The Serengeti-Mara Great Migration

The holy grail of African safaris is the great wildebeest migration that takes place each year in the Serengeti-Mara ecosystem of Tanzania and Kenya. This post describes some of what we learned while researching and what we experienced while on safari in the Serengeti (see our post) and the Mara (see our post).

During severe droughts in the Serengeti, there is so much death that vultures forget how to fly.

Maasai Elder


The Serengeti-Mara Ecosystem

The Maasai Mara National Reserve and the Serengeti National Park are part of the same continuous ecosystem that also includes the the Ngorongoro area and the Grumeti (Singita) reserves in the west. The ecosystem also includes Game Reserves where licensed trophy hunting is permitted including Maswa, Ikoma, Kijereshi, and Loliondo, and others. The ecosystem has 3 seasons: the dry season (June to October), a short-rains season around November and December, and the long-rains, around April and May.

Migration is an adaptation to seasonally changing food resources — if the wildebeest had an endless supply of grass and water in the south, they would not migrate. The grass disappears during the dry season.

For safari goers, there are 3 main habitats where the wildebeest mega-herds are to be found (see map for reference):

The 3 main migratory habitats in the Serengeti-Mara

1 Northern Serengeti (and Maasai Mara): this area has some tall-grass savanna, rolling hills and forests in the Mara’s highlands. The northern landscape has some winding seasonal watercourses (korongos) and picturesque mounds of weathered granite (kopjes) that predators use to survey and ambush prey. The northern area usually receives more rainfall than the southern plains and the Mara River is usually permanent which support the herds through the rest of the dry season (although the Mara River did dry up in 2009). The migrating herds move into the northern Serengeti anywhere from June to September, depending on the year, and usually stay until mid-November.

2 Western Corridor (and central Serengeti): this area is west of Seronera, and is a relatively remote part of the Serengeti, that lies between the Grumeti and the Mbalageti rivers. It has tall-grass savanna and woodlands, some forests along the Grumeti river and more kopjes. This area harbors a small resident population of wildebeest of about 25,000. Once the southern plains dry out in May or June, some northbound herds opt for the high rainfall areas of the western corridor before continuing to the northern Serengeti.

3 Southern Serengeti Plains: This area is diverse with vast treeless short-grass plains in the south east, some marshes and lakes, although the lakes become too salty to drink during dry season. Migrating wildebeest move onto the southern plains following the onset of the short rain season, and remain until the food supply is depleted. Within these plains they move around to where grazing is best while avoiding waterlogged areas. The fertile soil is nutrient rich from the ash left behind by the ancient volcanoes, an important element for lactating wildebeest and their calves. Calving normally occurs between January and March. There is virtually no water source other than rain here, so when the rain stops and dry season begins, the fast drying plains force the herds to move elsewhere.

Note: there’s a much smaller migration where wildebeest migrate north from the Mara river to the Loita plains, but that’s not on the scale of megaherds in the Serengeti.

On the Nasa satellite map above, the overlayed dotted white line represents the approximate outline of the ecosystem, while the solid white line delineates the protected areas inside protected parks and areas. The image was taken in early February, between the short-rains and long-rains, and the green shading represents vegetation (grassy plains, savanna, woodlands, etc). See our post here for why the weather is so important to the timing of the migration.


The Mara and Grumeti rivers

In addition to the 3 primary migrating habitats above, there are 2 important rivers which impact the migratory movements:

The Mara river is important for safari planning because it is the only perennial river in the ecosystem and serves as a magnet and life support for resident wildlife and migrating herds from the south. It also is home to a population of about 4,000 hippopotamus.

The Grumeti River, which flows from the wooded savanna of the central and northern hills and empties into Lake Victoria. In a typical dry season (non-drought) the Grumeti is reduced to a series of very shallow muddy pools, but in a drought year the Mara river is the only viable water source.

TRIP TIP: The water flow of the Grumeti river declines to zero within a few weeks after the end of rains, while water flow in the Mbalageti declines even faster, therefore both the number of river crossings and the associated drama are greatly reduced if your lodge is in this area after the rains stop.


