If you were asked to pick the closest living relative of a hippopotamus, your mind would probably jump to a pig. They both have stout, round bodies, stubby legs, non-ruminant snouts, and a general love for wallowing in mud.
For over a century, naturalists thought the exact same thing. But modern genetics revealed a plot twist that blew classical biology wide open: hippos are far more closely related to blue whales than they are to pigs.
When you start to investigate the origin of the name for a Hippopotamus, and what people of different countries call this well known African animal, and you start to delve into the genomic secrets of its genome, you start to realize that one language in particular might have had fore-knowledge of its genetic family relationships that only became well known in the 1990's.
The word for a Hippo in Afrikaans is the word: SEEKOEI (which can be directly translated as : Sea Cow, or 'Cow of the Sea').
Just how close this animal is to a "Cow of the sea", is what I want to introduce to you in this article.
In fact, if you sat down a hippo, a pig, and a 100-foot-long blue whale for a family reunion, the hippo and the blue whale would be first cousins—while the pig is just a distant relative who showed up for the free food.
The Great Plot Twist
For decades, scientists lumped hippos into the pig family tree based purely on appearances. But in the late 1990s, geneticists analyzing DNA sequences discovered something astounding: whales didn't just evolve near even-toed ungulates (hoofed animals)—they evolved inside their family tree.
Around 55 million years ago, a small, four-legged, hoofed mammal—not unlike a modern miniature deer—decided to start spending more time in rivers and swamps to escape predators and hunt for aquatic food.
Over millions of years, this single lineage split into two paths:
One group stayed semi-aquatic on land, eventually producing the modern hippopotamus.
The other group plunged headfirst into the open oceans, losing its hind legs and transforming into cetaceans (whales, dolphins, and porpoises).
Pigs branched off millions of years before this split even happened.
5 Mind-Blowing Similarities Between Hippos & Blue Whales
While a 3-ton river beast and a 150-ton ocean giant look entirely different on the surface, beneath the skin, they share anatomical and physiological "secrets" that no other land mammals possess.
1. Hairless Skin and Missing Sweat Glands
Unlike pigs, which are covered in coarse bristles, both hippos and blue whales have abandoned traditional hair and sebaceous (oil/sweat) glands.
Whales are completely hairless, relying on thick blubber for insulation.
Hippos have virtually no body hair. To survive on land without sweat glands, hippos secrete a unique red-orange fluid often called "blood sweat." It acts as a natural sunscreen, moisturizer, and antibacterial lotion!
2. Underwater Nursing Direct into the Mouth
Feeding a baby underwater is a massive engineering challenge—water can easily mix with milk or flood the calf's lungs. Both hippo mothers and blue whale mothers solve this with the exact same mechanism: voluntary milk squirts. The mother contracts specialized muscles around her mammary glands to pump dense, fat-rich milk straight into the baby's mouth underwater, allowing the calf to drink without swallowing seawater.
3. Underwater "Talk" Without Vocal Cords
If you've ever heard a whale song, you know cetaceans make some of the loudest sounds on Earth. But neither blue whales nor hippos have traditional mammalian vocal cords. Instead, both species use complex systems of internal air sacs around their larynx to generate low-frequency booms, clicks, and rumbles that travel effortlessly through water. Hippos can even emit sounds that travel through water and air simultaneously—a trick known as amphibious vocalization.
4. Hidden Internal Testes
In almost all land mammals (including pigs, dogs, and humans), male reproductive organs are housed externally to keep them cool. Hippos and blue whales are notable exceptions:
Male hippos have retractable, internal sub-inguinal testes.
Blue whales have completely internal abdominal testes.
This keeps their bodies smooth and hydrodynamic for underwater travel while protected from the crushing pressures of deep water.
5. Multi-Chambered "False Ruminant" Stomachs
Cows chew their cud using a complex multi-chambered stomach, but pigs have a simple single-chambered stomach (much like humans). Hippos and blue whales both possess complex, multi-chambered stomachs—despite having completely different diets! A hippo uses its three-chambered stomach to ferment grass, while a blue whale uses its three-chambered stomach to crush and digest millions of hard-shelled krill.
