can be a bit of a maintenance hassle but you floors, furniture and woodwork will love you for adding one.