Hippopotamuses, also called hippos, are living in groups of up to 40 animals, called a pod, herd, school or bloat. A male hippo is known as a bull, a female, a cow, and a baby, a calf. Male hippos challenge one another with threatening gapes. Since their habitat is often encroached upon by farmers and tourists, and because they are so territorial, the hippopotamus is the most dangerous animal.Baby hippos are born underwater at a weight between 25 and 45 kg.