The hippopotamus, Hippopotamus amphibius, is the only surviving member of a family of heavy-bodied, four-legged mammals that share a common ancestor with whales. The other species went extinct between three and five million years ago. Hippos are native to all parts of Africa south of the Sahara Desert. Only one group of about 200 wild hippos is found outside of Africa, and these are descendants of four animals that escaped captivity after being illegally imported into Colombia in the 1980s.
Hippos are the third largest land mammal after elephants and rhinoceroses. Standing about five feet high at their shoulders, they have a wide, barrel-shaped body supported by four short, stout legs. Males grow continuously their entire lives topping out at about one-and-a-half tons, while females stop growing at about 25 years old, and remain slightly smaller. They have large canine tusks and short tails. Dense bones and a low center of gravity allow them to move their weight around easily on both land and while submerged. They spend much of their lives in large bodies of water, using their webbed toes to help propel them easily through the water. Their eyes, ears, and nostrils are located on the top of their heads, above the water’s surface when swimming or standing submerged. Their eyes have a nictating membrane keeping water out while still allowing them to see underwater, and their nostrils and ears can be closed underwater.
Hippos live in areas with rivers, lakes, or mangrove swamps that have nearby grasslands for foraging. They spend most of the day in water or mud to stay cool under the tropical sun. Their gray-blue-black skin is about 2.5″ thick and has almost no hair. Glands in the skin secrete a reddish-orange liquid that covers their entire body, acting as a sunscreen and inhibiting the growth of bacteria on their skin. In addition, should the animal be out of water too long, the liquid provides hydration to keep the skin from cracking. They can walk or run on land, reaching speeds over 15 mph. They cannot jump, but can climb steep banks. Underwater, their stout legs and feet and webbed toes help to spread out their weight and allow easy movement across the bottom. Hippos are unable to breathe underwater, but can stay submerged for about six minutes before needing to push off the bottom and raise their head above the surface for a breath of air. They have been observed sleeping underwater, rising to breathe every few minutes without interrupting their sleep.
Groups of hippos travel together in pods, sometimes reaching 100 animals. Bulls with a group of cows and calves may establish a territory in water, but never on land; and when water sources dry up, the territories dissolve. Hippopotamus are quite skittish, sensitive to every animal close by. This may be due to having sensitive hearing, but poor eyesight. Hippos use aggressive behavior as a defense against other species and they kill about 500 humans a year. They do have some mutualistic relationships, especially with fish in certain deep water holes where the hippos stand almost submerged, with their mouth open, and let the fish clean parasites from their bodies and teeth. Both species benefit as the hippos are rid of parasites and the fish get fed.
Hippopotamuses are vegetarians, spending their days submerged in water to keeping cool, then foraging in nearby grasslands during the cooler evening hours. They walk about five miles each evening, eating 90 pounds of grass each day. Their incisors and canines in the lower jaw grow continuously throughout their lives, but these teeth are only used in combat. Their lips are flat and tough, able to tear off the grasses which are moved directly to their molars to be chewed. During times when food is scarce, a hippo can regurgitate food and chew it a second time to get more nutrients. Hippos tend to follow the same routes through water and grasslands every day, and by doing so, they make a huge impact on their habitat, shaping paths on land and in the water. These openings make it easier for water to circulate and other species to move through the area.
Females, known as cows, start breeding at around six years of age. Hippos mate and give birth in secluded areas of water. After an eight month gestation, the cow will find a quiet, shallow pool and give birth to a 110 pound baby. Mothers are very protective of their calves, although for short periods, all of the young in a pod may be kept together and guarded by only a few adults. Calves feed on mother’s milk until they are a year old, and they will remain near the mother for six to seven years. Communication within the pod consists of a bellow starting as a high-pitched call and ending with a throaty thrumming vibration. They may also exhale loudly, especially when threatened, or emit a series of clicking noises that can be heard underwater, the purpose of which is not yet understood. Hippos have the unique ability to hold their heads partially out of water and make sounds that carry long distances in both air and water at the same time.
Hippopotamuses are considered vulnerable, with a stable population of 120,000. Their two biggest threats are unregulated hunting and poaching for both meat and the ivory of their large canine teeth. As war rages in parts of central Africa, very few protections are enforced. Animal predators are few in number due to their immense size and aggressive nature, but calves may be attacked by lions, Nile crocodiles, or spotted hyenas. Learn more about these magnificent animals by visiting a zoo or natural history museum near you.
