Hippo defecation is so nasty it can kill swathes of fish at once. The fish are essentially suffocated to death by poop, which is not a pleasant way to go by any standard.
A team at Yale University had the unenviable task of testing the feces-infested waters of 171 hippo pools in the Mara River, East Africa, over three years, documenting a total of 55 flushing flows. (These occur when the rate of river flow doubles.) The results have now been published in the journal Nature Communications.
Forty-nine times out of 55, oxygen levels in the river depleted. In 13 cases, oxygen levels dropped to such an extent that fish died as a direct result.
So, what's happening? When a single hippo goes about its business, its poop sinks down to the riverbed. In the Mara River, there are 4,000 hippopotami and together they produce 8,500 kilograms (18,700 pounds) worth of poop in a single day, littering the riverbed with feces. The bacteria that decompose the feces also consume oxygen and, if there are too many, they can deoxygenate the river to such an extent that it can starve other aquatic life of the gas – a phenomenon known as hypoxia.
But that’s not all. The influx of hippopotami and their feces is not dangerous under normal circumstances. The water stays in the hippo pools and doesn't spread to other parts of the river where it can harm fish. Things only become a problem when there is heavy rainfall. This can cause flushing flows that force water into areas further downstream and into fish habitats.
In addition to starving fish of oxygen, it pollutes the water with all sorts of nasty chemicals – chemicals including ammonium and hydrogen sulfide, both of which can be deadly to fish.
“Human sewage, drought, hog farms, or cattle pens all can lead to hypoxia but we show it also can be caused by wildlife in unregulated rivers,” David Post, senior author and professor of ecology and evolutionary biology at Yale University, explained in a statement. “And oxygen is the master variable for all aquatic life.”
To collect the samples, the team used remote-controlled boats. This is because hippos can be dangerous and some 500 people are killed by the animals every year, making them the deadliest large land mammal on the planet. Then, using microcosms, experimental stream arrays, modeling, and whole ecosystem manipulation, they determined the links between flushing of hippo pools and hypoxia.
The researchers point out that this is a natural occurrence and, while it can cause a temporary hazard to marine life, it also helps clean out the river and provides a source of food (the fish) for other animals.