Why Are There So Many Wildebeest Compared to Other Animals in The Serengeti?

Herds crossing into Kenya.

Having been on safari for the last couple months, I’m unworthy of being called a co-author of this blog considering the wonderful posts that Colin has been writing. In my travels I have been to the Serengeti ecosystem four times in the last few months, three times in Serengeti and once in Maasai Mara and of course we have followed the spectacular herds of wildebeest.

When you’re driving through hundreds of thousands of wildebeest, or watching tens of thousands plunge into the Mara river because the grass is greener on the other side, its hard to wonder why there are so many of them. Why not zebra, topi, kongoni, impala, dikdik or one of the other antelopes?

So, I thought I would explore this topic and discovered this wonderful paper online, which you can download if you want to read a more scientific explanation. (Click here )

Part of Colin’s themes has been that there are things that shape or influence the environment, and that the environment then shapes the species in it. It’s a two-way interaction that steers what happens. E.g. When there is predation on plants they evolve defense mechanisms like thorns or chemicals.

So, what is it about the Serengeti that promotes these massive herds of wildebeest?

The simple answer:

Climate and soils.

The Serengeti ecosystem extends between two geologically significant features:

In the east, are the rift valley volcanoes that blew volcanic ash over the eastern part of the Serengeti, starting millions of years ago. These became the extremely fertile short grass plains between Maswa and Piyaya.

The short grass plains of Piyaya- the volcanoes in the distance.

In the west, Lake Victoria gives the north-western Serengeti a much higher rainfall (1200mm) than south-eastern Serengeti (500mm), especially when everywhere else is dry.  

Put these two factors together and you have high quality grazing every month of the year. In the wet months of the year (Feb, March, April), the soils in the short grass plains make the grass particularly excellent grazing with extra dose of calcium and phosphorous - perfect if you are a wildebeest trying to make milk for your calf. In the dry season- well, you migrate to where its raining and you find green grass which is much more nutritious than dry grass. (Wildebeest need 30% more energy, 5 times as much calcium, 3 times more phosphorous and 2 times as much sodium when they are lactating than pregnant and the short grass plains are perfect.)

A newborn wildebeest in Piyaya. It stands within 20 minutes
 to suckle. The milk is a high-cost to the mother but she
survives because of the minerals in the grass.

So, now we understand that the whole 25,000km2 Serengeti ecosystem always has nutritious grass (and drinking water) somewhere at all times of the year. The next question we have to investigate is- why wildebeest? Why not zebra, topi, kongoni, eland etc. etc?

The simple answer:

                   Wildebeest are special.

As you might know, wildebeest belong to a tribe of antelopes called the Alcelaphines. This means they are fairly closely related and if you want to know how close, well, they are about 4 million–year-old cousins. All of them are ruminants, which means they have a four-chambered stomach that they use to digest cellulose. Rumination is a very efficient way of extracting nutrients from plants but each species will have it’s own efficiency and Coke’s hartebeest are actually the most efficient of the three species. So why isn’t it Coke’s hartebeest?

Topi in the long less nutritious grass on the Lamai wedge

We can start by looking at the mouth structure of these animals and realizing that wildebeest actually have a mouth that is perfect for eating grass that is 3cm high, which is when the grass has the highest levels of protein.

The next thing they do is chose the parts of the grass that are also more nutritious- the leaves and fresh shoots. Coke’s hartebeest and topi eat more stems and leaf sheaths than wildebeest, zebra survive on almost only stems. But there’s a lot more grass stems than grass leaves so you would rather expect zebra populations to be in the millions but they aren’t- what is actually happening, is that zebras suffer very high losses of young, so predators keep zebra numbers down.

Now, you might ask, why aren’t wildebeest populations kept low by predators?

Answer: Synchronized reproduction and rumination.

80% of wildebeest calves are born in 3 weeks in February= 250,000 wildebeest calves= 500 per hour. It is an amazing sight. In scientific terms: extreme synchronous breeding outstrips predator’s ability to limit wildebeest recruitment.

Calves are most vulnerable when they are very young but they reach a certain age when they become equally vulnerable as the other wildebeest. There is a limit to how many calves predators can take per day, so by all having their babies at the same time, more calves have the chance to live past the age where they are vulnerable. Topi and hartebeest do not have as synchronized breeding as wildebeest.

Zebra on the extra nutritious short grass plains.

As we mentioned before, wildebeest are ruminants. They spend about 8hrs a day grazing so they have 16hrs a day to look for predators. Zebra on the other hand, spend 15hrs a day grazing so they only have 9hrs to look for predators. This is because they are hind-gut fermentators. This is obviously simplified.

Now, we’ve established the benefit of synchronized breeding but there are other advantages to being a wildebeest. Serengeti’s short grass plains are the best place for the females to get the nutrients they need to lactate, but they are also a great place to spot predators, which also helps to reduce the number of calves killed before they are out of the vulnerable stage.

Finally, calves are born precocial with a very strong imprinting instinct. The mother and calf learn to recognize each other immediately by smell and the calf stands as soon as it can and then stays as close to its mother as possible. The calf then also tends to run on the hidden side of the female so that predators have a harder time seeing them. The effect= reducing predation.

Wildebeest calve's coats change color to look like their
mothers at 2 months. Predation drops drastically.

There are other minor influences and for more details download the paper, but to try to sum it up in a sentence: The Serengeti’s unique climate and soils provide the perfect conditions to allow wildebeest to live in such large migratory herds because of wildebeest’s unique biology.

