Thanks for the followups. It's not a subfloor and they aren't tongue and groove. My main concern is if anything can be done to fill the gaps or do we have to carpet the floors?
The house is in SW South Dakota (Rapid City) and was built in 1960 - ironically by my now deceased grandfather. It doesn't look like there was ever any finish on the floor, and the wood looks like it's in very good condition. The boards are 2 1/4" (or close to that). Am I destined to live in carpet the rest of my life? Replacing the wood floors isn't an option for us right now.