These large African mammals spend most of their life in water - including when giving birth, suckling young and mating.

Meaning of scientific name
The name 'hippopotamus' is derived from the ancient Greek for 'river-horse'.

Life span
Hippos live for about 45 years (49 in captivity).

Statistics
Length: 3.3-3.45m, Height: 1.4m, Weight: male: 1.6-3.2 tonnes, female: 1.4 tonnes.

Physical Description
Hippos are fat, heavy-bodied animals, with short, stumpy legs. They have an enormous head with huge jaws filled with tusk-like canine and incisor teeth. They have a gape of 150 degrees, compared to 45 in humans. Hippos have a short, tufted tail. Hippos are almost hairless. Their skin is grey-brown to blue-black on the upper surface, and pinkish below. The eyes, ears and nostrils are all positioned on top of the head so that they can be almost totally submerged, but still be able to use all of their senses. When diving, the ears and nostrils can be closed. They also have webbed toes.

Distribution
Hippos live in West, Central, East and South Africa.

Habitat
During the day they inhabit rivers and lakes. At night, they graze on short grasslands.

Diet
Hippos feed on grass, but have very occasionally been seen eating small animals or scavenging. They eat about 40kg of grass a night, which is only 1-1.5 per cent of their body weight - their lifestyle is energy-efficient.

Behaviour
Hippos have a semi-aquatic lifestyle and can remain underwater for over five minutes. Out of water, they are fast runners, and if they feel threatened, will race back to the safety of the water. This can sometimes result in attacks on humans, and it is advisable never to get in between hippos and their aquatic habitat. During the day, they form family groups of 10-20 (but up to 100) consisting of females and their offspring. At night, they are solitary. They occupy a home range within the territory of a dominant male. They sometimes gather in large groups to share wallows - temporary watering holes in between their feeding grounds and permanent water source. Dominant males mark their territory by wagging their tail and scattering dung around. Other males are allowed to enter the territory as long as they behave submissively and do not try to mate with the females. If fights do break out, they can be very bloody, with wounds inflicted by the sharp canines, and may result in death for the loser.

Reproduction
Dominant males mate (in the water) with the females in their territory. After a gestation of 240 days, typically only one calf is born, sometimes on land, but usually underwater. The mother is fiercely protective of her calf, which can occasionally be taken by big cats and hyenas. Calves remain with their mothers for about five to eight years, but are weaned after 6-8 months.

Conservation status
Hippos are abundant and are not listed by the IUCN Red List of Threatened Species 2002.

Voice
They communicate using a variety of grunts and deep rumbles.

Notes
Hippos do not have sebaceous glands, but do have mucus-secreting glands, which produce a pink fluid that keeps their skin moist, and may also protect against infection and prevent sunburn. If hippos are out of the water for too long, the skin cracks and burns. It is generally accepted that whales are related to artiodactyls (even-toed ungulates), and it is possible that hippos are the group most close to them.