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• Male common hippopotamuses may have tusks longer than 1 m, including 122 cm. If the animal loses a tusk on the opposite jaw, the tusk does not wear down and grows to enormous sizes, curving increasingly backward.
• A female common hippopotamus can become pregnant only 17 months after giving birth to a calf. Approximately 63 % of calves are born during the rainy season.
• The common hippopotamus is the only mammal capable of producing sounds both on land and in water. According to IUCN experts, this fact highlights its uniqueness among mammals.
• A common hippopotamus calf cannot hold its breath longer than 40 seconds. After birth, the mother pushes it to the surface, otherwise it may drown.
• The common hippopotamus can lead predators into water to escape. An adult hippopotamus was attacked by three lions, but he dragged them into the water, forcing the predators to swim for survival.
• A hippopotamus calf can suckle underwater. It closes its nostrils and tightly presses its ears.
• The common hippopotamus cannot live long after its molars are completely worn down. Studies have not found hippopotamuses older than 42 years, although in captivity they live up to 50 years.
• Different sources indicate different record masses of the common hippopotamus — from 3500 to 4500 kg. Adult males weighing up to 3 tons are not rare.
• The common hippopotamus can rapidly restore populations in the absence of threats, achieving annual growth of 10%. Its short gestation period is 8 months, and females give birth every two years.
• The common hippopotamus can hold its breath for up to 10 minutes. An adult hippopotamus usually holds its breath for 3–5 minutes, often up to 6 minutes.
• Hippopotamuses cannot undertake long terrestrial migrations. If water bodies dry up, they suffer greatly if suitable habitats are far away. In the 1930s, a zoologist observed thousands of hippos lying in the mud of dried-up water bodies during drought.
• The hippopotamus’s nostrils, eyes, and ears are slightly raised and aligned in one plane, allowing it to breathe, see, and hear while almost fully submerged. By exposing only the top of its head, it can still perceive its surroundings.
• In male common hippos, the testes are concealed, making it difficult to distinguish them from females by primary sexual characteristics from a distance. In males, the testes are inside the body and not visible externally.
• Adult male hippos graze alone at night without fearing predators. Due to their large size, strength, and long tusks, they can confront even several lions acting together.
• The common hippopotamus competes in weight with the white rhinoceros for the second place among land animals, only surpassed by the elephant. Sometimes it is even called the second largest land animal after the elephant.
• The common hippopotamus has an encephalization quotient of approximately the same as armadillos, 0.7. This is significantly lower than in humans, where it is 5 (or 8 according to other calculations), and in the bottlenose dolphin, where it is 3.6.
• Common hippos on Madagascar have decreased in size due to long-term isolation on the island. Their relative brain volume also decreased, as is often the case with animals in island isolation.
• The common hippopotamus can roar even when its mouth and nostrils are closed. It produces a roar while diving and being underwater.
• The common hippopotamus can run at speeds up to 30 km/h, but usually moves at a slow canter. Its fastest gait is a heavy and fairly slow canter.
• Male common hippos fight each other in fierce battles that can last for hours. In these fights, they tear each other with their tusks and strike with their heads, causing serious injuries.
• Male common hippos have well-developed large tusks forming swellings on the face behind the nostrils. These swellings make it easy to distinguish a male from a female, even if only the head is visible above water.
• The skin of the common hippopotamus can become hard and used for polishing diamonds. The process of preparing the skin for this purpose takes up to 6 years.
• Common hippos and crocodiles can behave aggressively toward each other. Large male hippos and females with small young may attack crocodiles, and crocodiles may eat hippo calves.
• The common hippopotamus can drown a lion to protect its calf. In 2022, a male hippopotamus in Botswana drowned one of the lions when they tried to cross a river.
• The common hippopotamus has skin glands that secrete a reddish slimy secretion during intense heat. These glands become active when a hippopotamus cannot submerge in water for a long time.
• The canine teeth of male common hippopotamuses grow throughout their lives and can reach a length of 64.5 cm. They have no roots and can weigh about 3 kg.
• The common hippopotamus sprays feces and urine, which is of exclusive importance. It does this by making fast propeller-like rotational movements of the tail, both on land and in water.
• The common hippopotamus is always born alone, weighing 27–50 kg, usually around 40 kg. The female can give birth on land, building a nest of trampled grass and shrubs before giving birth.
