Are Hippos Related to Pigs? Their True Evolutionary Link

Hippos and pigs are distant cousins within the broader group of even-toed ungulates, but they are far less closely related than their barrel-shaped bodies and bristly appearance might suggest. The closest living relatives of hippos are actually whales and dolphins. That finding, firmly established by genetic evidence over the past two decades, overturned more than a century of classification that lumped hippos in with pigs and peccaries based on superficial physical similarities.

Why Hippos Were Grouped with Pigs for So Long

Before DNA-based methods transformed animal taxonomy, biologists sorted mammals by comparing visible anatomy: skull shape, tooth structure, foot bones, and organ layout. By those measures, hippos looked like they belonged near pigs. Both have stocky builds, relatively simple stomachs compared to ruminants like cattle, four-toed feet with each toe ending in a hoof-like structure, and large, forward-facing lower canines. Their teeth wear in similar patterns, and both groups are omnivores or at least not strict grazers in the way cattle and antelope are. Anatomical studies even found structural parallels in internal organs between pigs, peccaries, and hippos.

This led to the traditional grouping called “Suiformes,” which placed Suidae (pigs), Tayassuidae (peccaries), and Hippopotamidae (hippos) together as a natural group within Artiodactyla, the order of even-toed hoofed mammals. The logic was reasonable given the evidence available: if three families share a long list of physical features, they probably share a recent common ancestor. That logic turned out to be wrong in this case, because many of those shared features were either ancestral traits retained from a much older common ancestor or were coincidental resemblances shaped by similar lifestyles.

How Genetics Rewrote the Hippo Family Tree

The revolution came in the 1990s and 2000s, when molecular biologists started comparing DNA sequences across mammal lineages. One landmark study analyzed short and long interspersed elements, which are chunks of DNA that insert themselves into the genome and stay put over evolutionary time. Because these insertions are essentially irreversible, they act as clean markers of shared ancestry. The results were striking: hippos and cetaceans (whales, dolphins, and porpoises) formed a single group to the exclusion of all other artiodactyls, while pigs and peccaries clustered together in a completely separate branch.¹

Follow-up studies using larger datasets and more sophisticated methods confirmed this arrangement. A comprehensive phylogenomic analysis found that Cetacea nests within Artiodactyla as the sister group of Hippopotamidae, and that camels were the earliest branch to diverge from the rest of the group.² In other words, hippos are more closely related to a blue whale than to a warthog. The broader grouping that contains both the old Artiodactyla and Cetacea is now called Cetartiodactyla, reflecting the fact that whales evolved from within the even-toed ungulates rather than alongside them.

This rearrangement did not happen overnight or without pushback. Paleontologists, who rely on bones and teeth rather than DNA, initially resisted the hippo-whale connection because the known fossil record did not provide an obvious series of intermediate forms linking the two. But as more fossils turned up and more genes were sequenced, the molecular evidence became overwhelming. By the 2010s, the hippo-cetacean sister relationship was essentially consensus across both molecular and morphological studies.³

Where Pigs Actually Fit

Pigs and peccaries do belong to the same broad order as hippos, but their branch split off much earlier. Suidae (true pigs, including wild boar, warthogs, and bushpigs) and Tayassuidae (peccaries, which are New World animals sometimes confused with pigs) diverged from each other roughly 37 million years ago.⁴ Together, these two families sit on a branch of the artiodactyl tree that diverged from the hippo-whale lineage even further back in time.

Think of it this way: if you traced a hippo’s ancestry backward and a pig’s ancestry backward, you would eventually reach a shared ancestor, but that common ancestor lived tens of millions of years ago and looked nothing like either a modern pig or a modern hippo. By contrast, the shared ancestor of hippos and whales lived much more recently, and subsequent fossils have started to fill in that gap.

The old “Suiformes” grouping that united pigs, peccaries, and hippos is now considered an artifact of convergent evolution and ancestral retention. Many of the features that made these animals look alike are primitive traits that the earliest artiodactyls had and that some lineages simply kept while others, like cattle and deer, modified. The pig-hippo resemblance is real in a superficial sense but misleading about actual kinship.

