What is the closest living relative to the hippopotamus?

The closest living relatives of hippopotamuses are cetaceans—whales, dolphins, and porpoises.

This relationship may seem surprising because hippos are large, land-dwelling mammals, while cetaceans live entirely or mostly in water. Genetic evidence, molecular systematics, blood-protein studies, and fossils all place hippos and cetaceans together within the even-toed ungulates, or artiodactyls. Their shared branch is often called Whippomorpha.

Hippos are not descended from modern whales, and whales are not descended from modern hippos. Instead, both groups evolved from a now-extinct common ancestor. The hippo and cetacean lineages separated tens of millions of years ago, after branching away from the lineage that led to ruminants such as cattle, deer, and giraffes.

Earlier naturalists grouped hippos with pigs because of similarities in their teeth and body form. DNA and fossil discoveries overturned that classification. Fossils of early whale relatives, including Pakicetus, helped clarify the transition of cetaceans from land-dwelling ancestors to aquatic mammals. Today, two living hippo species remain: the common hippopotamus and the pygmy hippopotamus.