Which creature is a land animal closely related to marine mammals, carries its own pharmacy in its skin, and is the latest social media star?
The Hippopotamus!
Baby hippos are having a moment on social media. From Mr. Mars Potato Jones and his mother Posie at Tanganiyka Wildlife Park in Kansas to Moo Deng at Thailand’s Khao Kheow Open Zoo, hippos are some of the latest online animal celebrities. Inspired by Tania Roa’s 2021 Featured Creature on the hippo, we’re revisiting these fascinating ecosystem engineers.
River Horse
Despite their resemblance to large water pigs or even cows, hippopotamuses are named from the ancient Greek meaning for “river horse.” Their closest living relatives are cetaceans—whales, dolphins, and porpoises—forming the clade Whippomorpha within even-toed ungulates (artiodactyls). Genetic studies reveal shared DNA sequences unique to hippos and cetaceans, confirming they diverged from a common ancestor around 52–47 million years ago in the Eocene. Fossil evidence traces hippos to anthracotheres, semiaquatic artiodactyls from the late Eocene (~40 million years ago), with the hippopotamid lineage solidifying in the late Miocene (~7.4 million years ago) via forms like Epirigenys and Bothriogenys. This makes hippos the end of Africa’s longest terrestrial cetartiodactyl lineage, while cetaceans took to full oceans.
While they make look like descendants of sauropsid dinosaurs, they evolved post-extinction (after 66 million years ago) from synapsid-mammal stock. They do have a distant Triassic cousin, the giant synapsid Lisowicia bojani (208 million years ago), a 9-ton, hippo-like herbivore that rivaled early dinosaurs in size.
Who Needs Walgreens?
While hippos have thin, hairless skin prone to cracking in sub-Saharan sun, they’ve adapted over time and developed specialized mucous glands that secrete a viscous, oily-red-orange fluid that acts as a built-in sunscreen. Often mislabeled as “blood sweat,” the secretion starts off colorless and oxides into a reddish hipposudoric acid and orange norhipposudoric acid, both non-benzenoid aromatics derived from homogentistic acid. These pigments create a UV absorbent, retain moisture, and exhibit antibiotic activity against bacteria like Pseudomonas aeruginosa and Escherichia coli. This “built-in pharmacy” evolved for hyper-arid protection, hinting at bio-inspired human antimicrobials.
“Mother and Young Hippo, Uganda” by Rod Waddington is licensed under CC BY-SA 2.0 
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Land Cetaceans Dropping Nutrient Bombs
Hippos spend about 16 hours daily submerged in order to thermoregulate and have adapted sophisticated sensory awareness capabilities. Their eyes, nostrils, and ears remain above water, while their jawbones detect hydro-vibrations under the surface. This 360° awareness enables them to communicate with other hippos and maintain contact with their pod, detect predators and threats, and navigate murky waters with low visibility.
Hippos can hold their breath underwater for approximately 5 minutes, keeping their nostrils and ears sealed against the water. They don’t technically swim; their pachyosteosclerotic (ultra-dense) bones prevent buoyancy, so they “hop” along riverbeds, walking in depths up to 5m despite their 3-ton mass.
Nocturnal grazers, hippos act as ecosystem engineers, consuming short grass, and defecating massive dung loads directly into waterways. Their waste delivers nitrogen, phosphorus, and silica, often at 10x higher concentrations than the surrounding grasslands. The silica boost alone fuels diatom algae blooms that support entire food webs!
Hippos at Risk
Hippopotamuses were once found throughout more than half of the African continent. Unfortunately, they are now classified as Vulnerable by the International Union for the Conservation of Nature due primarily to habitat loss and poaching, with population declines ranging from 10,000-18,000 since 2008.
Climate-driven droughts are exacerbating loss of hippo habitats, causing literal downstream ecosystem impacts from the reduction in nutrient cycling. Without hippo dung delivering concentrated nitrogen, phosphorus, and silica to rivers, diatom algae blooms collapse, slashing fish biomass by up to 88% and disrupting food webs.
Sources
- UC Berkeley, French scientists find missing link between the whale and its closest relative, the hippo
- Baby Hippo Mars Mission Control
- International Fund for Animal Welfare – Hippopotamus
- 10 Fascinating Facts About Hippos
- Ancient hippo-like reptile was a giant to rival the dinosaurs
- Effects of the hippopotamus on the chemistry and ecology of a changing watershed
- Hippos and Whales: Unlikely Cousins
- Red Sweat: Hippo skin oozes antibiotic sunscreen
- World Land Trust – Hippopotamus
