Hippo ears signal danger to paddlers on rivers and lakes in Africa. But they are a good omen too because hippos play a vital role in preserving watery ecosystems through their excrement, scientists have discovered.
Hippo faeces pump large amounts of silicon – which is crucial to oxygen-producing algae – from the land into rivers and lakes.
“This ecosystem is in danger because there are fewer and fewer hippos. In the long term, this could lead to food shortages at Lake Victoria, for example,” said researcher Patrick Frings, from the German Research Centre for Geosciences GFZ.
Hippo numbers have shrunk up to 90% in the past few decades in Africa with habitat loss and hunting. Even their natural enemy, the crocodile, is hardly a threat compared with humans.
In the wild the “river horses” graze on dozens of kilograms of short grass on land at night, away from the sun which can burn their skin. In the daytime they release enormous batches of poo in the water while resting under the surface.
Weighing about 3,500kg, they are second in size only to elephants in Africa.
“Hippos differ from other large grazing animals in the savannah,” said the first author of the study, biologist Jonas Schoelynck from the University of Antwerp.
“The nutrients in the excrements of most grazers largely end up back in the savannah again ... hippos act as a kind of nutrient pump from the land to rivers and lakes.”
A lab analysis of samples of plants, water and hippo excrement in this study found silicon in the plants eaten by these semi-aquatic mammals.
The grazing hippos in the study absorbed 800kg of silicon a day, pumping half of this into the Mara River – an estimated 76% of the total silicon in the river.
The Mara River flows nearly 400km through the Maasai Mara Nature Reserve in southwest Kenya.
More than 4,000 hippos – of about 115,000 to 130,000 hippos in Africa, classified as vulnerable – live in the Maasai Mara.
“Our results are completely new,” Frings said. “So far, it has not been assumed that grazing wild animals could have such an influence on the transport of silicon from land to lakes.”
A recent study in the Mara River showed that a sudden large influx of hippo waste could be toxic to aquatic life.
But the latest results demonstrate the protective role that hippo faeces also play. The oxygen-producing algae needing silicon “form the basis of the food chain in many water ecosystems”. If they collapse and are replaced by pest algae, fish can die from oxygen depletion.
Remember that, the next time you’re charged by a hippo during a sunset paddle on the Zambezi River or closer to home watching palm-nut vultures on Lake St Lucia.