Monster Life reactions

Today's dumb question: what happens if you don't nurture your monsters when they get those hearts over their heads?

Can they de-level? Die? Run away? Any other ill effects?
Also, the game tells me I'd get more experience by nurturing monsters that want it - but I don't. Nurturing gives 10 exp regardless.

I don't think there are any ill effects for not nurturing them when they want it. I skipped a day without clicking on them with the hearts up, and it's still there a day after, so I'm guessing it just stays there forever.

It's pretty vague by the game's word, but I think the experience it meant is for the individual monster's exp to level, not your farm's. But of course, you get more waru per nurture click the higher level they are.