At first, Chris Dutton and Amanda Subalusky had no idea why the fish were dying.

At a bridge on the border between Kenya and Tanzania, they noticed that whenever the Mara River rose by a few feet, dead fish would wash up on its banks, sometimes in the thousands. Storks, vultures, crocodiles, and hyenas made short work of the carcasses, so “if you weren’t there to see it, you’d never know it was happening,” says Dutton. Local rangers knew about the die-offs, but they blamed the events on farmers who sprayed pesticides in upstream fields.

It wasn’t the farmers. Through an increasingly bold set of experiments, involving remote-controlled boats, computer simulations, a makeshift dam, and vast tankers of excrement-filled water, Dutton and Subalusky identified the real culprits: hippos.

The duo, who are married, published their results in a paper with the remarkably polite title of “Organic matter loading by hippopotami causes subsidy overload resulting in downstream hypoxia and fish kills.” To translate: Hippos sometimes poop so much that all the fish choke to death.

At night, hippos wander into grasslands to graze. During the day, they return to rivers to keep cool and protect themselves from sunburn. As they wallow, they constantly urinate and defecate. Every day, the 4,000 or so hippos in the Mara deposit about 8,500 kilograms of waste into a stretch of river that’s just 100 kilometers long. “Down at the bridge, you can put a net in the water for a few seconds, and the entire middle will just be coated with hippo feces,” says Dutton. “There’s hippo feces everywhere. Over the rocks. Over the bottom.”

In the dry season, when the Mara becomes narrower and shallower, certain stretches of it become especially thick with hippos—and their dung. Hippos are aggressive and dangerous, so only the foolhardiest of researchers would wade into these so-called hippo pools. Instead, Dutton and Subalusky deployed a remote-controlled boat armed with sensors. It revealed that the mud and water at the bottom of these hotspots is a stagnant mess of ammonia, methane, hydrogen sulfide, and other chemical grotesqueries. It’s also starved of oxygen: Almost all of the gas is consumed by bacteria as they slowly digest the accumulated hippo poop.