How aggressive are hippos towards each other? If they have too many, couldn't a few be shipped out to, say, Longleat? I'm sure the lake there could hold more than two hippo, it's just a matter of whether they get on (that's if Whipsnade ever wanted to resume breeding common hippo).
hippos are very aggresive towards each other. many fights lead to the death. i dont think tht spot and sonia would get along with more hippos as they areold and have had only each others company for at least 15yrs