* They breathe air: Hippos need to come to the surface to breathe, unlike fish that can extract oxygen from water.
* They are land mammals: Hippos give birth on land, nurse their young on land, and generally spend time foraging and socializing on land.
* They are adapted to both environments: While they are comfortable in the water and can hold their breath for a relatively long time, they have also adapted to land life with strong legs and hooves.
Therefore, hippos are considered semi-aquatic because they are adapted to both aquatic and terrestrial environments, but they are not fully aquatic.