Are Hippos Related to Whales? The Surprising Family Tree

How can a hippo be related to a whale?

At first glance a hippo looks nothing like a blue whale or a dolphin. But relationships between animals are based on shared ancestry, not on looks. Both hippos and whales belong to a large group of mammals called the even-toed hoofed mammals, or artiodactyls, which also includes pigs, camels, cattle, deer and giraffes.

Whales and dolphins long ago lost their hind legs and hooves as they adapted to the ocean, so they look very different from their relatives on land. But their DNA and their fossil ancestors show that they are part of the same family tree. Because of this, many scientists now combine whales and hoofed mammals into a single group, the Cetartiodactyla.

Within that group, hippos and cetaceans form their own branch, sometimes called Whippomorpha, a name that combines “whale” and “hippo”.

What is the evidence?

Several independent lines of evidence point the same way.

DNA studies. From the 1990s, scientists comparing genes across hoofed mammals kept finding that whales grouped with hippos. One of the strongest studies, published in 1999 by Masato Nikaido, Alejandro Rooney and Norihiro Okada, looked at pieces of “jumping DNA” that insert themselves into the genome. Once such a piece is inserted, it is almost never removed, so animals that share the same insertion at the same spot almost certainly share an ancestor. Hippos and whales shared insertions that no other hoofed mammals had.

Fossil ankle bones. In 2001, fossils of early whales with legs, from Pakistan, showed a distinctive “double-pulley” ankle bone. This bone is a hallmark of artiodactyls, proving that whales descend from hoofed mammals.

Early whale relatives. In 2007, J. G. M. Thewissen and colleagues described Indohyus, a small, deer-like animal from India about 48 million years old. It had thick bones, an adaptation for wading, and ear features found in whales. It showed that the whale line began with small, partly aquatic hoofed mammals.

How long ago did hippos and whales split?

Most estimates place the common ancestor of hippos and whales at roughly 54–60 million years ago, often rounded to about 55 million years. After the split, the whale line evolved quickly: within about 10–15 million years there were fully aquatic early whales.

The hippo line’s story is less clear, because there is a long gap in its fossil record. True hippos only appear around 20 million years ago or less, and the oldest well-known hippos lived in Africa between about 15 and 9 million years ago.

Where do anthracotheres fit in?

For many years, scientists have thought that hippos descended from anthracotheres, a family of extinct hoofed mammals that lived across Asia, Europe, Africa and North America. Some anthracotheres were semiaquatic and looked like slim hippos with narrower heads.

A 2015 study of a fossil called Epirigenys, found in Kenya and about 28 million years old, supported this view. It suggested that hippo ancestors came to Africa from Asia around 35 million years ago and that hippos evolved from one branch of anthracotheres. Not all details are settled, but the overall pattern is widely accepted.

What do hippos and whales have in common?

Hippos and cetaceans share several features:

Some of these similarities may come from their shared ancestry. However, a 2021 study of skin genes found that hippos and cetaceans lost hair and skin oil glands through different genetic changes, which suggests that they adapted to water separately. Their last common ancestor may have lived on land or only partly in water.

What does this mean for pigs?

Hippos were long classified with pigs, mainly because of similar teeth. Until the early 20th century many naturalists considered them close cousins. Modern studies show that this was a mistake: pigs and peccaries branched off much earlier.

In fact, ruminants such as cattle, deer and giraffes are more closely related to hippos and whales than pigs are. So a hippo is closer to a whale than to a cow, and closer to a cow than to a pig.

Are whales fish, then?

No. Whales are mammals, like hippos: they breathe air, give birth to live young and feed them milk. Their family tree within the hoofed mammals is one of the clearest pieces of evidence for that. Read more in Are whales fish? and in our profile of the pygmy hippopotamus, the hippo’s only living relative in its own family.