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Hippopotamus Facts, Hippo Sweat, and a Natural Sunscreen

Linda Crampton is a writer and experienced science teacher with an honors degree in biology. She enjoys writing about science and nature.

A female common or river hippopotamus and her calves in Tanzania

A female common or river hippopotamus and her calves in Tanzania

Large and Impressive Animals

A common or river hippopotamus is a powerful, heavy, and very impressive animal. Adult males may weigh up to 7,000 pounds, while females are about half this weight. The skin of a hippopotamus is especially interesting to scientists. It secretes a thick red liquid that is often called “hippo sweat.” The liquid acts as a sunscreen and also kills bacteria.

Hippopotamuses are semi-aquatic animals that live in and around lakes, rivers, and swamps in Africa south of the Sahara. During the day, they spend most of their time in the water, resting or interacting with other members of their herd. At night, they move over land, grazing on grass as they do so. They are sometimes seen grazing during the day as well.

Hippos can move fast, despite their short legs and stocky, rather ungainly body. They can also be aggressive. In fact, they are considered to be one of the most dangerous mammals in Africa.

The interesting face of a river hippopotamus

The interesting face of a river hippopotamus

The Common or River Hippopotamus

Sources that should be reliable say that the name ”hippopotamus” comes from the Ancient Greek words for “river horse.” Despite this name, hippos are more closely related to whales and dolphins than to horses. The scientific name of the common hippopotamus is Hippopotamus amphibius, which reflects its dual existence in water and on land. It's often known as the river hippopotamus. It belongs to the class Mammalia, like us, and the order Artiodactyla. Humans belong to the order Primates within the class Mammalia.

A hippo has a stocky, barrel-shaped body, a big, wide head, and a square snout. It also has short legs. Each foot has four webbed toes and is known as a hoof. The animal's mouth is huge and contains very long and curved canine teeth. A hippo has small ears and a short tail. Its skin is a mixture of grey, brown, and pink in color and is almost hairless.

Two species of hippopotamuses exist. The smaller pygmy hippopotamus (Choeropsis liberiensis) lives around swamps in the forests of West Africa. Like the common hippopotamus, its skin produces a red secretion that acts as sunscreen. Unfortunately, the pygmy hippo is endangered.

A river hippopotamus and a companion

A river hippopotamus and a companion

Daily Life of a Common Hippopotamus

Hippos live in groups called herds. The groups are also known as pods, bloats, and schools. A herd generally consists of females, their young and a breeding male who is territorial. Bachelor males may be solitary or may be part of the herd. Herds generally consist of around ten to forty animals but may be much larger.

Interestingly, the herd breaks up at night when the animals begin their search for food on land. The males aren't territorial while the hippopotamuses are grazing. The animals travel individually, except for a mother with a young calf, who travels with her youngster. Hippos can move fast when necessary and can outrun humans.

Hippos are classified as herbivores or plant eaters. Some interesting observations are coming to light, however. Researchers have observed the animals eating the dead bodies of baby elephants, impala, and even other hippopotamuses. Hippos are thought to be scavengers rather than predators. The amount of meat that they eat and the frequency of their cannibalism are unknown.

How Hippos Avoid the Heat

Hippos don’t have sweat glands, despite the use of the term "hippo sweat." They spend much of their day in the water or wallowing in mud to escape the hot African sun. They move on to the banks of a river or lake only if the temperature is low enough.

The eyes, ears, and nostrils of hippopotamuses are located high on their heads. This enables them to breathe and observe their environment while staying partially submerged in water.

When the temperature falls at dusk, the animals become active. The main component of their diet is grass. They spend up to six hours feeding during the night and may travel as far as six miles looking for food. Although they spend a lot of time in the water, they eat few, if any, aquatic plants.

Skin Secretion and Sunscreen

Red Sweat

Hippo skin is virtually hairless and would soon burn when exposed to intense sunlight if it was unprotected. The skin secretes an oily liquid that acts as sunscreen. The secretion is colorless, but after a few minutes, turns red. When it's mixed with mucus on the skin of a hippopotamus, it stays red for several hours and then turns brown as the pigments polymerize (form chains).