The Mara Triangle and Lamai Triangle (Northern Serengeti)

There are two triangular wedges of land that mirror each other across the border (see map below). These are both excellent areas for migration-related safaris. If there was no international border, it would just be one area, with the Mara river bordering on the east and the park’s edge (defined by the Oloololo Escarpment) on the west.

The Mara Triangle is in the Maasai Mara on the west side of the Mara River. It has wildlife all year-round, including the Big 5. Many migrating herds (but not all of them) enter Kenya within the Triangle. The 3 sides of the triangle includes the border with Tanzania to the south, the Mara River to the east and to the west is a 400m high escarpment (called Oldoinyio, or Oloololo or Siria Escarpment).

The Lamai wedge is on the Tanzanian side, immediately south of the Mara Triangle. The Lamai triangle has freshwater and green grasslands that draw as many wildebeest and zebra during the dry season as the Mara Triangle from July to November.

Both of these areas offer some of the best wildlife viewing in the entire ecosystem, because the migrating herds are drawn to the Mara river during dry season, criss-crossing it for months.


Migration dynamics

Not all species migrate in the Serengeti. A species that defends territory from intruders from its own species is not considered migrant. The migrating species includes wildebeest, zebra, gazelles and eland. Some of these species, especially gazelles become resident. There are also some resident wildebeest populations in Ngorongoro Crater and in some parts of the north and western Serengeti where there is enough water to support them.

Resident species on the Serengeti includes impala, topi, giraffe, buffalo, hippo, warthog, and most of the predators. They may move temporarily to another area but usually return to a particular home range.

Some species, like wild dogs, elephants, and cheetahs are neither resident nor migrant. A recent census of the ecosystem sponsored by Paul Allen revealed about 6,000 elephants, with highest densities in the west and north, and about 55,000 buffalo, with densities evenly distributed, but slightly higher in the south.

The star of the migration is the wildebeest (gnu) because there are 1.5 million of them and they can aggregate in groups of 50 individuals or mega herds of 400,000. The word wildebeest is Afrikaans, and means ‘wild cattle’, although they are actually from the antelope family. They travel up to 40 km in a single day.

The migration has many co-stars, including the zebra, with its much smaller population of about 200,000. The wildebeest and zebras often travel together especially when they are in wide open areas. The zebras’ digestion system is different than the wildebeest, the latter feeding on the shorter parts of the grass after the zebras have cropped the grass short. (click here to see our post on the Botswana zebra migration).


Why are there more Wildebeest than Zebras?

Wildebeest usually don’t have calves in isolation; instead they prefer to form small groups (crèches) with other pregnant females. Calves born in these crèches are more easily defended against predators by the group of females. The bigger the wildebeest herd, the better the survival rate.

Wildebeest calves are precocious, and can stand within about 5 minutes of being born. They can walk within the first 30 minutes of birth, always staying within a few feet of their mothers in the herd. Predators can’t easily isolate and kill individual wildebeest calves.

Wildebeest births are synchronized over a 6 week period, with the majority of births occurring at the beginning of the wet season (mid January to early March).

The high number of calves overwhelms the predators –they can only eat so much in that short a period.

Zebras do not calve communally, but prefer to give birth in isolation, hiding their calves for many days before re-joining the herd. The unattended calves and foals are at increased vulnerability to predators.

Zebra foals 15 minutes after birth, and can walk 60 minutes later.

Scientific studies (see sources below) have concluded that the population of Serengeti zebras is suppresed primarily by high foal mortality, either by direct predation, or by malnutrition due to death of mother.

Zebra births are not synchronized, because gestation period is longer than a year, and mares therefore cannot foal in the same season each year.

A low number of foals are born throughout the year and available to predators for more of the year.

50% of zebra foals are killed within first year.


Serengeti Predators

The two most numerous predators in the ecosystem are hyenas, estimated at about 9,000, followed by lions at about 3,000. The least numerous are cheetahs at about 250, because the mortality rate of cubs is up to 95%, which is why many female cheetahs are unsuccessful at raising a single cub to maturity.

