The hippopotamus (Hippopotamus amphibius) is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two extant species in the family Hippopotamidae, the other being the Pygmy hippopotamus. Its name comes from the ancient Greek for "river horse".
Aside from elephants and rhinos, the hippopotamus is the largest land mammal. It is also the largest extant land artiodactyl. Despite their physical resemblance to pigs and other terrestrial even-toed ungulates, the closest living relatives of the Hippopotamidae are cetaceans (whales, dolphins, porpoises, etc.), from which they diverged about 55 million years ago. Hippos are among the most dangerous animals in the world due to their highly aggressive and unpredictable nature.
Hippos are recognizable for their barrel-shaped torsos, wide-opening mouths with large canine tusks, nearly hairless bodies, pillar-like legs, and large size. The eyes, ears, and nostrils of hippos are placed high on the roof of their skulls. This allows these organs to remain above the surface while the rest of the body is submerged. The hippo's jaw is powered by huge masseter and digastric muscles which give them large, droopy cheeks. The jaw hinge allows the animal to open its mouth at almost 180°. The canines and incisors are used mainly for combat instead of feeding. Hippos rely on their flattened, horny lips to grasp and pull grasses which are then ground by the molars. Hippo skin is 6 cm (2 in) thick across much of its body with little hair. The animal is mostly purplish-grey or blue-black, but brownish-pink on the underside and around the eyes and ears. Their skin secretes a natural, red-colored sunscreen substance that is sometimes referred to as "blood sweat" but is neither blood nor sweat. This secretion is initially colorless and turns red-orange within minutes, eventually becoming brown. This natural sunscreen cannot prevent the animal's skin from cracking if it stays out of water too long.