Early observers referred to the liquid as "blood sweat," but we know today that it doesn't contain blood. In addition, scientists point out that although the liquid is referred to as sweat, it's produced by subdermal glands and not by sweat glands in the dermis.

Blocking UV Light

Two pigments have been found in the skin secretion of the hippopotamus—hipposudoric acid, which is red, and norhipposudoric acid, which is orange. Researchers have discovered that both pigments absorb ultraviolet light. The skin secretion also contains crystalline structures that scatter light.

Fighting Bacteria

Hipposudoric acid has been found to be strongly antibacterial, even at low concentrations. Unfortunately, the isolated pigments from the sweat are unstable. When they are in a hippo's secretion, they are much more stable.

Potential Applications

The antibiotic sunscreen is intriguing for researchers. They would like to replicate some of its components in order to protect humans. They will probably need to identify the nature and role of other chemicals and structures in the mixture and deal with stability problems before a manufactured product (or products) is of use to us.

The red pigment also has antibiotic activity: at concentrations lower than that found on the hippopotamus's skin, it inhibits the growth of the pathogenic bacteria Pseudomonas aeruginosa A3 and Klebsiella pneumoniae.

— Yoko Saikawa et al, "Nature"

Hippos can't swim, and their bodies are too dense to float. Nevertheless, the animals can move very effectively underwater.

Locomotion and Life in the Water

Movement Underwater

Hippos have several methods of locomotion underwater. They prefer to move through shallow water and stay close to the bottom. They also enter deeper water, however.

The animals often walk along the bottom of lakes and rivers but may do a slow-motion trot or gallop instead. They also use a method called “punting” to propel themselves through the water. They push off from the bottom and then glide gracefully for a short distance just above the ground until they land. At this point, they push themselves off again. They sometimes use their hind feet to push themselves upwards to the water’s surface.

Other Aquatic Behavior

Adult hippos can hold their breath for up to five minutes, or perhaps for longer. Their ears are sealed off, and valves close their nostrils when they are submerged.

Scientists have discovered that hippos make clicking sounds when they are underwater. Researchers are uncertain about the purpose of these clicks. They may be used for communication or echolocation.

Aggression in Hippopotamuses

Hippos are aggressive animals. This is especially true for the dominant males. Females are also aggressive if they're protecting their young. Males are often photographed giving huge yawns. These yawns are a threat gesture and display the hippo’s large, curved canine teeth. The teeth are up to one and a half feet long in males.

Hippos have other ways to issue threats in addition to yawning. They vocalize by producing snorts, grunts, loud bellows, and a sound known as wheeze-honking. Bellows as loud as 115 decibels have been recorded. A hippopotamus has a flat tail, which males use like a paddle to distribute excrement around their territory. They sometimes aim the excrement at rival males in order to assert their dominance. The hippo’s skin is frequently wounded during fights, but the oily secretion by skin glands helps to prevent infections.

Hippopotamuses are extremely dangerous to humans. They have been recorded running up to thirty miles an hour. The animals have poor eyesight but a great sense of smell. A male will attack humans if he feels that his territory is being invaded, and a female will attack if she feels that her baby is being threatened. Although the number of human deaths each year from hippopotamus attacks is hard to confirm, there are reportedly many human fatalities from encounters with the animals. They will attack humans that are either on land or in boats.

Reproduction and Lifespan

Mating and Birth

Hippos mate in water and give birth either in shallow water or on land. A female usually produces a single calf after an eight-month gestation period. Twins are rare. If the calf is born in water, the mother pushes the baby to the surface for its first breath. The father of the calf is usually the dominant male of the herd, although he will sometimes allow a subordinate male to mate with a female.

The Calf

The female leaves her herd to have her baby and stays alone with the calf for one to two weeks. During this time, the calf develops a strong bond with its mother, which seems to be one of the purposes of the separation. Eventually, the mother returns to the herd with her baby.

