Which mammal group is the hippopotamus’s closest living relative?

Cetaceans are the hippopotamus’s closest living relatives.

Cetaceans are the mammal group containing whales, dolphins and porpoises. Modern genetic and fossil evidence places hippos within the even-toed ungulates, or artiodactyls, and shows that their closest surviving relatives are cetaceans. This relationship can seem surprising because hippos live on land and in freshwater, while whales and dolphins are fully aquatic.

The connection becomes clearer through evolutionary history. Hippos and cetaceans share a common ancestor, and fossil discoveries such as anthracotheres help link early hippo relatives with the cetacean lineage. Molecular studies comparing genes and proteins also support the close relationship. Scientists therefore group hippos and cetaceans in the broader clade Whippomorpha, although hippos are not descended from modern whales.

A common mistake is to identify pigs as hippos’ closest relatives because both were traditionally grouped as “pachyderms” in older popular classifications. That grouping is not a modern scientific family. Hippos are more closely related to whales than to pigs, despite their very different lifestyles and body shapes.