The Fossil Bridge Between Hippos and Whales

If hippos and whales are sister groups, there should be fossils connecting them. The most important candidates are the anthracotheres, a diverse family of semi-aquatic and terrestrial artiodactyls that flourished from roughly 46 million years ago into the Miocene. Multiple lines of evidence now point to a subfamily of anthracotheres called the Bothriodontinae as the stem group from which modern hippos evolved.⁵ The discovery of a fossil species called Epirigenys lokonensis in Kenya provided further support for this hypothesis, offering a transitional form between anthracotheres and the earliest true hippos.

Anthracotheres were widespread across Africa, Asia, and Europe. Many of them were large, semi-aquatic animals with broad snouts and eyes positioned high on the skull, traits they share with living hippos. Their body plan suggests a lifestyle that involved wading and foraging in rivers and swamps, which fits neatly with the idea that hippos inherited their aquatic tendencies from ancestors who were already spending time in water.

Whales, meanwhile, descended from a different lineage of early artiodactyls that were fully terrestrial before beginning the transition to aquatic life around 50 million years ago. The hippo-whale common ancestor was probably a land-dwelling or semi-aquatic even-toed ungulate, and the two lineages then adapted to water independently: whales went fully aquatic, while hippos settled into their current amphibious lifestyle. This parallel path toward water makes the genetic relationship between them less bizarre than it first sounds.

What Hippos and Whales Share at the Genetic Level

The relationship between hippos and whales is not just an abstraction drawn from a family tree. Their genomes carry specific parallel changes that reflect their shared affinity for water. A genomic comparison identified eight skin-related genes that have been inactivated in both cetaceans and hippos, including genes linked to sebaceous glands, hair follicles, and the process by which skin cells harden. Both groups have largely lost their body hair, lack functional sebaceous glands, and have modified the outer layers of their skin in ways consistent with spending extensive time submerged.⁶

Interestingly, the mutations that knocked out these genes are different in the two lineages. None of the inactivating mutations are shared between cetaceans and hippos, which means the gene losses happened independently after the two lineages diverged. The estimated timing supports this: the skin-gene changes in cetaceans date to about 46.5 million years ago, while the same changes in hippos occurred around 30.5 million years ago, about 16 million years later.⁶ So while hippos and whales are genetically close, their aquatic adaptations are the result of convergent evolution on top of shared ancestry rather than traits inherited from a single water-loving ancestor.

Pigs share none of these aquatic skin adaptations. Domestic pigs can swim, and wild boar occasionally forage in shallow water, but their skin biology is fundamentally different. Pigs retain dense body hair, functional sebaceous glands, and a keratinization program designed for life on land. This is another illustration of why the pig-hippo comparison breaks down once you look past surface-level body shape.

Built for the Water in Ways Pigs Are Not

Hippos have a suite of anatomical features that only make sense when you appreciate their aquatic ancestry. One of the most revealing is their bone structure. Hippo limb bones are unusually dense, with compact bone replacing the spongy tissue that fills the interior of most mammal bones. This condition, called osteosclerosis, increases the animal’s overall body density, which helps it stay on the bottom when walking along riverbeds. The extra ballast from denser limb bones stabilizes the hippo while submerged, functioning like built-in weights that counteract buoyancy.⁷

This is a trait hippos share conceptually with early whales, whose fossils also show increased limb-bone density during the transitional period when they were moving from land to sea. Dense bones are common in semiaquatic and aquatic mammals as a means of buoyancy control.⁷ Pigs have no such adaptation. Their bones are built like those of other fully terrestrial mammals, with lighter cancellous tissue in the interior optimized for load-bearing on land rather than underwater stability.

