For wildebeest, the yearly migration across the Serengeti can mean life or death.

Predators such as crocodiles and big cats lie in wait as the herd of more than one million makes its 1,000-mile loop across the savannas of Tanzania and Kenya.

But one of the most serious—and overlooked—threats of this overland migration is drowning. With thousands crossing the Mara River at the same time, scores of the antelopes are swept away by the current. (Read how wildebeest know when to migrate.)

So for the first time, scientists have estimated how many wildebeest actually die each year: An average of 6,250, equivalent to the mass of 10 blue whales.

Not only that, Amanda Subalusky, a postdoctoral researcher in aquatic ecology at the Cary Institute of Ecosystem Studies in New York, and colleagues revealed these decaying bodies provide vital nutrients to Serengeti waterways. (Learn more about great animal migrations in National Geographic magazine.)

“When a drowning occurs, we drop everything and start studying them,” says Subalusky, whose study appeared this week in the Proceedings of the National Academy of Sciences.

THE CIRCLE OF LIFE

Figuring out how over two million pounds of rotting wildebeest contributes to the river ecosystem is no easy feat.

For the study, Subalusky and her team hauled drowned animals out of the water to dissect them and figure out their exact nutrient composition.

They also put parts of the wildebeest in the river, protected by special croc-proof decomposition cages, to see how long it took each part—skin, flesh, and bones—to decay in the environment.

Chemical measurements of local water and fish were taken to see how much the nutrients of the wildebeest were incorporated into the environment. The team discovered only a small portion of the bodies are gobbled up by crocodiles—they can only eat.

DEATH’S LEGACY

The drowned wildebeests’ major contribution to the ecosystem is their skeletons. Wildebeest bones take around seven years to decay, slowly releasing phosphorous, an element critical for plant and animal growth. A slimy film of bacteria called a biofilm grows on the bones and provides food for fish in the river.

“There is this legacy of a drowning event through the bones of an animal that could contribute to the ecosystem for decades,” says Subalusky, whose research was funded by the National Geographic Society during her graduate studies at Yale University.

Knowing how the dead wildebeest impact the environment is timely, because it shows what would be lost if wildebeest populations or migrations declined, says Kendra Chritz, a geochemist at the Smithsonian’s National Museum of Natural History who was not involved in the study.

“There are many large-bodied African mammals that no longer migrate, or have had their migrations cut short, because of human population growth and land-use change,” says Chritz. (Read: “Jane Goodall on Why We Should Help the Serengeti.”)

While the wildebeest migration is no longer threatened by a now-defunct plan for a Serengeti highway, migrations of other large mammals around the world like springbok and bison have declined or stopped completely.

UNBALANCED BUDGET

While the new calculations account for where about half of the wildebeest nutrients end up, the budget is not completely balanced. “We can’t account for what happens to about half of the carbon and nitrogen,” Subalusky explains.

Chemical analysis shows that fish in the river are eating up some of these nutrients, but it’s unknown how much.

As a whole, a terrestrial migration having so much impact on an aquatic ecosystem is a somewhat novel concept, and needs to be further explored she says.

“We once saw the wildebeest just as a pulse of carcasses that were available for a short time and then gone,” she says. The new study “changed our sense of scale of the issue.”

The Great wildebeest migration – When is the Best Time to Witness the River Crossings?

This is one of the most common questions that tourists ask their travel agents and tour operators. The thing to understand is that the exact timing of the migration is driven by the weather so an unusually dry or wet season can cause a variation in the timing. That said, the months of May to October are the most likely periods to witness the river crossings.

• May and June — As the rainy season comes to a stop, the south and eastern plains of the Serengeti start to dry out, prompting some of the herds to move towards the Western Corridor and the Grumeti River, which they must cross as they head north towards the Masai Mara plains.

• ‍July to October — By this time, the herds are congregating around the Mara River and crossing in large numbers.

What Makes the Great Migration River Crossing?

In what is one of the most quintessential wildlife experiences on the planet, the annual Great Migration in Tanzania and Kenya is an event unlike any other. Picture millions of wildebeest, zebras, gazelles, elands, and other antelope species making this incredible journey across Tanzania and Kenya every year, giving birth and facing scores of different predators on both land and water. For many tourists, witnessing such a spectacle is an emotional experience that could never be fully captured by mere words or photos. Thus, making this one of the most sought after safari tour. Whether you are on a family safari holiday or going on a solo trip this unique phenomenon shouldn’t be missed!

