Who is the enemy of the hyenas?

The adult hippo's only real enemy (other than man) is his mate. They bite, maim, and kill each other. The bulls fight over females in breeding season.

Some people say that the main enemies of the hippo are crocodiles, lions and hyenas but I don't think they would dare attack an adult hippo.
Not much can live and take on a hippo. I did see on Animal Planet a crock take a baby hippo from it's mother, but she gave him a fight. They are really mean animals and hundreds of humans each year are attacked and killed by hippos.