- 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.
- 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.
- 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 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.
- 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.
- 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 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 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 mouth of the common hippopotamus opens to 150 degrees. The width of the jaws reaches 60–70 cm.
- 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.
- 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.
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