Hippopotamus

## What's in a name?

Watch a hippo, on land or in the water, and you’ll soon discover that this roly-poly is one of the animal world’s great characters. It is the most rotund land mammal and spends its daytime hours in lakes, pools, mudholes, or in the preferred moving waters of rivers.

The name “hippopotamus” comes from a Greek word meaning “water horse” 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 and the much smaller pygmy hippo.

## 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. Hippos can even sleep underwater, using a reflex that allows them to bob up, take a breath, and sink back down without waking up.

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, which are slightly webbed, 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 remain in 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," as 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; even large wounds don't seem to get infected despite the filthy water wild hippos sometimes live in.

## A challenging life

In the unpredictable African wilderness, hippos face many perils, such as disease and drought. A full-grown adult doesn’t have much in the way of natural predators. Lions have been known to take down hippos, but it depends upon how many lions, the age of the hippo, and how far it is from the water.

But simply reaching adulthood is a challenge. Crocodiles, lions, hyenas, and leopards are all potential threats while growing up - but the most dangerous thing to a young hippo is another hippo. When you live in a group that may contain as many as 100 individuals, disagreements are bound to happen. Sometimes young hippos get caught in the middle of violent clashes between adults and can be bitten or crushed - occasionally by their own parent.

## A little goes a long way

During daylight hours, hippos spend almost all of their time wallowing in shallow water. In the evening, 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 mixed vegetables, and on special occasions, melons.

## Cute, but not cuddly

Despite the hippos' cute appearance, they are among the most dangerous and aggressive of all mammals. Their canine and incisor teeth grow continuously, with canines reaching 20 inches (51 centimeters) in length. Hippo males especially use their canines for fighting. To ward off enemies, a hippo may yawn, scoop water with its mouth, shake its head, rear up, lunge, roar, grunt, chase, and make a loud wheezing sound, all of which are threat displays. A hippo can kill people if it's provoked or feels threatened. But the impressive tusks and canine teeth are used mainly for defense or fighting with other hippos.

## 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 adult females and several adult males, but there is one dominant male. He has the right to mate with all adult females in his herd, although he sometimes allows subordinate males in and around his territory to mate. The dominant male reminds other hippos of his territory by flinging his dung as far as possible with his fan-shaped tail!

When rival males meet, they stand nose to nose with their mouth open as wide as possible, up to a 150-degree angle! This is called "gaping," a way to size each other up. Usually, the smaller male retreats 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.

## Sounding off

Hippos are one of the noisiest animals in Africa: some hippo vocalizations have been measured at 115 decibels, about the same volume as being 15 feet away from the speakers at a rock concert! They also use snorts, grumbles, "wheeze-honking," and other ground-shaking - and sometimes subsonic - vocalizations to communicate to other hippos. As one starts calling, the rest of the family “sounds off” down the line.

## 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 gestation period for such a large mammal 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 nears the time to give birth, she leaves the herd for one or two weeks to give birth to her young and create a bond with her baby. She is comfortable giving birth in water or on land.

If the baby 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 leave the water at night to graze. Mothers nurse their babies, even underwater, for about eight months.