With a broad mouth, a hairless body, and a set of semi-aquatic habits, the common hippopotamus (Hippopotamus amphibius) has always struck humans as vaguely comical creatures. Found only in sub-Saharan Africa, a hippo in the wild can be almost as dangerous (and unpredictable) as a tiger or hyena.

Fast Facts: Hippopotamus Scientific Name: Hippopotamus amphibius

Hippopotamus amphibius Common Name: Common hippopotamus

Common hippopotamus Basic Animal Group: Mammal

Mammal Size: 11–17 feet

11–17 feet Weight: 5500 pounds (female), 6600 pounds (male)

5500 pounds (female), 6600 pounds (male) Lifespan: 35–50 years

35–50 years Diet: Herbivore

Herbivore Habitat: Sub-saharan Africa

Sub-saharan Africa Population: 115,000–130,000

115,000–130,000 Conservation Status: Vulnerable

Description

Hippos aren't the world's largest land mammals—that honor belongs, by a hair, to the largest breeds of elephants and rhinoceroses—but they come pretty close. The biggest male hippos can approach three tons and 17 feet, and apparently, never stop growing throughout their 50-year life span. The females are a few hundred pounds lighter, but every bit as menacing, especially when defending their young.

Hippopotamuses have very little body hair—a trait that puts them in the company of humans, whales, and a handful of other mammals. Hippos have hair only around their mouths and on the tips of their tails. To make up for this deficit, hippos do have extremely thick skin, consisting of about two inches of the epidermis and only a thin layer of underlying fat—there's not much need to conserve heat in the wilds of equatorial Africa.

Hippos do, however, have very delicate skin that needs to be protected from the harsh sun. The hippo produces its own natural sunscreen—a substance called "blood sweat" or "red sweat," it consists of red and orange acids that absorb ultraviolet light and inhibit the growth of bacteria. This has led to the widespread myth that hippos sweat blood; in fact, these mammals don't possess any sweat glands at all, which would be superfluous considering their semi-aquatic lifestyle.

Many animals, including humans, are sexually dimorphic—the males tend to be larger than the females (or vice-versa), and there are other ways, besides directly examining the genitals, to distinguish between the two sexes. A male hippo, though, looks pretty much exactly like a female hippo, except that males are 10 percent heavier than females. The inability to easily tell whether a particular animal is male or female makes it difficult for researchers in the field to investigate the social life of a lounging herd of hippos.

Wikimedia Commons

Species

While there is only one hippopotamus species—Hippopotamus amphibius—researchers recognize five different subspecies, corresponding to the parts of Africa where these mammals live.

H. amphibius amphibius, also known as the Nile hippopotamus or the great northern hippopotamus, lives in Mozambique and Tanzania;

H. amphibius kiboko, the East African hippopotamus, lives in Kenya and Somalia;

H. amphibius capensis, the South African hippo or the Cape hippo, extends from Zambia to South Africa;

H. amphibius tchadensis, the West African or Chad hippo, lives in (you guessed it) western Africa and Chad; and the Angola hippopotamus; and

H. amphibius constrictus, the Angola hippo, is restricted to Angola, Congo, and Namibia.

The name "hippopotamus" derives from Greek—a combination of "hippo," meaning "horse," and "potamus," meaning "river." Of course, this mammal coexisted with human populations of Africa for thousands of years before the Greeks ever laid eyes on it, and is known by various extant tribes as the "mvuvu," "kiboko," "timondo," and dozens of other local variants. There is no right or wrong way to pluralize "hippopotamus:" some people prefer "hippopotamuses," others like "hippopotami," but you should always say "hippos" rather than "hippi." Groups of hippopotamuses (or hippopotami) are called herds, dales, pods, or bloats.

Habitat and Range

Hippos spend most of each day in shallow water, emerging at night to travel to "hippo lawns," grassy areas where they graze. Grazing only at night allows them to keep their skins moist and out of the African sun. When they're not grazing on grass—which at night takes them into the African lowlands several miles away from the water and for periods of five or six hours at a stretch—hippos prefer to spend their time fully or partially submerged in freshwater lakes and rivers, and occasionally even in saltwater estuaries. Even at night, some hippos remain in the water, in essence taking turns at the hippo lawns.

