Herd Mentality
Hippos make defensive use of the idea that there is safety in numbers. They live in herds consisting of anywhere from 10 to 30 members. This includes a male, or bull, who is the group's leader, the cows with which he mates and the resulting offspring. Sometimes the dominant male will allow another bull to join his herd but only as a subordinate. By staying together in groups, hippos watch out for threats to themselves and others in their herd.