Hippopotamus

About the hippopotamus

The hippopotamus, often shortened to hippo and sometimes called the common, Nile, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. Its name comes from the Ancient Greek for "river horse." It is one of only two living species in the family Hippopotamidae, the other being the much smaller pygmy hippopotamus. After elephants and rhinoceroses, the hippo is the next-largest land mammal, and it is also the largest living land artiodactyl, the group of even-toed hoofed mammals.

Despite a passing resemblance to pigs and other terrestrial even-toed ungulates, the hippo's closest living relatives are actually cetaceans — whales, dolphins, and porpoises — from which hippos diverged roughly 55 million years ago. Hippos are instantly recognizable for their barrel-shaped torsos, wide-opening mouths armed with large canine tusks, nearly hairless bodies, short legs, and sheer bulk. Adults average around 1.5 t (1.7 short tons) for bulls and 1.3 t (1.4 short tons) for cows, and exceptionally large males have been recorded at 2.66 t (2.93 short tons). Males keep growing throughout their lives, while females reach their maximum weight at about age 25.

Hippo bodies are shaped by life in the water. Their skeletons are graviportal, built to carry enormous weight, and their dense bones and low center of gravity let them sink and walk along riverbeds and lake bottoms rather than float. Their eyes, ears, and nostrils sit high on the skull so those organs stay above the surface while the rest of the body is submerged; the nostrils and ears close underwater, and a nictitating membrane covers the eyes. Hippo skin runs about 6 cm (2.4 in) thick over much of the body and carries almost no hair. It is mostly purplish-gray to blue-black, with brownish-pink patches on the underside and around the eyes and ears. The skin secretes a natural, reddish substance sometimes called "blood sweat," though it is neither blood nor sweat — two acidic pigments in the secretion inhibit bacterial growth and absorb ultraviolet light, giving the hippo a built-in sunscreen. That sunscreen cannot stop the skin from cracking if a hippo stays out of water too long.

Behavior and diet

Hippos inhabit rivers, lakes, and mangrove swamps, where territorial bulls each preside over a stretch of water shared with a group of five to thirty cows and calves. Mating and birth both take place in the water. Hippos spend the day cooling off submerged in water or mud and emerge at dusk to graze on grasses on land. While hippos rest close together in the water, grazing is a solitary activity, and hippos generally do not show territorial behavior once they are out on land. They are regarded as among the most dangerous animals in the world because of their aggressive and unpredictable nature.

Feeding is built around a powerful jaw. Huge masseter and digastric muscles, combined with a hinge set far back in the skull, let a hippo open its mouth 100 to 110 degrees. Extensions at the back of the jaw create extra surface area for muscle attachment, producing the large, droopy cheeks that let a hippo reach that gape without tearing tissue. The lower jaw's incisors and canines grow continuously — incisors can reach 40 cm (16 in) and canines up to 50 cm (20 in) — but these teeth are used mainly for combat rather than eating, and the rigid jaw cannot move side to side for chewing. Instead, hippos use their flattened, horny lips to grasp and pull grasses, which are then passed back to the molars and ground down. A hippo is considered a pseudoruminant: it has a complex three-chambered stomach but does not chew cud.

Reproduction and social structure are tied to that same bull-led group, with both mating and birth occurring in the water within a bull's territory.

Range and habitat

The hippopotamus is native to sub-Saharan Africa today, but its range was once far larger. Hippopotamus amphibius reached Europe around 560,000 to 460,000 years ago, during the Middle Pleistocene, and during warmer interglacial periods it spread through northern France, Great Britain as far north as Stockton-on-Tees, Belgium, the Netherlands, and western Germany, in addition to a stronghold across southern Europe. Fossil evidence also places hippos across the Arabian Peninsula during wet periods of the Pleistocene, with remains found in Saudi Arabia's Nafud and Rub' al Khali deserts, in Shanidar Cave in northern Iraq, and in the historical Levant. During the "Green Sahara" period of the early Holocene, roughly 10,000 to 4,000 years ago, hippos ranged across a Sahara that was then savanna crossed with rivers and lakes, as shown by remains and rock art depicting the animals.

In the modern range, hippos live in rivers, lakes, and mangrove swamps across sub-Saharan Africa. The youngest European fossil records date to the Late Pleistocene, around 40,000 to 30,000 years ago, and the species had disappeared from Egypt and North Africa by the historical period, leaving its present distribution confined to Africa south of the Sahara.

Conservation status

The hippopotamus is listed as Vulnerable (VU). It is threatened by habitat loss and by poaching for its meat and for its ivory canine teeth. No specific protection programs are documented beyond identifying these two pressures as the primary drivers of concern for the species.