Despite their physical resemblance to pigs and other terrestrial even-toed ungulates, the closest living relatives of the hippopotamids are cetaceans (whales, dolphins, porpoises, etc.

During the day, hippos remain cool by staying in water or mud, emerging at dusk to graze on grasses.

The Latin word hippopotamus is derived from the ancient Greek ἱπποπόταμος (hippopótamos), from ἵππος (híppos) 'horse' and ποταμός (potamós) 'river', together meaning 'horse of the river'.

[7]: 2  A study examining mitochondrial DNA from skin biopsies taken from 13 sampling locations found "low, but significant, genetic differentiation" among H. a. amphibius, H. a. capensis, and H. a. kiboko.

Several lines of evidence, first from blood proteins, then from molecular systematics,[12] DNA[13][14] and the fossil record, show their closest living relatives are cetaceans (whales, dolphins, and porpoises).

[12] One branch would evolve into cetaceans, possibly beginning about 52 million years ago, with the protowhale Pakicetus and other early whale ancestors collectively known as Archaeoceti.

[16] The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of which in the late Eocene would have resembled skinny hippos with comparatively smaller, narrower heads.

From 7.5 to 1.8 million years ago, a possible ancestor to the modern hippo, Archaeopotamus, lived in Africa and the Middle East.

Evidence from the archaeological site Aetokremnos continues to cause debate on whether or not the species was driven to extinction or even encountered by humans.

[36][7]: 3, 19  Their skeletal structures are graviportal, adapted to carrying their enormous weight,[7]: 8  and their dense bones and low centre of gravity allows them to sink and move along the bottom of the water.

[7]: 17  A folded orbicularis oris muscle allows the hippo to attain an extreme gape without tearing any tissue.

[42][43] Regardless of diet, all hippos secrete these pigments so food does not appear to be their source; rather, they may be synthesised from precursors such as the amino acid tyrosine.

[48] The oldest living hippopotamus in captivity is Lu the Hippo, from the Ellie Schiller Homosassa Springs Wildlife State Park.

[1] Genetic evidence suggests common hippos in Africa experienced a marked population expansion during or after the Pleistocene, attributed to an increase in water bodies at the end of the era.

These findings have important conservation implications, as hippo populations across the continent are currently threatened by loss of access to fresh water.

[66] As of 2016, the Virunga hippo population appears to have increased again, possibly due to better protection from park rangers, who have worked with local fishermen.

[68] A population of hippos exists in Colombia, descended from captive individuals that escaped from Pablo Escobar's estate after his death in 1993.

Their numbers grew to 100 by the 2020s and ecologists believe the population should be eradicated, as they are breeding rapidly and are an increasing menace to humans and the environment.

[7]: 4–5, 49–50 Hippos engage in "muck-spreading" which involves defecating while spinning their tails to distribute the faeces over a greater area.

[82] Hippos mate in the water, with the cow remaining under the surface,[7]: 63  her head emerging periodically to draw breath.

Cases where large lion prides have successfully preyed on adult hippos have been reported, but it is generally rare.

[93] Cut marks on bones of H. amphibius found at Bolomor Cave, a site in Spain preserving fossils dating from 230,000 to 120,000 years ago, provides evidence for Neanderthal butchery of hippopotamuses.

[96] 4,000–5,000 year art showing hippos being hunted have been found in the Tassili n'Ajjer Mountains of the central Sahara near Djanet.

[102] Small boats can easily be capsized by hippos and passengers can be injured or killed by the animals, or drown in the water.

Modern hippo enclosures also have a complex filtration system for the animal's waste, an underwater viewing area for the visitors, and glass that may be up to 9 cm (3.5 in) thick and capable of holding water under pressures of 31 kPa (4.5 psi).

[101]: 158–159 In Egyptian mythology, the god Set takes the form of a red hippopotamus and fights Horus for control of the land, but is defeated.

The goddess Tawaret is depicted as a pregnant woman with a hippo head, representing fierce maternal love.

According to a San story, when the Creator assigned each animal its place in nature, the hippos wanted to live in the water, but were refused out of fear they might eat all the fish.

[101]: 122–123  Ever since Obaysch inspired the "Hippopotamus Polka", hippos have been popular animals in Western culture for their rotund appearance, which many consider comical.

The film Hugo the Hippo is set in Tanzania and involves the title character trying to escape being slaughtered with the help of local children.