Strange Anecdotes You Probably Didn't Know
1. Hippos Don't Actually Swim—They Walk on the River Floor!
Despite spending up to 16 hours a day in water, hippos cannot swim. Their bones are so dense and heavy that they sink like stones. Instead, they sink to the riverbed and walk or bound along the bottom in a slow-motion moonwalk. This heavy-boned adaptation (pachyosteosclerosis) is identical to the dense bones found in ancient fossil whales like Ambulocetus before they developed light, buoyant bones for open-ocean swimming!
2. The Ankle Bone That Solved the Mystery
For over a century, scientists argued about whether whales came from extinct meat-eating land animals or hoofed grazers. The smoking gun came when paleontologists uncovered the fossil ankle bone (astragalus) of an early whale called Pakicetus. It had a unique "double-pulley" shape found in only one group of animals on Earth: artiodactyls (even-toed hoofed mammals like hippos). That single ankle bone officially locked whales into the hippo family tree.
3. The Shared Loss of Vitamin C
Deep in their DNA, both hippos and blue whales carry the exact same broken gene for synthesizing Vitamin C (GULO). Somewhere in their shared ancestral swamp 50 million years ago, a mutation turned this gene off. Because their aquatic diets provided plenty of nutrients, the broken gene was passed down to both modern hippos and modern whales!
In order to determine the chromosomal similarities I used the Visual Genome Browser software to determine the synteny between the Chromosomes of the Hippopotamus and the chromosomes of the Blue Whale.
The Dot-plot graphs shows a green downwards sloping line for all chromosome runs which are aligned in the same direction along the chromosome and a blue upwards slanted line for runs of the chromosomes which are aligned in the backwards/anti-parallel orientation. (This usually happens during evolutionary time when a chromosome breaks and then gets fixed by the cellular repair machinery by joining the opposite end of the chromosome to the break position)
Dot Plots
GCF_009873245.2 (BlueWhale) runs along vertical axis
GCF_030028045.1 (Hippo) runs along horizontal axis
NC_045785.1 (chr1) BLUE WHALE
Chromosome 1's genes have been spread across Chromosome 3 and Chromosome 1 of the Hippo genome.
Blue Whale Chromosome first / Hippopotamus Chromosome Second
NC_045785.1 (chr1) / NC_080188.1 (chr3)
NC_045785.1 (chr1) / NC_080186.1 (chr1)
NC_045786.1 (chr2) BLUE WHALE
NC_045786.1 (chr2) / NC_080187.1 (chr2)
NC_045786.1 (chr2) / NC_080189.1 (chr4)
NC_045787.1 (chr3) BLUE WHALE
NC_045787.1 (chr3) / NC_080186.1 (chr1)
NC_045787.1 (chr3) / NC_080200.1 (chr15)
NC_045788.1 (chr4) BLUE WHALE
Here the BLUE WHALE Chr4 is spread among the Hippo Chr 6 and Chr 10
NC_045788.1 (chr4) / NC_080195.1 (chr10)
NC_045788.1 (chr4) / NC_080191.1 (chr6)
NC_045789.1 (chr5) BLUE WHALE
NC_045789.1 (chr5) / NC_080188.1 (chr3)
NC_045789.1 (chr5) / NC_080198.1 (chr13)
NC_045790.1 (chr6) BLUE WHALE
Here for example the BLUE WHALE Chromosome 6 is fully found on the Hippo Chromosome 2.
