They will only take torpor damage by headshots with a catapult (haven't tried cannon yet). It adds around 450 torpor but does 250 damage per hit. so if it's health gets low, drag a body or carcass next to it and it will eat it and heal it for a large amount of health, but it's torpor will remain the same. A magnifying glass helps with determining it's health and torpor levels, also it does an animation when hit in the head from what I have seen.