Hippopotami, or “hippos”, are large semi-aquatic mammals that spend most of their time in the water. They are herbivores but they are quite aggressive in defending their territory and their great size makes them dangerous opponents. This is compounded by the fact that, in spite of the bulky appearance, they can run faster than a human on dry land. Hippos can’t float, though they can propel themselves to the surface of the water for a breath of air, and they can stay submerged for 15 minutes at a time.