• A female hippopotamus named Huberta covered more than 1600 km in 3 years. This happened on the territory of present-day South Africa.
• The common hippopotamus does not tolerate the presence of even its own kind and drives away all approaching large animals. It can fight with elephants or rhinoceroses.
• The hippopotamus loses water through its skin much faster than purely terrestrial mammals. This happens even when it is not in water.
• The head of the common hippopotamus makes up a quarter of its mass, which can reach 900 kg. The animal has a large, blunt head that appears rectangular in profile.
• The red secretion from the skin glands of the common hippopotamus helps heal wounds on its hide. The secretion contains pigments with antiseptic properties and also repels insects, especially numerous near water bodies.
• The common hippopotamus has a three-chambered stomach with a volume up to 500 liters. The hippopotamus's stomach is three-chambered and very large even by the standards of ungulates.
• The breathing rate of the hippopotamus is 4–6 times per minute, but after surfacing it increases several times. The breathing of a hippopotamus that surfaces after a long time underwater becomes more frequent.
• The common hippopotamus can recognize its kin by the scent of urine inhaled underwater. A submerged animal inhales some water containing the urine of other hippos through its nostrils.
• Common hippos trample paths in rock if they use them over many years. Such paths can reach a depth of up to one and a half meters.
• The common hippopotamus grazes on land, biting grass with its hardened lips right down to the root. In areas of intensive grazing, the grass is literally clipped short by them.
• The hippopotamus emits a loud bellow up to 110 decibels. On a quiet day, it can be heard over water for many hundreds of meters.
• Hippopotamus excrement alters the composition of water and riverbeds in Colombia, threatening the survival of capybaras and manatees. The country has no predators that could control hippopotamus numbers.
• The common hippopotamus has the thinnest skin at the base of the tail, so in zoos injections are given in this part of the body. The syringe needle can reach the veins there.
• The common hippopotamus can cross sea straits, as shown by the presence of a population on the Bijagós Islands. The presence of a water body is a necessary condition for hippopotamus habitation on these islands.
• The common hippopotamus swims across a 30-kilometer strait to reach the island of Zanzibar. Such a crossing occurs with little effort.
• The mouth of the common hippopotamus opens to 150 degrees. The width of the jaws reaches 60–70 cm.
• The hippopotamus roar is one of the most characteristic and recognizable sounds of African wildlife. It is used for communication with kin through short guttural roaring sounds.
• The common hippopotamus drives out the dominant male if he enters a group of females with young. Even the dominant male can be driven out by the females.
• The common hippopotamus intestine reaches a length of up to 60 m, providing a higher degree of food digestibility compared to elephants. The hippopotamus diet by weight is half that of other thick-skinned animals, such as rhinoceroses.
• When surfacing after a long dive, a hippopotamus exhales air, and in cool weather, vapor from the exhaled air forms noticeable fountains. The same phenomenon occurs in whales' exhalation.
• The common hippopotamus often snorts and forcefully exhales air through its nostrils, which usually indicates irritation or aggression. This can also be a warning signal, for example, when predators approach.
• The skin of the common hippopotamus is extremely thick — up to 4 cm. In zoos, when suturing wounds, metal wire is used instead of thread.
• The common hippopotamus has very few fine and short hairs on its body — about 25 per 100 cm² on the back, and even sparser hair on the sides and belly. The hippopotamus's hair is so sparse that it is almost impossible to see even from close distance.
• Nile crocodiles and common hippopotamuses usually do not conflict and sometimes jointly drive away other animals from water bodies. Hippopotamus females leave their young with crocodiles to protect them from lions and hyenas.
• The common hippopotamus can be considered an opportunistic predator. Recently, there have been proposals to classify it as such, despite its diet consisting of aquatic or terrestrial grass.
• The common hippopotamus received its scientific name due to its ability to live in two environments. The species name amphibius means "amphibious," that is, living both on land and in water.
• Hippopotamus trails in areas of their mass occurrence become one of the most noticeable features of the landscape, cutting through the shores every few dozen meters. Steep banks often have such ditches formed by years of constant use.
• Common hippopotamus becomes more aggressive in dense groups, especially when male fights for hierarchy positions intensify. Aggressiveness of hippos increases in such dense groups.
• Common hippopotamus can move away from water up to 3 km, but usually does not go beyond this distance. Cases were recorded when it grazed 3.2 km away from the water body.