Every year, 1.2 million blue wildebeest migrate across East Africa, and despite their extraordinary numbers, they can still take you by surprise. “It’s like magic,” says Amanda Subalusky from Yale University. “One day, there are these vast empty plains of really tall grass. The next morning, you’ll wake up and the plains will be full of animals as far as you can see. There’ll be black specks across the landscape, like someone has taken a pepper shaker to the savannah.”

The wildebeest, accompanied by some 200,000 zebra and antelope, travel clockwise through the Serengeti, tracking pockets of rainfall and grazing on about 4,500 tons of grass every day. It’s the largest overland migration in the world. It’s also one of the deadliest. In the northern part of their route, the wildebeest must repeatedly cross the mighty Mara River—and many fail.

Natural history documentaries like to show crocodiles killing the wildebeest, but being cold-blooded, crocs have small appetites and are easily sated. For every wildebeest that they kill, 50 more drown on their own. “They’re anxious and hesitant when they get to rivers,” says Subalusky. “The herd piles up on the bank, and hours or days can go by before they get up the courage to cross. Once one dives in, the rest follow and this herd mentality takes over.”

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.

“When you have 1.2 million animals going round an ecosystem, it’s hard to find anything that’s not impacted,” says Grant Hopcraft from the University of Glasgow, who also studies the migration. “The cool thing isn’t that the migration has an effect on the river, but that it’s a lasting effect.” A crossing could be over in an hour, but if just a few hundred animals die, they could change the river for months. “The long pulse of nutrients means that different species can take advantage,” says Hopcraft. “That leads to diversity and diversity leads to resilience.”

Other migrating animals provide valuable pulses of nutrients, too. Pacific salmon are the best example. They migrate from the oceans to their birth rivers to mate, spawn, and die. Their bodies directly feed eagles, seals, and bears. But their contribution extends well beyond the riverbanks because their carcasses, dragged into the forests by bears, also nourish mosses, trees, insects, songbirds, wolves, and more.

But the Mara wildebeest differ from the Pacific salmon in two important ways. The first is quantitative: they provide four times as much flesh by weight. The second is qualitative. “Unlike salmon, no part of the wildebeest life cycle requires that they die en masse in a river,” says Janice Brahney, a freshwater ecologist at Utah State University. “But the fact that [such deaths] occur nearly annually suggests that their impact on the ecosystem might be just as significant. It would be interesting to investigate what might happen should this influx of wildebeest nutrients not occur for a period of time. With climate change, how might the frequency of these events change in the future? Has the frequency of these events already changed?”

Subalusky certainly thinks so. These kinds of mass drownings used to be far more common, before humans slaughtered their way through the world’s large herds. Take the American bison. Today, there are around 30,000 individuals left in the wild. But before the 1800s, there were at least a thousand times as many—between 30 and 60 million of them. “We found references in Lewis and Clark’s journals to bison drownings in the US, with thousands of carcasses piling up as they crossed frozen rivers,” says Subalusky. “These were potentially regular events in the recent past.”

So, when we watch the wildebeest migration, we’re looking through a window into the global past—into a planet where mega-herds routinely traversed bodies of water and lost their lives in huge numbers, to the benefit of the surrounding ecosystem. And that, Subalusky says, should force us to reconsider our view of what those ecosystems ought to look like.

“What is a pristine river?” she asks. “We picture a crystal-clear babbling brook, but that has low levels of nutrients. The Mara suggests that maybe some ‘pristine’ rivers, back when large animals were really common, were full of dead bodies.”