This Hippo skull has fibre glass tusks.Hippos have a flexible social system defined by hierarchy and by feed and water conditions. Usually they are found in mixed groups of about 15 individuals, but in periods of drought large numbers are forced to congregate near limited pools of water. This overcrowding disrupts the hierarchical system, resulting in even higher levels of aggression, with the oldest and strongest males most dominant. Hippos are unpredictable. If they are encountered away from the safety of water, anything that gets between them and their refuge may be bitten or trampled.