Hippopotamus

The hippopotamus is a two-tonne grazing animal that has arranged its entire life around one weakness: its skin cannot handle the sun. So it spends the day submerged and does all its living at night.

Overview

Hippos need two things close together: deep, permanent water to rest in, and open grassland to feed on. They are found along rivers, lakes, and swamps across sub-Saharan Africa, in groups — called pods, bloats, or schools — of ten to thirty animals, sometimes many more, presided over by a dominant bull.

Physical characteristics

A hippo is built like a barrel on stubby legs, with a huge head that takes up a third of its length. The eyes, ears, and nostrils sit high on the skull so the animal can see, hear, and breathe while almost entirely underwater. The nostrils close and the ears fold shut when it submerges.

The skin is nearly hairless and secretes hipposudoric acid — the reddish “blood sweat” — which blocks UV light and kills bacteria. Out of water for too long in the day, a hippo’s skin dries and cracks.

The jaws open to nearly 180 degrees, revealing lower canines that grow throughout life and can reach 50 cm. These are weapons, not feeding tools: hippos fight each other, and threaten rivals, with a wide “gape” display.

A life split in two

• By day, hippos rest in the water — floating in the shallows, standing on the bottom, or hauled out on a sandbank in a tight group. This conserves water, keeps the skin cool and moist, and supports the animal’s weight.
• At night, they leave the water alone and walk up to 10 km along regular trails to graze. A hippo eats around 40 kg of grass, cropping it with its wide, muscular lips rather than its teeth. Their dung fertilises both the grassland and the rivers, moving nutrients between the two and feeding fish and insects.

Behaviour

Hippos are far more social in the water than on land, where they feed alone. In the pod, bulls defend stretches of shoreline and will fight savagely over them, slashing with their canines. Hippos communicate with a range of grunts, bellows, and wheezes, some produced above water and some below, and a “chorus” can travel down a river from pod to pod.

Reproduction and life cycle

Mating and birth both happen in the water. After an eight-month pregnancy a single calf is born, able to swim before it can walk and nursing underwater by clamping the nostrils and ears shut. Mothers are fiercely protective and keep calves away from aggressive adult males. Young hippos stay with their mothers for several years.

Threats and conservation

The hippo is listed as Vulnerable and declining:

• Habitat loss. Rivers are being dammed, diverted for irrigation, and drained, removing the deep permanent water hippos cannot live without.
• Poaching and hunting for meat and for the ivory in their canine teeth, demand for which rose after restrictions on elephant ivory.
• Human–hippo conflict, as farms expand onto the grazing land and hippos raid crops, leading to retaliatory killing.

Populations are relatively secure in large, well-managed parks in eastern and southern Africa but have collapsed across much of West and Central Africa.