Hippopotamus (Hippopotamus amphibius)

Genus: Hippopotamus
Species: H. amphibius
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family: Hippopotamidae

The Hippopotamus (Hippopotamus amphibius also known as river-horse, dual), from the Greek ‘éððïðüôáìïò (hippopotamos, hippos meaning "horse" and potamos meaning "river"), is a large, plant-eating African mammal, one of only two extant, and three or four recently extinct, species in the family Hippopotamidae.

Characteristics

Hippopotamuses (hippopotami is also accepted as a plural form by the OED), also called hippos, are gregarious, living in groups of up to 40 animals, called a pod, herd, school or bloat. A male hippo is known as a bull, a female, a cow, and a baby, a calf. A hippo's lifespan is typically 40 to 50 years. Female hippos will reach sexual maturity at 5 to 6 years, and have a gestation period of 8 months.

Hippos average 3.5 meters (11 ft) long, 1.5 meters (5 ft) tall at the shoulder, and weigh from 1,500 kg to 3,200 kg (3,300 to 7,000 lb). They are approximately the same size as the White Rhinoceros, and experts are split on which is the next largest land animal after the elephant. Male hippos appear to continue growing throughout their lives, whereas the females reach a maximum weight at around the age of 25. Females are smaller than their male counterparts, and normally weigh no more than 1,500 kg. The value given above of 3,200 kg is often quoted as being the upper limit of weight for a male hippo. However, larger specimens than this have been documented, including one of which weighed almost 5000 kg. Even though they are a bulky animal, hippopotamuses can run faster than a human on land. There are estimates of its actual running speed varying from 30 km/h (18 mph) to 40 km/h (25 mph), or even 48 km/h (30 mph). The hippo can maintain these higher estimates for only a few hundred yards.

The eyes, ears, and nostrils of the hippo are placed high on the roof of the skull. This allows them to spend most of the day with the majority of their body submerged in the waters of tropical rivers to stay cool and prevent sunburn. For additional protection from the sun, their skin secretes a natural sunscreen substance which is red colored. This secretion is sometimes referred to as "blood sweat," but it is not actually blood, nor sweat. This secretion starts out colorless, turns red-orange within minutes, eventually becoming brown.

There are two distinct pigments that have been identified in the secretions, red and orange. The two pigments are highly acidic compounds. They are known as red pigment hipposudoric acid and orange one norhipposudoric acid. The red pigment was found to inhibit the growth of disease-causing bacteria, lending credence to the theory that the secretion has an antibiotic effect. The light absorption of both pigments peaks in the ultraviolet range, creating a sunscreen effect. Hippos all over the world secrete the pigments so it does not appear that food is the source of the pigments. Instead, the animals may synthesize the pigments from precursors such as the amino acid tyrosine.

As indicated by the name, ancient Greeks considered the hippopotamus to be related to the horse. Until 1985, naturalists grouped hippos with pigs, based on molar patterns. However evidence, first from blood proteins, then from molecular systematics, and more recently from the fossil record, show that their closest living relatives are cetaceans – whales, porpoises and the like. Hippopotami have more in common with whales than they do with other artiodactyls (even-toed ungulates), such as pigs. Thus, the common ancestor of hippos and whales existed after the branch-off from ruminants, which occurred after the divergence from the rest of the even-toed ungulates, including pigs. While the whale and hippo are each other's closest living relatives, their lineages split very soon after their divergence from the rest of the even-toed ungulates.