Hippo underwater locomotion itself is distinctive. Rather than swimming in the conventional sense, hippos walk or bounce along the riverbed in what researchers describe as a slow-motion gallop, pushing off with their hind legs and gliding forward. Their dense bones and overall body composition make them negatively buoyant, which is exactly what you would want if your survival strategy involves spending up to 16 hours a day submerged. Pigs placed in water float and paddle; hippos sink and stride.

The Red Sweat and Hippo Skin Chemistry

One of the most distinctive things about hippos is their so-called “blood sweat,” a reddish secretion that appears on their skin within minutes of emerging from water. Despite its alarming color, the substance is neither blood nor sweat in the conventional sense. It is a viscous, initially colorless fluid that gradually turns red and then brown as its pigments react with air. The pigments are unusual non-benzenoid aromatic compounds that turn out to have both antibiotic and sunscreen properties.⁸

This secretion solves a real biological problem. Hippos have nearly hairless skin and lack functional sebaceous glands, which in most mammals help waterproof and protect the skin surface. Without those glands or a fur coat, a hippo’s skin would be vulnerable to UV radiation and bacterial infection during the hours it spends on land. The pigmented secretion fills both roles at once, acting as a natural sunblock and an antimicrobial layer. It is a chemical workaround for a body that has shed the usual mammalian defenses in favor of aquatic efficiency.

Pigs, by contrast, are famously susceptible to sunburn precisely because they do retain pale, relatively hairless skin in many domestic breeds but lack any equivalent chemical protection. Wild pigs cope by wallowing in mud, which provides a physical sunscreen layer. The hippo solution is more elegant and more unusual, and it highlights how profoundly different the two animals are under the skin despite their passing resemblance above it.

Why the Misconception Persists

Even after two decades of clear genetic evidence, the idea that hippos are basically “water pigs” lingers in popular culture. Part of this is linguistic: the word “hippopotamus” comes from the Greek for “river horse,” which is not any more helpful taxonomically but at least does not invoke pigs. English-speaking cultures, however, have informally associated hippos with pigs for centuries, and the comparison is reinforced by genuinely shared features like broad, flat snouts, small eyes, thick skin, and a general absence of the graceful proportions people associate with other hoofed mammals.

Children’s books and zoo signage sometimes perpetuate the connection, and the older “Suiformes” classification still appears in some outdated reference material. The true relationship with whales is counterintuitive enough that people reasonably resist it. A hippo looks nothing like a dolphin. But evolutionary relationships are about shared ancestry, not shared appearance. A bat looks nothing like a cow, yet both are members of the same mammalian superorder. Convergent evolution and adaptation to different environments can make close relatives look wildly different and distant relatives look strikingly alike.

The pig-hippo confusion is actually a useful case study in why morphology alone can be misleading. Many of the physical similarities between pigs and hippos are either ancestral traits retained from the earliest even-toed ungulates or independent adaptations to similar ecological pressures. The genetic evidence cuts through those surface resemblances to reveal the actual branching pattern of the family tree, and on that tree, hippos sit firmly next to whales, far from the pig branch.

How Camels and Ruminants Fit into the Picture

The full family tree of Cetartiodactyla contains some other surprises worth knowing. Camels and llamas (Tylopoda) are the most basal living group, meaning they split off from the rest earliest.² After camels diverged, the tree branches into pigs and peccaries on one side and a large group containing ruminants (cattle, deer, giraffes, antelope), hippos, and whales on the other. Within the ruminants, chevrotains (tiny deer-like animals from Southeast Asia) were the first to diverge.¹

This means a cow is more closely related to a humpback whale than either is to a pig. And a giraffe is more closely related to a hippo than a hippo is to a warthog. These relationships feel absurd until you internalize the fact that whales evolved from within the even-toed ungulates rather than being an outside group that happens to share a few genes. Cetacea is deeply nested inside Artiodactyla, which is why the combined order is now called Cetartiodactyla.¹ The older classification, which kept whales in their own separate order, made them seem more exotic and distant than they really are. In truth, they are hoofed mammals that lost their hooves.