While every aspect of the Great Migration delivers a unique wildlife viewing experience, the river crossings are not to be missed. The dramatic event is one that showcases nature at its rawest. Life and death hang in the balance in a survival of the fittest among the large herds as they battle hungry crocodiles and hippos, as well as turbulent waters in their search for greener pastures.

Where the River Crossing Takes Place during the Great wildebeest migration

There are two major river crossings along the path of the Great Migration. They are the Grumeti River crossing in Tanzania and the Mara River crossing in Kenya. The Grumeti River is the first water obstacle that these bumbling herds will encounter on this journey. Here, large Nile crocodiles lie in wait as the herds are forced into the water in spite of the obvious threats. This predator activity is beyond anything you can imagine. It is a spectacular event to behold, but it is not as challenging as what they are about to face when crossing the Mara River further north.

The Mara River is almost 400 km long and is deep, wide, craggy and treacherous. It is also home to Africa’s largest crocodile population as well as a robust population of hippos. As the herds clamor and struggle to cross the river to get to the greenery on the other side, these large Nile crocodiles and territorial hippos await, unfolding what is without a doubt one of the most dramatic and chaotic scenes in the animal history of the world. In many ways, this river crossing represents the climax of a long and arduous journey. The rivers are torrential, swollen with water and the two game reserves are teeming with enough animals hungry for the flesh of the migrating beings.

The Great Wildebeest Migration in Africa – also known as the Gnu Migration, Serengeti Migration and Masai Mara Migration – is one of the last mass terrestrial wildlife movements left on the planet. It’s the chief reason why so many travelers venture to Kenya and Tanzania for a Migration safari, especially around mid-year.

The Migration is one of nature’s greatest paradoxes: timing is absolutely vital, but there is no way to predict the timing of the animals’ movements. We know that the wildebeest (and a smattering of zebra and antelope) will cross the Mara River – but nobody knows exactly when. We also know that rain will trigger the wildebeest to move onto fresh grazing – but nobody knows exactly when the rain will fall.

Fortunately, we’ve been planning Wildebeest Migration safaris in Africa since 1998. We’ve helped thousands of travelers to be in the best possible place at the best possible time for the best possible price. If you’re looking for expert planning advice, look no further. We’ve compiled all our specialist tips in this handy beginner’s guide to a Wildebeest
Migration safari..

1. Around half a million calves are born in two months

Roughly 400,000 wildebeest calves are born every year between January and early March in the southeastern plains of the Serengeti. It’s also a general calving season, as zebras and antelopes also give birth to young during this period. For many, this is considered the best time to visit the Serengeti, as it’s exciting to see the newborn animals.

The dangers of the Great Migration to its participants cannot be overstated. The wildebeests, zebras and antelopes must confront thirst, hunger, exhaustion and predators. Predators include lions, leopards, cheetahs, African wild dogs, spotted hyenas and crocodiles.

The most vulnerable members of the Great Migration herds include, of course, the young. Only one in three calves will make it back alive in a year to the Serengeti’s southeastern plains. So the continuation of the Great Migration is, in a sense, a numbers game.

2. The grunt of each wildebeest is unique

One of the other names for a wildebeest is gnu. The g is silent, so it’s pronounced ‘nu’. This is an onomatopoeic word for the grunting sound the animals make

What’s truly astounding is that each wildebeest has a unique grunt – a unique nu, if you will. These sounds help them to locate to each other. So when it’s dark, or there’s a large herd, a mother and foal, for instance, can find one another by listening for their unique nu’s.

3. Calves are able to stand minutes after birth

Wildebeest and zebra calves are amazing. They quickly stand after birth, and soon thereafter can start running. In fact, wildebeest calves can walk on their own within minutes of being born. And within days they can keep up with the herd and even outrun a lioness! Zebra foals can also pretty much walk and even run after an hour.

We call species like this precocial, meaning their young are born in an advanced state. The hoofed animals of the Great Migration are precocial to help them survive. They live with lions, leopards and other predators lurking around, watching, so if their young were unable to move, they’d literally be sitting targets.

Precocial babies need long gestation periods, since the fetuses must grow to greater maturity in utero. The gestation period of wildebeests and elands is nine months. For zebras, it’s 13 months!

4. Wildebeests rely on zebras for their survival

The animals of the Great Migration are grazers. The very reason for their non-stop migration is the search for green pastures.

