Matthew Poole and continuators on Job 40:20

20. Surely the mountains bring him forth food : where all the beasts of the field play .

Though this Creature be vastly great and require much food and no Man careth for it, yet God provides for it out of his own stores, and makes even desert Mountains to afford him sufficient sustenance. The Hippopotamus also though he live most in the Water fetched his food from the Land, and from the Mountains or Hills. which are nigh unto the River Nile.

They not onely feed securely, but sport themselves by him or with him, being taught by experience that he is gentle and harmless and never preys upon them.