Nursing and Weaning

Babies can nurse in water and on land. When they nurse underwater, their nostrils and ears are closed. The calves sometimes ride on their mother’s back as she moves through the water. Weaning begins when the calf is six to eight months old, but the baby begins eating grass only one month after birth.


Calves live with their mother for several years. After this time, they live a more independent life and may leave their herd to find a new one. Common hippopotamuses can live for as long as forty or fifty years.

A mother river hippopotamus and her calf

A mother river hippopotamus and her calf

Newborn river hippopotamus calves can hold their breath for only forty seconds and can't stay underwater as long as their mother.

The Vulnerable Hippopotamus Population

The International Union for Conservation of Nature (IUCN) has created a “Red List” that indicates how close a species is to extinction. In order of seriousness, the categories in the Red List are:

  1. Least Concern
  2. Near Threatened
  3. Vulnerable
  4. Endangered
  5. Critically Endangered
  6. Extinct in the Wild
  7. Extinct

Hippos are classified as "Vulnerable" due to the destruction of their habitat as land is cleared for agriculture and human settlement. The hippopotamus is also hunted for its meat and the ivory in its large canine teeth or tusks. Its population was last assessed in 2016.

There's still much to learn about river hippopotamuses. They are difficult to study due to their aggressive tendencies and their fondness for travelling underwater. Learning more about the hippo is important so that this fascinating animal can continue to survive on Earth.

An underwater hippopotamus at the San Diego Zoo

An underwater hippopotamus at the San Diego Zoo


  • Information about the hippopotamus from the San Diego Zoo
  • "Hippopotamus Underwater Locomotion: Reduced-Gravity Movements for a Massive Mammal" from the Journal of Mammalogy, Oxford University Press
  • "The truth about hippos: carnivore or herbivore?" from BBC Earth
  • "Hippo's 'Magic' Sweat Explained" from the BBC (British Broadcasting Corporation)
  • The sweat of the hippo from the Nature journal
  • Studies on the red sweat of the hippopotamus from Pure and Applied Chemistry
  • Hippopotamus amphibius entry from the International Union for Conservation of Nature

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2010 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on September 20, 2020:

Thank you, Peggy. I appreciate your comment very much.

Peggy Woods from Houston, Texas on September 20, 2020:

What a fascinating article, Linda. I was aware that they could run fast and that many people are supposedly killed by hippos each year. But I learned so much more about them by reading your excellent article. I am glad that I could still comment on it. I am surprised that this is not on the Owlcation niche site.

Linda Crampton (author) from British Columbia, Canada on March 24, 2015:

Thank you for the vote and comment, peachpurple. Hippos have interesting behaviour. I think they are more complex than many people realize.

peachy from Home Sweet Home on March 24, 2015:

hippo mummy are alike humans, taking care of their babies, voted up

Linda Crampton (author) from British Columbia, Canada on May 16, 2014:

Thank you very much for the comment and the share, ologsinquito! I appreciate your vote, too.

ologsinquito from USA on May 16, 2014:

This is such an interesting article. Of course they'd need some sort of protection from the hot African sun, although most people, myself included, never gave this much thought. Voted up and shared.

Linda Crampton (author) from British Columbia, Canada on October 26, 2011:

Thanks so much the comment and votes, Tina. Yes, it would be terrible if the hippopotamus became extinct. They are very interesting creatures and move so gracefully underwater.

Christina Lornemark from Sweden on October 26, 2011:

Very interesting article and so well put together! The Hippopotamus are amazing animals and it would be a disaster if they where extinct! I learned many interesting news about these big giants. I love the “moonwalk”, so cool:) Thanks Alicia,

Vote up, interesting


Linda Crampton (author) from British Columbia, Canada on October 25, 2011:

Thank you very much for the comment, september girl. Hippos do have a strange appearance, but I think that they're interesting animals too! I'm looking forward to seeing what researchers do with the discovery of the sunscreen in the hippo's skin secretion.

september girl on October 25, 2011:

Although I have always thought they were quite ugly animals they are interesting. Thanks for sharing your info and the videos. I learned something new today. : )