Lions on the Serengeti plains depend on the resident prey populations when the migrating herds are elsewhere. They don’t migrate long distance following the wildebeest because their range is limited by nurturing their underdeveloped cubs and their efforts to maintain their territories. Lion prides have the highest reproductive success when their territories are in areas where 2 rivers join (confluence) because they have water, shade trees, and ambush spots.

Hyenas are more likley to track migration than lions for 2 reasons: (1) they can lope distances of up to 40 kms daily, which the lions can’t do; (2) their social system allows ‘commuters’ to pass through another hyena’s territory.

Crocodile attacks at river crossings are a big tourist attraction. The crocs of the Mara River are known to be some of the biggest Nile Crocodiles in Africa. In the off-season crocodiles can control their metabolism to a state of near-hibernation and only need to feed once or twice a year.


Why do the wildebeest migrate?

The massive wildebeest herds migrate from an area after they’ve eaten all the grass in that area or when water sources runs dry. If those herds never ran out of water or nutrient rich food in their favourite area (the short-grass plains in the south), they would not migrate. They don’t walk across the country for the scenery. They generally migrate in the direction of the rains because rains provide fresh green grasses and drinking water in marshes, creeks and rivers. But why don’t the wildebeest migrate straight north, instead of taking a detour into the western corridor. And how do they decide?

Scientists suggest wildebeest may be responding to a combination of 5 triggers: (1) the so-called green-wave hypothesis (herds surf from one green vegetation patch to another); (2) proximal environmental cues such as rain storms; (3) perceived predation risk; (4) memory.

Some recent research suggests that nutrient-rich grass is a bigger magnet for wildebeest migration than the smell of rain. The short grasses in the southern plains are available only for a few months of the year, but they are of high nutritional value when available and required for the lactating female wildebeest. In the western corridor, the long-grass in the savanna dries more slowly and tends to retain crude protein well into the dry season, which may explain why the wildebeest detour into the western corridor instead of a straight linear northward movement as predicted by rainfall.


Where and when do the wildebeest migrate?

The graphic below was plotted from monthly aerial surveys and overlaid with labels for the seasons and peak calving. The grey dots represent the migration pattern of wildebeest over 3 years, from 1969 to 1972. The pattern will change from year to year, and especially in very dry or very wet years.

As can be seen in the graphic above, the wildebeest spend the wet season (November to May) in the southern Serengeti plains, and the dry season in the northern Serengeti (July to October). In between, they’re transitioning between those two areas. If rains are delayed, the herds may temporarily move into the Western Corridor on their way south.


Sources

  1. Annual mass drownings of the Serengeti wildebeest migration influence nutrient cycling and storage in the Mara River (2017) …..
  2. Attribution of extreme weather events in Africa: a preliminary exploration of the science and policy implications (2015) …..
  3. Cross-boundary human impacts compromise the Serengeti-Mara ecosystem (2019)…..
  4. Long-term ecological changes influence herbivore diversity and abundance inside a protected area in the Mara-Serengeti ecosystem (2019)…..
  5. Opposing Rainfall and Plant Nutritional Gradients Best Explain the Wildebeest Migration in the Serengeti (2006) …..
  6. Population regulation of African buffalo in the Mara–Serengeti ecosystem (2015)…..
  7. Rainfall trends and variation in the MaasaiMara ecosystem and their implications for animal population and biodiversity dynamics (2018)…..
  8. Serengeti wildebeest population dynamics: regulation, limitation and implications for harvesting (Ph.D. Thesis, 1996) …..
  9. The Social Organisation and Population Ecology of the Plains Zebra (1969) …..
  10. Spatiotemporal aspects of coexsitence in savanna carnivores (Ph.D. Thesis, 2014) …..
  11. Spatial distribution of Serengeti wildebeest in relation to resources (1999) …..
  12. Unesco report on the Serengeti National Park: 42 COM 7B.96 (2018) …..
  13. What limits the Serengeti zebra population? (2004) …..