Hippopotamus

• 1. Introduction

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  The hippopotamus - whose name comes from the Ancient Greek for "river horse" - is one of the stranger animals alive on Earth today. At first glance, it looks something like a very large pig. But its closest living relatives are whales and dolphins. That surprise alone should tell you something about how far this animal's story reaches back into geological time.

  Hippos are the third-largest land mammals on the planet, after elephants and some rhinoceros species. A bull can weigh around 1.48 tonnes, and exceptionally large males have reached 2.66 tonnes. Yet for all that mass, they carry it on legs short enough to look almost comical, and their dense bones actually help them sink and walk along riverbeds rather than swim.

  They spend their days in rivers, lakes, and mangrove swamps across sub-Saharan Africa, surfacing to breathe every few minutes, and only emerge at dusk to graze alone on grass. They are widely considered among the most dangerous animals in the world. And they have shaped human culture - in ancient Egypt, in Greek scholarship, in Yoruba ritual, and in Disney films - for thousands of years. How one animal can be simultaneously so familiar and so strange is what this documentary is about.

• 2. Relatives in the Water

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  Until 1909, naturalists confidently grouped hippos alongside pigs, based on shared molar patterns. The reclassification that followed ranks among the more surprising reversals in zoological history.

  Evidence from blood proteins, then from molecular systematics and DNA, eventually confirmed that the closest living relatives of hippos are the cetaceans: whales, dolphins, and porpoises. The two lineages diverged roughly 55 million years ago. Before that split, hippos and whales shared a common semiaquatic ancestor that had itself branched off from other even-toed ungulates somewhat earlier.

  One branch of that ancestral split gave rise to the protowhale Pakicetus and the group collectively known as Archaeoceti, which eventually became the fully aquatic cetaceans. The other branch became the anthracotheres, described in the fossil record as large four-legged animals that, in their earliest forms, resembled lean hippos with narrower heads. All branches of the anthracotheres, except the one that evolved into Hippopotamidae, died out during the Pliocene.

  An early hippopotamid, the genus Kenyapotamus, lived in Africa from roughly 15 million years ago. A possible ancestor to the modern hippo, Archaeopotamus, lived in Africa and the Middle East from around 7.5 million years ago. The oldest confirmed records of the genus Hippopotamus date to the Pliocene, roughly 5.3 to 2.6 million years ago. The discovery of Epirigenys in East Africa, thought to be a descendent of Asian anthracotheres, suggests hippo ancestors entered Africa from Asia rather than evolving there from scratch.

• 3. Body Built for Two Worlds

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  Hippo skin runs 6 cm thick across much of the body, and underneath it sits a skeleton described as graviportal: bones designed to bear enormous weight, dense enough that the animal sinks rather than floats. That density, combined with a low centre of gravity, lets hippos walk and bounce along riverbeds instead of swimming. Despite their reputation, it is actually debated whether hippos can swim at all.

  An adult hippo needs to surface for air at least every six minutes. Young hippos must breathe every two to three minutes. Remarkably, hippos can sleep submerged, rising intermittently to breathe seemingly without waking. The nostrils and ears seal shut underwater, while nictitating membranes cover the eyes. The eyes, ears, and nostrils themselves sit high on the skull's roof, so an almost entirely submerged hippo can still perceive its surroundings.

  The jaw opens at 100-110 degrees, powered by massive masseter and digastric muscles. On the lower jaw, the incisors grow to 40 cm and the canine teeth to 50 cm. These are used for combat, not feeding. For grazing, hippos rely on broad, flat lips to pull grass, which then passes to molars with complex enamel folds. The stomach has three chambers, but hippos do not chew cud; they are classified as pseudoruminants.

  Perhaps the most unusual feature of hippo biology is a skin secretion often called "blood sweat" - though it is neither blood nor sweat. Initially colourless, it turns red-orange within minutes, eventually becoming brown. Researchers have identified two pigments: red hipposudoric acid and orange norhipposudoric acid. Both inhibit bacterial growth, and both absorb light in the ultraviolet range, functioning as a natural sunscreen. Every hippo produces these pigments regardless of diet, suggesting they are synthesised internally, possibly from amino acid precursors such as tyrosine.

• 4. Life Inside the Pod

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  Hippo pods can swell to over 100 individuals, but the social bonds inside them are almost nonexistent among adults. They rest in close physical proximity yet develop little attachment to one another. The challenge of studying their social dynamics is compounded by the fact that hippos are not sexually dimorphic: cows and young bulls look nearly identical in the field.

  Territorial bulls each control a stretch of water - ranging 250-500 m in lakes or 50-100 m in rivers - and have breeding access to all cows within that territory. Younger males may remain in the group as long as they defer to the dominant bull. When the water dries up, territories are abandoned entirely. Crucially, bulls hold territory only in water, not on land; when hippos emerge to graze at dusk, they do so as individuals with no territorial behaviour.

  Fighting between bulls involves using the incisors as shields to block attacks and the large canines as offensive weapons. When a habitat shrinks or a population grows too large, bulls sometimes commit infanticide, though this is uncommon under normal conditions. The most common hippo vocalisation is the "wheeze honk", a call that begins as a high-pitched squeal and deepens into a resonant sound that travels long distances. Hippos recognise individual voices and react more strongly to the calls of strangers than to familiar individuals - a pattern researchers call the dear enemy effect.

