Hippos are the third largest land animal, after elephants and white rhinos. Despite their large size and rolly-polly looks, they are fast and furious -- and are considered one of the deadliest animals in Africa.
The truth is, hippos are extraordinarily aggressive. They live in groups called schools or bloats (or sometimes pods or sieges) and jostle for position in the social ladder. The big "yawns" they make are actually aggressive displays, showing off their sizable and sharp teeth. It doesn't take much to set a hippo off on a tantrum, and fights are a daily occurrence.
Not only do they go for each other, but a hippo will charge anything it perceives as a threat, even cattle grazing nearby or people either on land or even when in boats traveling along a river. Just when it will charge is anyone's guess -- hippos are famously unpredictable. In fact, only in November of last year, a hippo attacked a boat carrying school kids across a river in Niger. Twelve of the children plus a villager were killed. Whether on foot or in a boat, anyone in hippo territory should consider themselves at risk. Hippos can run a surprising 14 miles per hour for short distances, so it isn't easy to outrun one even on land. Ultimately, hippos are responsible for killing around 3,000 people every year.
Despite the fact that hippos are so deadly to human, humans are the ones causing hippos to fast disappear as a species. Hippos have lost vast portions of their habitat to human settlements and are now confined primarily to protected areas.