The family Hippopotamidae, belonging to the order Artiodactyla (even-toed ungulates), comprises large, semi-aquatic mammals commonly known as hippopotamuses. This family includes two extant species: the common hippopotamus (Hippopotamus amphibius) and the smaller pygmy hippopotamus (Choeropsis liberiensis). Hippopotamids are characterized by barrel-shaped bodies, short legs, large mouths with prominent tusks, and thick skin that helps retain moisture and protect against the sun. They spend much of their time in rivers, lakes, and swamps to stay cool and are primarily herbivorous, feeding on grasses. Hippopotamids play a significant ecological role in their aquatic habitats by influencing water vegetation and nutrient cycles.