Hippopotamus Facts, Hippo Sweat, and a Natural Sunscreen
Large and Impressive Animals
A common or river hippopotamus is a powerful, heavy, and very impressive animal. Adult males may weigh up to 7000 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 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 Common or River Hippopotamus
The name ”hippopotamus” comes from the Ancient Greek word for “river horse.” Despite their 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 to 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. It has small ears and a short tail. Its skin is a mixture of grey, brown, and pink in colour and is almost hairless.
Two species of hippopotamus 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 a sunscreen. Unfortunately, the pygmy hippo is endangered.
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 and 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 hippos 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 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 water, they eat few if any aquatic plants.
Skin Secretion and Sunscreen
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 a sunscreen, however. The secretion is colourless to begin with but quickly turns red. Early observers referred to the liquid as "blood sweat", but we know today that it doesn't contain blood.
Two pigments have been found in the skin secretion of the hippopotamus—hipposudoric acid, which is red, and norhipposudoric acid, which is orange. Hipposudoric acid has been found to be strongly antibacterial, even at low concentrations. The skin secretion absorbs ultraviolet radiation and also contains crystalline structures that scatter light. The hippo's antibiotic sunscreen is intriguing to scientists, who would like to replicate the sunscreen, or at least some of its components, for humans to use.
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
Hippos have several methods of locomotion underwater. They prefer to move through shallow water and to stay close to the water 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.
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 for 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.
Hippos are extremely dangerous to humans. They have been recorded running at 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
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 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.
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 swims. 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 as long as forty or fifty years.
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:
- Least Concern
- Near Threatened
- Critically Endangered
- Extinct in the Wild
Hippos are classified as "Vulnerable" due to the destruction of their habitat as land is cleared for agriculture and for 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.
- 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
- Hippopotamus amphibius entry from the International Union for Conservation of Nature
© 2010 Linda Crampton