River horses sweat sunscreen and cousin whales
The common hippopotamus of sub-Saharan rivers is Earth's third-largest land mammal after elephants and rhinos—yet its closest living relatives are whales and dolphins, split some 55 million years ago. Barrel-bodied and aggressive, hippos cool in water by day and graze alone at dusk.
Adults average about 1.5 tonnes for bulls and 1.3 for cows (means near 1.48 and 1.365 t), with exceptional males to 2.66 t; length can reach roughly 2.9–5.05 m including a short tail, shoulder height about 1.3–1.65 m. Nearly hairless skin 6 cm thick secretes clear fluid that turns red-orange then brown—"blood sweat" that is neither—bearing acidic hipposudoric and norhipposudoric pigments that block ultraviolet light and inhibit bacteria. Eyes, ears, and nostrils sit high so a submerged body can still sense air; nostrils and ears seal underwater. Jaws gape 100–110 degrees; lower canines may grow to 50 cm, used in fighting more than feeding. A three-chambered stomach makes them pseudoruminants that do not chew cud.
Territorial bulls hold a water stretch and five to thirty cows and calves; mating and birth occur in water. On land they rarely defend turf while grazing. Five named subspecies based on skull geography saw only limited genetic support among three tested lineages. Until 1909 naturalists classed hippos with pigs; blood proteins, DNA, and fossils redirected kinship to cetaceans via anthracothere ancestors entering Africa from Asia about 35 million years ago. Kenyapotamus lived 15–9 million years ago; modern Hippopotamus amphibius is clearly Middle Pleistocene, with possible earlier records.
Holocene Madagascar lost three dwarf hippo species within the last thousand years, likely helped by human hunting; Mediterranean islands hosted further dwarfs, including Cyprus's late-surviving form. Huge Hippopotamus gorgops exceeded 4,000 kg. Today habitat loss and poaching for meat and canine ivory threaten an animal already counted among the world's most dangerous.
Call it a river horse if you like—the genome votes marine mammal.
Source: Hippopotamus
