What Lies Beneath

By Dr Ian Whyte

I was only ever chased up a tree by an animal once in my 37 years in the Kruger National Park, by a hippo that had come to graze on the lawn in my garden. Though there is little data to corroborate the reputation hippos have as being the mammal that kills more humans than any other in Africa, and i do wonder heather this reputation is deserved, I wasn’t taking any chances. It may well have been the case when humans and hippos tended to live along-side one another and potential for conflict was far greater. Hippos attacking fish boats, raiding crops or simply grazing around villages are all situations with the potential for fatal consequences to humans. These days I suspect elephants have taken over this role.  Humans and elephant populations are growing rapidly in many parts of the continent and conflict is on the rise. Be as it may, hippos remain dangerous animals capable of inflicting fearsome injuries and have been responsible for many human deaths. In most instances attacks on boats or people on the edge of the water are by mothers with small calves.  You only have to look at their teeth to understand why hippos are so fearsome.  Their huge canine teeth, measuring up to 300 millimetres in the lower jaw and 150 millimetres in the upper, grow continuously. These teeth protrude from the skull in such a way that they wear off against each other, keeping them razor sharp. The jaws open wide enough to encompass a human torso. In a bite from a hippo the upper and lower canines would probably meet inside the body, punching two huge holes right through it, almost certainly a fatal injury. Most field guides will tell you that hippos can stay submerged for up to six minutes, but rangers in the Kruger have recorded times of up to eight minutes. Once alarmed, the animals are wary of exposing themselves and will breath by cautiously allowing only the tip of the nostrils to break the surface of the water. So as not to give their position away, they do this in almost complete silence, compared to the explosive release of air normally heard from relaxed animals in the water. Hippo bulls are very territorial. They maintain a territory, usually consisting of a good pool with sandbanks for sunbathing, for many years. The territory is confined to a stretch of river and its immediate embankments, and does not extend to the grazing areas away from the river. Each territory has its associated females and calves that return to the same pool during the daylight hours. This status quo remains while the bull holds tenure, which he does for as long as he can fend off rivals. Displacement by a younger, more vigorous bull results in a upheaval in which the new contender may kill the calves sired by the previous bull. This behaviour ensures the new bulls genes are passes on into future generations and not those of his predecessor. Fighting is generally ritualised with much posturing and open-jawed displaying of the canines.  But sometimes more serious fights erupt in which males may inflict serious wounds by slashing at each other with their sharp canine teeth. These may last for several hours and are accompanied by weird vocalisations such as squeals, grunts and roars. Such battles can result in the death of one, or sometimes both, of the protagonists. Given their size, hippos have an extraordinarily short gestation period of eight months. Growth of the foetus in the womb is incredibly fast. Black rhinos, which are similar in size to hippos, have a gestation time almost twice as long, yet rhino calf weighs less at birth, about 40 kilograms, compared to a newborn hippo at about 50 kilograms. Calves may be born at any time of the year but the majority are born during the mid-summer, rainy months. When the time comes to give birth, female hippos move away from the herd and remain separated for about a fortnight. The calves are capable of going into deep water and can suckle underwater within minutes of being born. Initially the calf stays in shallow water near a shore while the mother grazes nearby. Growth of the first 10 years of life is also rapid. The average increase in weight is in the order of 100 kilograms a year, so that at 10 years old, a hippo can weigh in excess of 1200 kilograms. Males and females are similar in size, the male being only slightly larger than the female.

Wild magazine summer 2013/2013