Again, my thoughts aren't to do anything because IF the boards should expand either on their own or with the help of man-induced humidity into the house, whatever you put between the boards will be forced up. Then, again, the boards will either buckle (and possibly create NEW problems) or just force up whatever it is you put in between the boards.
If you have 1/4" gaps with boards that are Planks vs. boards that are strips, then that's not too abnormal. BUT, if you have 1/4" gaps with strip boards, then I'd consider your gaps to be larger than 'normal'. You didn't say what is 'under' the floor (basement, crawlspace, slab, or what.) Since you're right on the joists, then the boards are reacting 'directly' with whatever is in the 'air' directly below. You could buy yourself a Hygrometer for under $10 or even under $5 to see what the humidity is in the 'space/room' under the floor. NOrmally, 35-45% is good.