The Hippopotamus, commonly known as Hippo, is a large, mostly herbivorous, semi-aquatic mammal, often considered to be the second largest after the elephant and is comparable in size and weight to the rhinoceros. They live along the rivers and lakes throughout sub-Saharan Africa and are often seen basking on the banks or sleeping in the rivers, lakes, and swamps next to the grasslands. The Hippopotamus is a Greek word which stands for river horse as they spend up to 16 hours a day submerged in water to keep their massive bodies cool under the hot African sun.
They can see clearly and breathe freely while mostly submerged as their eyes and nostrils are located high on their heads. Under the water, the hippos tap their feet along the ground to propel themselves, and when they come out of the water, they secrete a red-colored substance to cool their hairless skin. Although the secretion is referred to as blood-sweat, it is actually neither of those fluids. It is actually a skin moistener and natural sunblock that filters out ultraviolet radiation. After leaving the water at sunset, the hippopotamuses graze on grasses to consume about 65 to 70 kg of grass, which is comparatively low, considering their enormous size.
Despite their physical resemblance to pigs and other even-toed hoofed animals, their closest relatives are cetaceans, an entirely aquatic group of mammals that included the whales and dolphins. However, unlike most other semiaquatic animals, hippos have very little hair on the skin, and although often seen basking in the sun, hippos lose water rapidly through the skin and become dehydrated without periodic dips. They are destined to retreat to the water to keep them cool as they do not sweat. However, the skin is 2 inches (6 cm) thick, providing it a great protection against predators.
The hippopotamus has a bulky, barrel-shaped torso with short and columnar legs, an enormous head, a long muzzle, wide-opening mouth that can gape 150° revealing large canine tusks. The lower canines and lower incisors, used for combats, are sharp and enlarged, especially in males. While the incisors can reach 1 foot 4 inches (40 cm), the canines reach up to 1 foot 8 inches (50 cm). The nearly hairless body has a short tail and four toes on each foot, with a nail-like hoof in each toe. The adult male species are usually 10 to 15 feet long, stand 5 feet (1.5 m) tall, and weigh around 1,500 kg, which is roughly 30 percent more than females. However, very large males weighing around 3,000 kg in captivity have been reported.
Nevertheless, despite their bulky bodies, the hippos can gallop at 30 km per hour (19 mph) over short distances but normally trot. As a scrotum is not present, the testes of the males descend only partially, and the penis retracts into the body when not erect. The vagina of the female hippo is ridged, and two large diverticula, bulging pouches, protrude from the vulval vestibule, containing the opening to the urethra and the vaginal opening.
While the female hippos reach sexual maturity at five to six years, males reach maturity at around 7.5 years. A battle may erupt if a strange bull invades territories in the mating season. Although the opponents may engage in combat by slashing upward at each other’s flank with the lower incisors, the aggression mostly includes making noise, splash, bluff charges, and a yawning display of the teeth. However, occasional wounds can be fatal despite the thick skin. The hippos mate in the water with the female submerged for most of the encounter and periodically emerging her head to draw breath.
After a gestation of eight months, the female usually gives birth to a single calf weighing about 45 kg, although twins also occur. The calf can close its ears and nostrils to nurse underwater; but must swim to the surface to take the first breath. However, it also suckles on land when the mother leaves the water. Often the calf also rests on its mothers' backs when the water is too deep. While weaning starts between six and eight months after birth, most calves are fully weaned after a year.
Although the hippos are commonly viewed as harmless, they can be dangerous, and with their powerful jaws and sharp teeth, they can crush a crocodile. There was a time when the hippos had a broader distribution, but now live in eastern, central, and southern sub-Saharan Africa, where their populations are in decline. In captivity, they may live up to 60 years, but rarely more than 40 years in the wild.