AZ Animals (US)
Story by Sharon Parry
Wildebeest are a common sight in eastern and southern Africa, where they spend their days grazing grasses and sipping water from rivers in the Serengeti. At certain times of the year, however, they do something extraordinary, it is called the Great Migration. Around two million wildebeest gather and undertake a perilous journey, which includes crossing the deadly Mara River. This is the largest mammalian migration on Earth. Thousands of them will not survive; this is literally the wildebeest’s race for life. Let’s find out more about why they do it and the dangers they face in and around the water.
Why Wildebeest Migrate:
The wildebeest’s Great Migration is a 500-mile trek that takes place during the wet season. The short grass plains are the preferred grazing grounds for wildebeest when the grasses are green and nutritious, as the open landscape allows them to spot approaching predators. This makes it an ideal place for giving birth to their young, who are always the most vulnerable members of the herd. However, this idyllic situation does not last. As the rains cease, the plains dry up, and the herds are forced to move to access food. They move north to where the rainfall is highest, but the grasses are not so nutritious. After spending the dry season in the north, they need to move back down to the preferred grasslands in the south as the rains return.
Great Migration Timings:
There are no exact timings for this awesome mass movement of millions of animals; each year, it can be slightly different. However, it always follows a general pattern. The short November and December rains trigger a movement to the main calving grounds. The herds stay here until the end of the long rains, which occur around the end of April and early May. As early as March or April, the wildebeest may start moving around, looking for greener pastures.
Between May and July, the herds prepare for their epic trek. The exact starting date of the migration, however, can be anywhere between late April and early June. As the herd gathers and starts to move, it can stretch for 25 miles and is one of the world’s most spectacular natural sights. Before they even get to the Mara River, they have to cross the Grumeti River. They often congregate on the southern bank and can stay there for two weeks before crossing.
Crossing the Mara River:
The next big obstacle is the mighty Mara River, and the herd crosses this in July or August. This is where many of the iconic images of the mass migration are captured. It is also where tourists flock to witness the spectacle. Visitors have shared their experiences on TripAdvisor, describing how the wildebeest seem to follow one leader to get across the water. Sadly, eyewitnesses also describe many dead wildebeest both in the water and on the rocks.
The wildebeest continue on to the Masai Mara in Kenya, but about half remain in the Mara-Serengeti area, crossing the river multiple times for reasons that are not fully understood. The rest of the herd must also cross the river again on their return journey. They head back south between October and November, returning to the calving grounds where the entire cycle begins anew.
Death at the Mara River Crossing:
The wildebeest’s crossing of the Mara River is frantic and desperate. Describing the scene in an article for the Wilder Magazine, which he founded, Jan Fox describes how “hundreds of lanky, bearded herbivores hurl themselves off the steep bank opposite us into the river. On the other side, they are swept into a frenzied mass and clamber up jagged rocks to join the rest of the herd.”
Of the 1.5 million wildebeest making the migration, around 300,000 die. So, how do these animals perish during the river crossing? Here are the main causes of wildebeest mortality during the most dangerous moments of their lives.
Predation
Millions of ungulates in one spot are a fantastic feeding opportunity for predators. Terrestrial carnivores such as lions, leopards, cheetahs, and hyenas will take the opportunity to hunt members of the herd. The easiest to pick off will be the young, the old, and the stragglers.
Once the wildebeest enter the water, they face another threat. Here, Nile crocodiles are waiting to launch themselves at their prey. The wildebeest may find it hard to spot them as they are the same color as the murky water.
Once they have left the riverbank, the wildebeest have to take their chances dodging those powerful jaws. At times, the river is teeming with hungry crocs lying in wait. Nevertheless, because crocodiles have a low metabolic rate, they are easily satisfied and feed only intermittently. A study of one region of the river found that crocodiles consume only 150 wildebeest throughout the entire migration period, accounting for just 2 percent of the wildebeest that die in the river.
Currents and Deep Water:
Wildebeest are very capable swimmers, but the Mara River crossing still holds many challenges for them. Sudden changes in depth may catch them off guard. Also, the current is very strong in places, and they risk getting swept away. Younger and weaker animals are most at risk of getting into serious trouble in these waters.
Chaos and Other Wildebeests - Stampede
Sometimes, the greatest threat is posed by other wildebeests. At times, the crossing can be utterly chaotic, with wildebeests hurling themselves into the water and then desperately clambering up treacherous slopes and rocks on the other side. In their eagerness to get across, they can easily trample other wildebeest or even push them underwater. Bottlenecks can form at certain entrance or exit points, and some wildebeest can become trapped or separated from the herd. This makes them more likely to be picked off by predators and more susceptible to being swept away.
Stress and Exhaustion:
Even for a strong and powerful animal like the wildebeest, this journey is challenging. The river crossing is especially taxing, and they do not all have the stamina to complete it. Additionally, the stress of crossing this hazardous water can weaken the animals’ immune systems, leaving them vulnerable to disease. It can also leave them with injuries that make them vulnerable to attacks by predators later in the journey.
Quantifying Mara River Crossing Deaths:
The mass crossings of the Mara River cause so many wildebeest deaths that they influence the storage and recycling of nutrients in the ecosystem of the river and the surrounding area. Scientists used historical reports and field surveys to measure the scale of mass drownings in Mara River migration crossings. They found that mass drownings occurred in the Kenyan portion of the Mara River in at least 13 out of 15 years from 2001 to 2015. From 2011 to 2015, a total of 23 mass drownings were recorded, with between 3,250 and 9,250 deaths per drowning. Despite the fact that 175,000 zebras migrated with the wildebeest each year, fewer than five zebra carcasses were found during the entire study period.
Not all crossings result in mass drownings:
These events are caused by a complex interaction of factors, including the amount of water in the river, the shape of the riverbanks, the size of the herd, and even tourist behavior. The scientists estimated that mass drownings contribute 1,096 ± 526 tons of biomass to the Mara River each year. Furthermore, when the carcasses are scavenged by birds, the nutrients are spread more widely throughout the terrestrial ecosystem. All parts of the carcass are recycled by the environment.
Even the bones, which remain in the river months after the rest of the body has decomposed, become covered in a biofilm of bacteria, fungi, and algae.
As is so often the case in nature, one species’ loss provides a gain for other life in the area.