  Hippos also hold their heads partially above the water and send out calls that travel through both air and water simultaneously, with individuals responding from both above and below the surface. Clicks produced underwater may have echolocative properties.

• 5. A Range That Once Spanned Continents

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  Hippopotamus amphibius arrived in Europe around 560,000-460,000 years ago. During interglacial periods such as the Last Interglacial, which ran from approximately 130,000 to 115,000 years ago, hippos reached as far north as Stockton-on-Tees in Great Britain and extended across northern France, Belgium, the Netherlands, and western Germany. The youngest European records come from Greece and the Rhine Graben of southwest Germany, dating to around 40,000-30,000 years ago.

  Fossil remains also place hippos across the Arabian Peninsula during wetter periods, including the Nafud desert of northern Saudi Arabia and Shanidar Cave in northern Iraq. Archaeological evidence indicates their presence in the Levant as recently as 3,000 years ago. During the African Humid Period, roughly 10,000-4,000 years ago, hippos ranged across the Sahara, then a savannah crossed with rivers and lakes. Rock art depicting hippos has been found in the Tassili n'Ajjer Mountains of the central Sahara, near Djanet.

  In Egypt, hippos were common along the Nile from the Sudan border to the delta for much of recorded history. Human persecution fragmented that range through the Ptolemaic and Roman periods and then the early modern period; by around 1600, hippos had become rare in the Egyptian Nile. Reports of the last hippo killed in Natal, South Africa, came at the end of the 19th century.

  One entirely separate, unplanned population exists today in Colombia. It descends from hippos that escaped from the private estate of Pablo Escobar after his death in 1993. Their numbers grew to 100 by the 2020s. Ecologists consider the population an ecological threat, and efforts to control it through sterilisation and culling are ongoing.

• 6. Threats and the Numbers That Follow

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  As of 2017, the IUCN Red List classifies the hippopotamus as vulnerable. The global population was then estimated at between 115,000 and 130,000 animals, with the listing noting a stable population trend at that time.

  The most severe documented collapse occurred in Virunga National Park in the Democratic Republic of the Congo. A population of around 29,000 hippos in the mid-1970s had fallen to between 800 and 900 by 2005. The cause was the disruption of the Second Congo War. Poachers identified in connection with the killing included Mai-Mai rebels, underpaid Congolese soldiers, and local militia groups. By 2016, the Virunga population appeared to have begun recovering, a development attributed partly to cooperation between park rangers and local fishermen.

  Hippos are listed under Appendix II of the Convention on International Trade in Endangered Species, meaning all international trade in hippo products - including body parts and derivatives - requires documentation from both exporting and importing authorities. Hippo meat is highly valued in parts of central Africa; the teeth are traded as a substitute for elephant ivory. Black-market sales of hippo meat are difficult for park authorities to track, even when the sale is illegal.

  Genetic evidence points to a significant population expansion during or after the Pleistocene, likely connected to an increase in freshwater bodies at the end of that era. That historical resilience now sits against a present in which loss of access to fresh water is the primary conservation threat across the continent.

• 7. Hippos and Human History

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  Cut marks on hippo bones at Bolomor Cave in Spain, a site preserving fossils from 230,000 to 120,000 years ago, show that Neanderthals butchered hippos. The earliest evidence of modern human interaction with the species comes from the Bouri Formation, where hippo bones bearing butchery marks date to around 160,000 years ago.

  The ancient Egyptians built a complex relationship with the animal. Tomb art of Egyptian nobles depicts humans hunting hippos. The god Set takes the form of a red hippo in conflict with Horus. The goddess Tawaret is shown as a pregnant woman with a hippo head, representing fierce maternal love. The first recorded instance of hippos kept in captivity for display dates to 3500 BC, in Hierakonpolis.

  The Greek historian Herodotus described the hippo in The Histories, written around 440 BC. The Roman naturalist Pliny the Elder included the animal in his Naturalis Historia, written around 77 AD. The Yoruba people named the hippo erinmi, meaning "elephant of the water", and used hippo ivory in divination rituals. The Chewa of Southern Africa incorporated hippo masks into Nyau funerary ceremonies.

  In the modern era, the first zoo hippo in recorded history arrived at the London Zoo on the 25th of May 1850. His name was Obaysch, and he attracted up to 10,000 visitors a day. His presence inspired a popular song, the "Hippopotamus Polka". A hippo named Huberta became a celebrity during the Great Depression by trekking a long distance across South Africa. In 1953, the novelty song "I Want a Hippopotamus for Christmas" became a hit for child star Gayla Peevey. The oldest hippo in captivity whose age is confirmed died in 2012: Donna the Hippo, who died at the Mesker Park Zoo in Evansville, Indiana, at age 61. Two hippos share the record for the oldest ever recorded at 65 years - Bertha, who lived at the Manila Zoo in the Philippines from its opening in 1959 until her death in July 2017, and Lu, born at the San Diego Zoo on the 26th of January 1960, who died at the Ellie Schiller Homosassa Springs Wildlife State Park in June 2025.