Diet

Hippos eat between 65–100 pounds of grass and foliage each night. Somewhat confusingly, hippos are classified as "pseudoruminants"—they're equipped with multiple-chambered stomachs, like cows, but they do not chew a cud (which, considering the huge size of their jaws, would make for a pretty comical sight). Fermentation takes place primarily in their fore-stomachs.

A hippo has an enormous mouth and it can open up to a whopping 150-degree angle. Their diets certainly have something to do with it—a two-ton mammal has to eat a lot of food to sustain its metabolism. But sexual selection also plays a major role: Opening one's mouth very widely is a good way to impress females (and deter competing males) during mating season, the same reason that males are equipped with such enormous incisors, which otherwise would make no sense given their vegetarian menus.

Hippos don't use their incisors to eat; they pluck plant parts with their lips and chew on them with their molars. A hippo can chomp down on branches and leaves with a force of about 2,000 pounds per square inch, enough to cleave a luckless tourist in half (which occasionally happens during unsupervised safaris). By way of comparison, a healthy human male has a bite force of about 200 PSI, and a full-grown saltwater crocodile tilts the dials at 4,000 PSI.

Behavior

If you ignore the difference in size, hippopotamuses may be the closest thing to amphibians in the mammal kingdom. In the water, hippos live in loose polygynous groups made up mostly of females with their offspring, one territorial male and several unallied bachelors: The alpha male has a section of beach or lake edge for a territory. Hippopotamuses have sex in the water—the natural buoyancy helps to protect the females from the suffocating weight of the males—fight in the water, and even give birth in the water. Amazingly, a hippo can even sleep underwater, as its autonomic nervous system prompts it to float to the surface every few minutes and take a gulp of air. The main problem with a semi-aquatic African habitat, of course, is that hippos have to share their homes with crocodiles, which occasionally pick off smaller newborns unable to defend themselves.

Although male hippos do have territories, and they squabble a bit, that is usually restricted to roaring vocalizations and ritual. The only real battles are when a bachelor male challenges a territorial male for rights over his patch and harem.

Reproduction and Offspring

Hippopotamuses are polygynous: One bull mates with multiple cows in his territorial/social group. Hippo females usually mate once every two years, and the bull mates with whichever cows are in heat. Although mating can occur throughout the year, conception only occurs from February to August. The gestation period lasts nearly a year, with births taking place between October and April. Hippos only give birth to one calf at a time; calves weigh 50–120 pounds at birth and are adapted to underwater nursing.

Juvenile hippos stay with their mothers and are reliant on mother's milk for nearly a year (324 days). Female juveniles remain in their mother's group, while males leave after they are sexually mature, about three and a half years.

WILLIAM WEST/Getty Images

Evolutionary History

Unlike the case with rhinoceroses and elephants, the evolutionary tree of hippopotamuses is rooted in mystery. Modern hippos shared a last common ancestor, or "concestor," with modern whales, and this presumed species lived in Eurasia about 60 million years ago, only five million years after the dinosaurs had gone extinct. Still, there are tens of millions of years bearing little or no fossil evidence, spanning most of the Cenozoic Era, until the first identifiable "hippopotamids" like Anthracotherium and Kenyapotamus appear on the scene.

The branch leading to the modern genus of hippopotamus split off from the branch leading to the pygmy hippopotamus (genus Choeropsis) less than 10 million years ago. The pygmy hippopotamus of western Africa weighs less than 500 pounds but otherwise looks uncannily like a full-sized hippo.

Conservation Status

The Internal Union for the Conservation of Nature estimates that there are 115,000–130,000 hippos in central and southern Africa, a sharp drop from their census numbers in prehistoric times; they classify hippos as "vulnerable," experiencing a continuing decline in area, extent, and quality of habitat.

Threats

Hippopotamuses live exclusively in sub-Saharan Africa (though they once had a more widespread distribution). Their numbers have declined most precipitously in the Congo in central Africa, where poachers and hungry soldiers have left only about 1,000 hippos standing out of a previous population of almost 30,000. Unlike elephants, which are valued for their ivory, hippos don't have much to offer traders, with the exception of their enormous teeth—which are sometimes sold as ivory substitutes.

Another direct threat to the hippopotamus is the loss of habitat. Hippos need water, at least mudholes, all year round to take care of their skin; but they also need grazing lands, and those patches are in danger of disappearing as a result of climate-change-driven desertification.

Sources