Mammals: Hippopotamus
 | Range: West, Central, East, and South Africa Habitat: rivers, lakes, and their surrounding grasslands |
What’s in a name?
“Hippopotamus” comes from a Greek word meaning “water or river horse.” But hippos are not related to horses at all—in fact, their closest living relatives may be pigs or whales and dolphins! There are two species of hippopotamus: the river, or common, hippo Hippopotamus amphibius (featured on this fact sheet) and the much smaller pygmy hippo Hexaprotodon liberiensis. The hippo is heaviest land mammal (after the elephant).Come on in, the water’s fine!
Hippos are definitely adapted for life in the water and are found living in slow-moving rivers and lakes in Africa. With their eyes, ears, and nostrils on the top of the head, hippos can hear, see, and breathe while most of their body is underwater. Hippos also have a set of built-in goggles: a clear membrane covers their eyes for protection while still allowing them to see when underwater. Their nostrils close and they can hold their breath for five minutes or longer when submerged. Yet despite all these adaptations for life in the water, hippos can't swim! They can't even float! Their bodies are far too dense to float, so they move around by pushing off from the bottom of the river or simply walking along the riverbed in a slow-motion gallop, lightly touching the bottom with their toes like aquatic ballet dancers.
Blood sweat
Hippos have unique skin that needs to be kept wet for a good part of the day. Staying out of the water for too long can lead to dehydration, so hippos try to stay in the water during the day. They don’t have true sweat glands; instead, hippos secrete a thick, red substance from their pores known as "blood sweat" because it looks like the animal is sweating blood. But not to worry! The blood sweat creates a layer of mucous that protects hippo skin from sunburn and keeps it moist. It is thought that this mucous may also prevent infections because even large wounds don't get infected despite the filthy water hippos sometimes live in.
A little goes a long way
During daylight hours, the hippo spends almost all its time wallowing in shallow water. In the evenings, after the hot sun has set, hippos come out of the water for a night of grazing—in fact, this goes on for about six hours! Despite their enormous weight, hippos eat an average of only 88 pounds (40 kilograms) of food a night. This amount is about 1 to 1.5 percent of their body weight. By comparison, the largest cattle eat 2.5 percent of their body weight each day. While hippos like to feed on patches of short grasses (called “hippo lawns”) close to water, they must sometimes travel several miles (kilometers) to find food, making long trips on land to new lakes or rivers. Their ears help them hear the sounds of falling fruit and their keen sense of smell helps them sniff out the tasty treats. Hippos are mostly inactive unless eating and this helps them conserve energy. At the San Diego Zoo, the hippos are fed herbivore pellets, alfalfa and Bermuda hay, lettuce, and on special occasions, melons.
Rub-a-dub-dub, baby hippo in a tub
The breeding season for hippos is linked to the dry season so that most births happen during the wettest time of the year. Hippos prefer to breed in the water but can also do so on land. Amazingly, the hippo gestation period is just 8 months—slightly shorter than the human gestation period—yet the hippo calf is about 10 times larger than a human baby! When the female, called a cow, nears the time to give birth, she will leave the herd for one or two weeks to give birth to her young and create a bond with her calf. She is comfortable giving birth in water or on land. If the calf is born underwater, the mother needs to push it to the surface to breathe. Newborn hippos are only able to hold their breath for about 40 seconds at a time. The mother stays in the water with her newborn for several days without eating, and she waits until her baby is strong enough before they dare to go out at night to graze. Mothers will nurse their babies, even underwater, for about eight months.What’s that I “herd”?
The hippopotamus is a social animal, living in groups of 10 to 30 animals. They have even been seen in much larger groups of up to 200 animals! The herd has several cows and several bulls (males), but there is one dominant bull. He has the right to mate with all cows in his herd, although he will sometimes allow subordinate bulls in and around his territory to mate. The dominant bull reminds other hippos of his territory by flinging his dung as far as possible with his fan-shaped tail! When rival bulls meet, they stand nose to nose with their mouths open as wide as possible, up to 150 degrees. This is called "gaping," a way to size each other up. Usually the smaller bull will then retreat without being pursued by the larger hippo. When two hippos do decide to fight, they slash out with their tusks or swing their enormous heads like sledgehammers while bellowing loudly. They have been known to die as a result of a very aggressive battle.
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