Sometimes, everything goes wrong. The river might be especially deep or strong at that point. The opposite bank could be slippery or steep. The herd might be too big. Aggressive tourists can push them to more dangerous crossing points. “If there’s anything that keeps them from getting out on the other side, they’ll start to pile up. And even as they’re drowning on one side of the river, there are still wildebeest following them in.”

The result is an annual mass drowning. “We’ve seen up to 300 carcasses wedged into the river bank in some places,” says Subalusky. “It’s quite a sensory experience. The smell is potent for a quarter mile, and lasts for weeks. There’s a ranger station nearby and they really hate it when the drownings happen.”

She and her colleagues, including husband Chris Dutton and supervisor David Post, spent five years studying the migrating wildebeest, counting their corpses as they floated downstream. Through their sometimes grisly work, they’ve shown that these drowning herds account for a shocking large proportion of the river’s nutrients. Disney symbolized the circle of life with a lion cub being held aloft by a monkey. It might have done better with a mound of rotting, sodden wildebeest carcasses.

“Even when people noticed these drownings, it’s easy to underestimate the size and frequency of them,” says Subalusky. Her team estimated that around five mass drownings (defined as events involving at least 100 dead wildebeest) happen every year. Together, these events create around 6,000 carcasses and 1,100 tons of dead meat—roughly like dumping ten blue whales into the river every year.

A pile of drowned wildebeest wash up on the bank of the Mara. (Amanda Subalusky)

To work out what happens to these bodies, the team analyzed water at various parts of the Mara, collected samples of fish and microbes, and used camera traps to count arriving scavengers. They found that the piles of bodies sustain life throughout the entire river basin.

Vultures and marabou storks, flying in from more than 100 kilometers away, roost among the dead and eat up to 9 percent of the wildebeest nutrients. At night, hyenas arrive. Insects lay eggs in the carcasses, creating food for mongooses and ibis. Within the water, fish swim up to eat their fill, getting up to half their diet from the wildebeest bodies. Crocodiles become fat. "I’ve got a photo of a juvenile crocodile basking on these wildebeest carcasses, and I think he looks so happy,” says Subalusky.

It takes a month for each wildebeest to decay away, but they continue to affect the Mara long afterwards. Subalusky’s team confirmed this by dragging three carcasses out of the river by hand, dissecting them, and analyzing the chemical composition of each body part.

They showed that the soft tissues like muscles and viscera make up just half of a wildebeest’s weight. Their bones make up the rest, and these take around 7 years to break down. During that time, they leach huge amounts of phosphorus into the river, fertilizing it. They also sustain mats of bacteria and algae that provide fish like catfish and barbels with up to a quarter of their diet.