Grasslands

They're the thing you probably think of first when you start thinking of savannah habitats, so probably a good place for me to start too. As we've already said, savannah is defined by grasses and all the grazing animals (by definition) depend on grass to a greater or lesser degree, so there must be something interesting to be said about grasses. Surely?! Not one of our guides in training had ever been brave enough to tell their clients about grass though, so certainly room for improvement.

Only ecologists could wander around Serengeti looking for grasses, surely?!

So, what is there to say? Well, it's probably worth first wondering why grass is so popular with so many animals, and how it survives being eaten all the time? The answer to the first part is probably as simple as there being a lot of grass out there, so clearly a niche waiting to be filled. The second is, perhaps, more interesting, even though we all know it anyway: the growing tip of grass is right at soil level, so you can eath the tips without damaging the growing point - not like most other plants at all. This is clearly a great evolutionary advantage for grass - the first grass that grew from the bottom must have had a much easier life than the other plants around. But it also raises the interesting prospect that, once evolved, the plant may actually harness grazers to keep the environment suitable for itself. The main competition grasses face on the savannah is from trees for water and (probably to a lesser degree) light. If grazers munching on grass encourage those same grazers to occassionally nibble (and thereby seriously damage) the seedling acacia nearby without suffering any serious cost themselves, they've effectively harnessed the animals to keeping the environment suitable for grasses. Neat trick for a bit of grass, huh?! Further more, we think there's a chance grasses may have also evolved flamibility for the same reason - by encouraging fire in the ecosystem (and grasses always carry the fires from place to place in the savannah), they keep seedling trees out of the picture in grasslands. As they suffer little from fire, this is again a neat trick (if it's true - there's some debate still, but the topic is an interesting one all the same!).

And then, despite first apperances, there's grasses and there's grasses. Which is to say, not all grasses are the same. In fact, there are somewhere between 9 and 10,000 grass species, depending on who's counting. And that includes (in only eight grass species) 70% of all crops - maize, wheat, sugar, millet, rice, etc. are all grasses. So there's somthing interesting to begin with!

Given the incredible diversity of grass species out there, it's no surprise that for grazing animals there are some pretty serious differences too, particularly in the dry season. One of the most important ecological divisions of grasses is into the two groups or "Sweet grass" or "Sour grass" - the familiar sweetveld and sourveld for our South African colleagues. This fundamental distinction is based on what the grasses do to the nutrients they've captured during the dry season. During the dry season all grasses stop growing and most above ground parts die, life remaining only in and near the roots. Indeed, for annual grasses, the entire plant dies during the dry season. But plants preparing for the dry season have two options - either they withdraw all the nutrient they can into the roots, leaving a very poor cellulose only dry matter in the dry season, or they don't and the nutrients remain available above the surface during the dry season. Obviously, grazing animals will prefer the latter. In general grass species do one of these or the other, but a few adapt their strategies slightly depending on how nutrient rich the soil is, being less careful about the nutrient that gets left to grazing animals in areas where the soils are richer.

This fundamental distinction between strategies for preparing for the dry season has huge impacts in grazing animals. Indeed, there's some evidence that the (now largely extinct) large mammal migrations of South Africa were primarily driven by animals leaving the sourveld in the dry (winter) season and moving to areas with better dry season grazing options. The same impacts can be seen in many areas of East Africa too.

This is far from the only difference between grasses, however, as even during the wet (growing) season there are considerable differences in the nutritional content of grasses growing in different soils. In particular, grasses growing on the recent volcanic soils associated with the Rift Valley (famously the short-grass plains of southern Serengeti and Ngorongoro, less widely know being the Simanjiro plains north east of Tarangire NP, etc.) are particularly rich in nutients. Indeed, it is largely the search among pregnant and lactating female wildebeest and zebra for grasses rich in Calcium (needed for growing healthy bones in particular)  and Phosphorus (needed for growth in general) that drive the famous migrations, with calving occurring precicely when the animals can use the nutrient rich grasslands in these areas.

Grasses can also be beautiful!

The other main variable that determines how palatable a particular grass might be is, of course, how well defended it is. Grasses use silica, a natural glass, to defend themselves - it's what makes grasses sharp enough to cut you as you walk past some. But not all species are as well defended as others, and (perhaps even more importantly) at different stages of growth the same plant may be better or worse defended. Silica takes time to produce and deposit once growing has begun, so new growth - as well as being rather nutrient rich - is typically rather poorly defended. Which is great for grazers! So you often find that once an area has attracted a number of grazers and they've kept it free of old, well defended course grasses, a grazing lawn develops where, just as in our garden lawns, as soon as the grass gets a bit high (and starts to defend itself), something comes along and chops the top off. In East Africa these grazing lawns ocur comonly on particularly rich grasslands (indeed, the whole of the Serengeti's short-grass plains may be a grazing lawn), but also on a smaller scale around local nutrient sources, like termite mounds. It's incredibly easy to find them at the end of the wet season, where most grass is tall, except at the grazing lawns. And a nice feature to point out, once you start seeing them. Interestingly, in areas with Hippos, grazing lawns are really common by rivers, they're very good at forming them. But in the few places where White Rhino are still relatively abundant they also play a very important roll in opening and maintaining grazing lawns that are then also utilised by many other species - this is a process that is now (sadly) lacking from many parts of Africa, and we can only wonder about the overall impact.

Grazing Lawn in Kruger, May 2011 - note characteristic very short grass among larger patches of tall, less palatable grasses. White Rhinos at work?!

Anyway, that's enough for now. I hope I've persuaded you that there are some interesting things to say, even about grass! And I'm sure there'l be more to come too...