THE ANIMAL FILES

Nile hippopotamus

Hippopotamus amphibius

The “river horse” has a surprising family tree.

A common hippo splits its day between water and land. The high-set eyes and nostrils fit a body that spends hours mostly submerged; the evening menu leads it back onto grass. Its name is a poor guide to its relatives, and its reddish skin secretion is not what it first appears to be.[1][3][2]

Follow the evidence ↓
Nile hippopotamus
Image: Bernard DUPONT from FRANCE · Commons · CC BY-SA 2.0
Daytime refuge
Rivers, lakes and shallow water[1]
Usual food
Grass grazed on land[1]
Red skin coating
Protective pigments, not blood[2][1]

A head built for a low profile

Eyes, ears and nostrils sit high on a hippo’s head. It can therefore keep much of its body underwater while looking, listening and breathing above the surface. At night it commonly leaves the water to graze. This makes the hippo a useful reminder that “aquatic” is not an all-or-nothing description: where an animal rests and where it feeds can be different places.[1]

The red coating is chemistry, not an injury

The skin releases a secretion that can turn reddish and later brown. Researchers isolated red and orange pigments and found sunscreen and antibiotic activity in them. The familiar nickname “blood sweat” is misleading: this is not blood leaking through the skin. The finding concerns properties of particular pigments; it does not mean a hippo cannot get sun damage or an infected wound.[2][1]

A relative is not an ancestor

The horse in “river horse” is a resemblance in a name, not a family-tree assignment. Among living mammals outside cetaceans, the hippopotamus family is closest to whales and dolphins. The two lineages share an ancestor; neither living group needs to be pictured as the unfinished version of the other. Genetics and fossil anatomy are more reliable guides here than a broad snout or a shared river habitat.[3]

A COMMON MIX-UP

Do those huge canine teeth mean a hippo mainly eats meat?

No. Its usual diet is grass, while the large canine teeth are important in fighting and defense. A conspicuous body part does not always serve the feeding role you first imagine.[1]

TRY A LITTLE REASONING

Which is better evidence of hippo–whale kinship: both use water, or matching genetic and fossil evidence?

Reveal the explanation

The genetic and fossil evidence. Similar habitats can produce similar adaptations in separate lineages. A family tree needs evidence of shared ancestry, not just a shared address.[3]

Sources & editorial notes

Original explanations, checked against the sources below. The image has its own credit and licence above.

  1. San Diego Zoo Wildlife Alliance — Hippo: skin, habitat and diet
  2. Saikawa and colleagues, Nature (2004) — Hippo pigments (abstract via PubMed)
  3. American Museum of Natural History — Whale and dolphin evolution

Last fact check:

One discovery leads to another

Blue whaleA giant with a very small menu.Balaenoptera musculus ↗Saltwater crocodileIts name says saltwater. Its range says more.Crocodylus porosus ↗PlatypusEyes shut. Dinner detector on.Ornithorhynchus anatinus ↗