How do you deliberately wake up a monster in a den?

In monster dens, I sometimes find a sleeping monster which I want to kill for loot. However, stepping on him doesn't trigger combat.

I've tried using monster abilities like Roar, jumping on them, running over them, harvesting everything in the room, etc., but I have no idea what wakes them up.

How do I deliberately wake up a monster? Or at least get into a fight with one and kill it instead of waiting around?

Solution 1:

You can also wake up a monster by collecting eggs from under its nose. After each egg collected there is a chance of waking it up -- it doesn't feel like it's a fixed number.