The wildebeests of the Great Migration are fussy eaters. They only eat the shoots of grass. Zebras, on the other hand, aren’t picky. They eat the fresh, long grass, and in so doing, ‘mow it down’. The wildebeests can then come in and access the shorter grass they need to survive.

So the roughly 200,000 zebras that travel harmoniously alongside the wildebeests of the Great Migration essentially enable the survival of the latter.

5. Nile crocs can survive on one or two feedings a year

Every year the herds of the Great Migration must cross the Mara and Grumeti Rivers of the Serengeti to follow their course towards greener grass. These river crossings are perilous occasions for a few reasons, not least because of the hungry presence of Nile crocodiles.

Nile crocs can weigh up to 750 kg (holy smokes!). And the crocs of the Grumeti are some of the biggest in Africa.

You would think such large beasts would need to eat constantly to survive. Yet, amazingly, some of the larger crocs can sustain themselves on just one or two wildebeest feeds per year!

The giant reptiles gorge themselves, and then go into semi hibernation. They actually slow their heart rate and metabolism. Amazing.

6. Wildebeest hooves leave a scent trail for others to follow

The Great Migration doesn’t consist of one, solid herd. Rather, there’s a main herd and satellite herds, all of which endlessly morph, splinter and coalesce over time.

The herd is at its most cohesive from late December to early March, when all the wildebeests (and other hoofed participants) gather together in the Serengeti’s southeastern grasslands. This is calving season, a vulnerable time, and they’re seeking protection in numbers from the ever-prowling predators.

When the animals are back on the move, however, the herd splinters and the animals spread out. This is especially the case when there’s plenty of green grass.

New evidence suggests that the glands in the wildebeests’ hooves secrete pheromones and faeces that stays on the ground. This allows their fellow wildebeests to follow their smell. And so the disparate herds can stay connected and link up.

7. Adult lions can eat 40 kg of wildebeest in a sitting

Lions don’t eat all the time, but when they do, they go for it! Lions usually eat every three to four days, averaging around 6 kg a feed. But they can go for longer than that – even for a week or so.

After a long spell without food, lions go big. In fact, a fully grown male lion can chomp down on 40 kg (88 lb.) of wildebeest in one sitting. That’s about a quarter of its own weight!

The crocodile infested Grumeti River is one of the first obstacles facing the wildebeests on their migration from Tanzania to the Maasai Mara during the Great Migration. Often overshadowed by her bigger sister, the Mara River, the Grumeti is no less impressive once the wildebeest arrive.                                                                                           

The Grumeti River spans the western corridor of the Serengeti National Park, flowing westwards into Lake Victoria. 

The western corridor stretches 50 km and comprises vast savannah grasslands, riverine forest, rolling hills and shady acacia groves. 

This area of the Serengeti is seldom visited due to its remoteness. However, those that do venture this far are rewarded with a great variety of year-round wildlife, including giraffe, hippo, cheetah, elephant, large Nile Crocodiles and good leopard and lion populations. 

Grumeti’s remote location means that you will not be distracted by a trail of safari vehicles. Around 30 safari vehicles gather on the banks of the Mara River during The Great Migration, but Grummet’s banks will be occupied by half as many vehicle

Although increasingly unpredictable, the marching column of wildebeest, zebra and antelope, usually spend two weeks between May and June crossing the Grumeti River. 

The neighboring Grumeti Reserve is one of the few places in the Serengeti where you can spend a few hours on foot exploring the African wilderness. 

Alternatively, glide across the early morning sky in a hot air balloon for a bird’s-eye view of the river and plains. 

There are only a few accommodation choices in the western corridor year-round, but in peak season mobile camps are set up that follow the migration. 

The Grumeti River is a great choice for those who want to avoid travelling in the peak season from mid-June to September, when the parks are busier and travel more expensive. 

The Grumeti River crossings usually occur within a 1 or 2-week window in May and June, but predicting the precise time is impossible, as the migration follows the rains. 

River crossings on the Grumeti usually occur as the long rainy season transitions into the dry. So intermittent rainfall is likely, which could affect roads and planned excursions. The grass is also typically longer and greener, making wildlife more difficult to spot. 

The Grumeti River is located in the remote western corridor of the Serengeti, which easily adds an extra 3-4 hours to your journey if driving – an important consideration if you are crunched for time. Alternatively, daily flights to Grumeti Airstrip are possible but can be expensive.