There are 2 species of hippopotamus both found only in Africa, although fossil evidence shows that the family was once more widely distributed in the southern parts of the Old World.
Both species of hippos are amphibious, spending much of their lives in water, and they have various adaptations for this mode of life, including nostrils that can be closed and specialized skin glands that secrete an oily, pink substance, which protects their virtually hairless bodies from external damage.
These enormous animals spend much of their time in water, where they stay out of the hot sun and take the weight off their legs. It swims and dives well and can walk along the river or lake bottom. Hippopotamuses lie in the water with only their nostrils, eyes and ears above the surface, and they can also submerge for up to 30 minutes, while walking on the bottom of the river or lake. Daytime hours are spent mainly resting in water or on the shore.
Hippos play a vital role in the ecology of inland waters, both by keeping down bankside vegetation and by excreting tons of fertilizing manure into the water, which encourages the growth of plankton and invertebrates and thus sustains the whole ecosystem.
Hippopotamuses eat only plant food, which does not contain a large amount of nutrients. The water supports some of the weight of the hippos’ massive bodies, so resting in water helps the animals conserve their energy and reduces the amount of food they need to eat. After dusk the hippos leave their watery refuge in search of grass, sometimes travelling more than 3km (2 miles) from water.
Hippopotamuses are gregarious animals and live in groups of up to 15 or so, sometimes more, led by an old male. Large males set up territories along river banks, which they defend against other males. Although neighbouring territory-holders are usually peaceful, confrontations sometimes occur. When this happens, the males challenge each other with displays, including lunging, splashing, scattering dung with their short tails and displaying their tusks with great yawns of their mouths. If neither male backs off, fighting occurs, and sometimes they attack each other with their tusks - which can lead to fatal injuries, or deep gouges at the very least. Older males sport many scars from their past battles.
The Hippopotamus's tusks - actually its canine teeth - can weigh up to 3 kg (6.5 lb) each, and are used in fighting. There have been some reports of hippos overturning small boats and biting the occupants to death.
All adults are fierce in defence of their young. Males are aggressive towards intruders, including young hippos. Females are aggressive when they have young, and many consider hippos to be among the most dangerous of all African mammals.
The young is suckled for about a year, and females usually give birth every 18 months to 2 years.
Distribution: Most of Sub-Saharan Africa, although most common in tropical grassland areas of East Africa.
Habitat: Deep water near reeds or grassland.
Food: Grass and other low-growing plants.
Size: 2 - 5 m (6.5 - 16.5 ft); 1 - 4.5 tonnes (2,200 - 9,900 lb).
Maturity: Between 6 and 14 years.
Breeding: Successful males breed with several females. Mating takes place in water. Females are pregnant for 240 days. Usually a single young born during rainy season, with 2 or 3 years between births.
Life span: Up to 40 years.
Status: Common in general, although rarer in western and Central Africa.