Hippo

The hippo (common hippopotamus, Hippopotamus amphibius) is one of the largest land animals on Earth - only elephants and some rhinoceroses are heavier. Adults are barrel-shaped, nearly hairless, and stand around 1.4 m at the shoulder, with bulls averaging around 1.5 tonnes and cows around 1.3 tonnes. Their skin is a purplish-grey or blue-black, with brownish-pink tones around the underside, eyes, and ears.

Hippos are found across sub-Saharan Africa, living in rivers, lakes, mangrove swamps, and estuaries. They spend most of the day in water to stay cool, emerging at dusk to graze on land. They are wild animals and are listed as vulnerable on the IUCN Red List, with an estimated wild population of between 115,000 and 130,000. They are also long-standing zoo favourites around the world.

Hippos belong to the family Hippopotamidae and are native to sub-Saharan Africa. Despite looking somewhat pig-like, their closest living relatives are actually whales, dolphins, and porpoises - the two lineages are thought to have split from a shared semiaquatic ancestor around 54 million years ago.

Ancient hippo relatives once ranged widely across Eurasia and Africa. Fossil evidence shows that hippos lived mainly in southern Europe during the Pleistocene, reaching north-western Europe - including Britain - during warmer interglacial periods, and their range once stretched across the Sahara when that region was greener and wetter. They disappeared from Europe around 30,000 to 40,000 years ago.

Humans have had a long relationship with hippos. Evidence of butchery by modern humans dates back around 160,000 years, ancient Egyptians depicted hippo hunts on tombs, and Greek and Roman writers described them. The first record of hippos kept in captivity dates to around 3500 BC in Egypt. In modern times, the first hippo at London Zoo arrived in 1850 and drew crowds of up to 10,000 visitors a day.

Hippos are considered among the most dangerous animals in the world. They are aggressive and unpredictable, and are known to charge and capsize boats, sometimes fatally. Despite their bulky build, they can move at reported speeds of up to 30 km/h on land, though this has not been confirmed scientifically.

During the day, hippos rest in water or mud in groups that can number over 100 individuals. However, social bonds between adults are weak, and grazing at night is a solitary activity. Bulls hold territories in water, each presiding over a stretch shared with a group of cows and calves. Younger males are tolerated as long as they defer to the dominant bull.

Hippos communicate through a range of sounds. Their most common call is a "wheeze honk" that carries over long distances through the air. They can also produce clicks underwater, which may have echolocative properties. Hippos are primarily grass-eaters, travelling 3-5 km each night and consuming around 40 kg of grass. They have a complex three-chambered stomach but do not chew cud.

A hippo's typical lifespan is 40 to 50 years, though some individuals in captivity have reached 65 years of age.

Hippo skin is around 6 cm thick but cracks if the animal stays out of water for too long. The skin produces a natural red-orange secretion sometimes called "blood sweat" - it is neither blood nor sweat, but a substance containing pigments that act as a natural sunscreen and inhibit the growth of harmful bacteria.

The parasitic flatworm Oculotrema hippopotami is known to infest hippo eyes, particularly the nictitating membrane. Hippos' stomach anatomy is not adapted to meat-eating, and occasional carnivorous behaviour is thought to result from nutrient deficiency or abnormal circumstances.

This is general awareness only, not veterinary advice. If you find an injured or unwell wild animal, contact a vet or a wildlife rescue.

How big do hippos actually get?

Bulls average around 1.5 tonnes and cows around 1.3 tonnes, though exceptionally large males have been recorded at over 2.6 tonnes. They stand roughly 1.4 m at the shoulder and can be over 3 m long from head to body. Male hippos appear to keep growing throughout their lives.

How long do hippos live?

Hippos typically live for 40 to 50 years. In captivity, some individuals have reached 65 years, which is the oldest age on record.

Are hippos really dangerous?

Yes - hippos are considered among the most dangerous animals in the world. They are highly aggressive and unpredictable, and have been known to charge and capsize boats, with fatal results. Their enormous jaws, powered by large muscles, can open to 100-110 degrees.

Can hippos swim?

Interestingly, it is debated whether hippos can actually swim at all. They move through water mainly by walking or bouncing along the bottom rather than swimming. Adults need to surface to breathe at least every six minutes, and they can even sleep underwater, rising to breathe without fully waking.

Is the "blood sweat" on a hippo's skin really blood?

No - the reddish secretion on hippo skin is neither blood nor sweat. It is a natural substance containing pigments that act as a sunscreen and inhibit the growth of disease-causing bacteria. It starts colourless and turns red-orange within minutes of being secreted.

Are hippos related to pigs?

Their nearest living relatives are actually whales, dolphins, and porpoises, from which the hippo lineage split roughly 54 to 55 million years ago.

The name hippopotamus comes from ancient Greek and means "horse of the river" - though their closest living relatives are actually whales, dolphins and porpoises.

Hippos can hold their heads partially above water and produce calls that travel through both air and water simultaneously - individuals respond to these calls whether they are above or below the surface.

The first hippo at London Zoo, named Obaysch, arrived in 1850 and was so popular he inspired a song called the "Hippopotamus Polka". Hippos have since appeared in the Disney film Fantasia, the board game Hungry Hungry Hippos, a Christmas song, and a poem by T. S. Eliot.

A hippo's lower canine teeth grow continuously and can reach up to 50 cm in length. These tusks are used mainly in combat between rivals, not for feeding.