A hippo is a large, four-legged mammal with an enormous head and mouth. The three species of modern hippos are the common hippopotamus or Nile hippopotamus, the pygmy hippopotamus and the two-toned riverine or bush Hippo. They are members of the family Hippopotamidae which also includes the extinct genus Choeropsis and all other similar animals from the Miocene through Pliocene epochs.
The word “hippopotamus” comes from the ancient Greek for “river horse”. The common name refers to their habit of spending long periods submerged in rivers and lakes to keep cool in hot climates, but they are actually more closely related to pigs than horses. Modern taxonomic classification places them as hollow-horned ungulates in the order Artiodactyla, but some authorities place them in their own separate order, Hippomorpha.
Hippos are among the largest living land mammals. A male common hippo can weigh up to 3200 kg (7000 lb) and be up to 4.5 m (15 ft) long from nose to tail, while a female typically weighs about half as much and is slightly smaller. Despite their size and weight, they are able to run at up to 30 km/h (19 mph).
All species of hippos have stout bodies with short legs, barrel-shaped torsos and large heads with wide mouths set on a thick neck supported by equally stocky legs. Their skin is tough and wrinkled; its colour ranges from slate grey, olive brown or reddish brown in pygmy hippos; deep purple-brown or chestnut brown in riverine hippos; blue-gray with pink highlights around eyes, ears & lips; pinkish gray along lower flanks & inner thighs; dark gray on back & outer thighs in lakeland subspecies of common hippo ; between light tan & creamy white along Baringo’s shoresline & below Kenya’s Tana River delta populations . All have pale undersides & paler areas around eyes , inside ears , lips , soles of feet . Common Hippopotamus’s hairless , oily skin secretes reddish -orange mucous that acts as sunscreen & waterproofing agent . Underwater , where it spends much time cooling off , its body temperature drops only slightly . Sweat glands open when air temperatures exceed 27 °C (81 °F ) ; pores close when waterlogged conditions would cause body heat loss .
While most terrestrial mammals must make do with about 10% lung capacity relative to their body size, adult pygmy Hippopotami have 50% more lung capacity per unit body mass than humans – aided by having one third as much fat per unit body mass – allowing them greater aerobic endurance during activities such as running