Hippos are the cause of the highest human mortalities as far as larger mammals are concerned. The skin is dark grey, smooth and naked, pink around the eyes and lips and in the folds. The head is massive with an enormous broad muzzle up to 50 cm wide, and a huge mouth in which the upper and lower canine teeth and incisors are enlarged into tusks. The eyes and ears are small, set far back and high on the head. The neck is short and thick, with heavy folds of skin, especially in mature bulls. The upper and lower canines work against each other, keeping the tips sharp. The body is an elongated barrel shape, carried on short, stocky legs. Each foot has four toes with thick nails. The tail is short and flat and is fringed at the end with thick, stiff bristles. Average shoulder height males 1,5 m and females 1,44 m. Average weight males 1,5 tons and females 1,32 tons. Length of lower tusk above the gum is males 22 cm and females 14 cm.
Hippos need water which is at least 1,5 m deep in which to submerge. They can survive temporarily in mud holes. If the water that it rests in is brackish, it needs fresh water for drinking. Preferred feeding areas are lawns of short grass within 1-2 km of water, but it is able to travel up to 30 km to reach grazing.
Spends the day resting in or near water to keep cool, protect the skin from sunburn, and to avoid biting insects. Leaves the water to sunbathe on the banks, especially during overcast, cool weather in summer, and warm sunny weather in winter. Hippo sweat contains a red pigment that acts as a sun screen. In water it stands or lies on the bottom with only the top of the back and the ears, eyes and nostrils exposed; young lie on their mothers as rafts in deep water. Hippos can swim on the surface or under water, and walk along the bottom, staying submerged for up to 6 minutes. Both ears and nostrils can be closed to stop water getting in. As it surfaces it exhales with a series of grunts. At dusk it moves out of the water to feed for 7-8 hours. Grass is cropped with the hard edges of the wide lips, leaving a short smooth lawn. May travel up to 30 km to feed and wander long distances.
Hippos live in schools of up to 30, containing females and their offspring, young males, and a single dominant bull. The bull defends the school’s stretch of water as a territory in order to monopolize access to the cows. If frightened on land, a hippo charges back to the safety of the water, and anything in the way is likely to be trampled. In the water, females attack to defend their babies, and dominant bulls attack to defend their females. Territories do not extend away from water. A bull may hold a territory for as long as 12 years. Neighbouring bulls meeting on their boundary stare fixedly at each other, turn, and spray dung and urine – much like politicians.
Fights over territory and females are savage, leaving serious wounds even in the thick skin on the neck. Battle injuries may be fatal but even in the filthy water that most hippos live in the wounds rarely become infected. Losers are driven out of the territory. Young males are expelled from the school by the dominant bull at about six years old; females remain in the school. Dominant bulls sometimes kill youngsters. Mothers and babysitters defend their offspring against these attacks. Yawning displays the tusks and is a dominance signal and sign to intruders of other species, including humans, to keep away. It also occurs in play fights and sparring between young animals.
Dominant bulls and maybe others scatter dung and urine by wagging their tails as they defecate which can build up into heaps where hippo paths leave the water. When the bull approaches another hippo it urinates and defecates into the water and the bull tests the water with his vomeronasal organ. On three occasions hippos have been seen to rescue animals of other species from drowning or attacks by crocodiles. The motivation behind this is a mystery. They have also been seen to attack and kill animals of other species.
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