NC_045790.1 (chr6) / NC_080187.1 (chr2)
NC_045791.1 (chr7) BLUE WHALE
Here again the BLUE WHALE Chromosome 7 can be fully found on the HIPPO Chromosome 8, but there are 2 parts which run in the same direction and 2 parts which run in the opposite direction. (As it was totally arbitrary in which directions geneticists would sequence the chromosome seeing that a chromosome can be sequences from either one of the 2 ends)
NC_045791.1 (chr7) / NC_080193.1 (chr8)
NC_045792.1 (chr8) BLUE WHALE
NC_045792.1 (chr8) / NC_080194.1 (chr9)
NC_045793.1 (chr9) BLUE WHALE
NC_045793.1 (chr9) / NC_080189.1 (chr4)
NC_045794.1 (chr10) BLUE WHALE
NC_045794.1 (chr10) / NC_080192.1 (chr7)
NC_045794.1 (chr10) / NC_080197.1 (chr12)
NC_045795.1 (chr11) BLUE WHALE
NC_045795.1 (chr11) / NC_080196.1 (chr11)
NC_045795.1 (chr11) / NC_080198.1 (chr13)
NC_045796.1 (chr12) BLUE WHALE
NC_045796.1 (chr12) / NC_080191.1 (chr6)
NC_045797.1 (chr13) BLUE WHALE
NC_045797.1 (chr13) / NC_080192.1 (chr7)
NC_045798.1 (chr14) BLUE WHALE
NC_045798.1 (chr14) / NC_080196.1 (chr11)
NC_045798.1 (chr14) / NC_080193.1 (chr8)
NC_045799.1 (chr15) BLUE WHALE
NC_045799.1 (chr15) / NC_080197.1 (chr12)
NC_045799.1 (chr15) / NC_080194.1 (chr9)
NC_045800.1 (chr16) BLUE WHALE
NC_045800.1 (chr16) / NC_080190.1 (chr5)
NC_045801.1 (chr17) BLUE WHALE
NC_045801.1 (chr17) / NC_080190.1 (chr5)
NC_045802.1 (chr18) BLUE WHALE
NC_045802.1 (chr18) / NC_080199.1 (chr14)
NC_045803.1 (chr19) BLUE WHALE
NC_045803.1 (chr19) / NC_080201.1 (chr16)
NC_045804.1 (chr20) BLUE WHALE
NC_045804.1 (chr20) / NC_080202.1 (chr17)
NC_045805.1 (chr21) BLUE WHALE
NC_045805.1 (chr21) / NC_080195.1 (chr10)
NC_045806.1 (chrX) BLUE WHALE
NC_045806.1 (chrX) / NC_080203.1 (chrX)
NC_001601.1 BLUE WHALE Mitochondrial chromosome
Y axis against HIPPO Mitochondrial chromosome on X- Axis
NC_001601.1 / NC_000889.1 (chrM)
Sequence Alignment Homology
Protein coding sequence alignments for key nuclear genes—such as beta-casein, von Willebrand factor, and myoglobin—show 90-95% amino acid identity between Hippopotamus amphibius and Balaenoptera musculus, vastly outranking sequence alignment scores between hippos and swine (Sus scrofa). Additionally, the structure of myoglobin in both species shows positive selection for high surface charge density, allowing dense muscle packing of oxygen for prolonged diving without aggregation.
Key Features of the Blue Whale MB Gene
Gene Symbol & Structure:
Symbol:
MB(NCBI Gene / Ensembl nomenclature).Structure: Like other mammalian globin genes, the cetacean MB gene consists of 3 exons separated by 2 introns. It translates into a monomeric hemoprotein composed of 153–154 amino acids.
Surface Charge Adaptation (Electrostatic Repulsion):
The blue whale MB gene exhibits key adaptive mutations that increase the positive surface net charge (Z) of the protein.
This high positive charge causes myoglobin molecules to electrostatically repel each other rather than clump or aggregate, allowing blue whale muscle cells to store myoglobin at concentrations up to 10–20 times higher than terrestrial mammals without precipitating.
Transcriptional Regulation:
Research on cetacean MB regulatory sequences shows that whale myoglobin genes actually possess relatively low basal promoter activity compared to terrestrial mammals.
High intracellular accumulation is driven primarily by extreme protein stability (slow turnover/degradation rate) alongside developmental expression during muscle maturation, rather than abnormally high transcription rates alone.
Here for example is a gene with more than 96% identity across
UTP20 gene (also known in humans as DRIM or 1A6) encodes a protein that is a crucial component of ribosome biogenesis—the fundamental process by which cells construct protein-synthesizing ribosomes.The Takeaway
The next time you see a blue whale surfacing in a ocean documentary or a hippo yawning in a river, remember that you're looking at two sides of the same evolutionary coin.
One chose the riverbanks; the other chose the endless sea. But deep inside their DNA, their bones, and their physiology, they remain eternal underwater cousins.
