Each year around the same time the ‘Great Wildebeest Migration’ begins in the Ngorongoro area of the southern Serengeti of Tanzania. A natural phenomenon determined by the availability of grazing. It is January to March when the calving season begins. A time when there is plenty of rain ripened grass available for the 500,000 zebra that precede 1.8 million wildebeest and the following 100,000 plains game.
In January to February preceding the long rainy season when wildebeest spend their time grazing and giving birth to approximately 400,000 calves within a 3-4 week period, which starts abruptly and is remarkably synchronized. Few calves are born ahead of time, the few that are as much as 6 months out of phase, hardly any will survive. (Estes 1992) The main reason for this being that very young calves are more noticeable to predators when mixed with older darker coloured calves from earlier in the previous year, and so are easier prey.
The calving grounds of the eastern Serengeti happen to be outside the hunting territories of most of the predators, such as hyena, cheetah, hunting dogs and lions although some losses to these predators can occur.
Wildebeest cows do not seek isolation during calving or afterwards. In migratory populations the cows will congregate in there hundreds on the calving grounds. Wildebeest society is much more structured at this time. Groupings of pregnant cows, cows that have calved, groups of yearlings recently separated from their mothers and bachelor herds, which are usually excluded by the territorial bulls from the calving grounds.
Expectant cows gather and drop their calves before midday (very few are born after midday). Labor lasts 10 minutes to one hour and may be interrupted at any time, should the cow be disturbed. When the calves head and trunk emerge, gravity will complete the process as long as the cow is standing. Giving birth usually occurs whilst the cow is lying on her side and can have visible contractions.
Once the calf is born the mother will lick her new-born and within around 10 minutes the calf will be on it’s feet, seeking it’s mothers udder. The mother may move away at this point which will encourage the calf to follow closely. Mothers and calves then group into nursery herds. Mix-ups and lost young can be a problem at first in large groups, as the instinct to follow means the calf will approach anything that moves, including predators.
It is important for the mother to stay with her calf for the first day or two, for the calf to be imprinted on her. The imprint process starts with the first successful suckling. Initially each mother will recognize her own calf by scent and sound they will actively reject all others. This means, the calves that become separated from their mothers, are doomed to starvation or predation.
Late March, April and May is when the herds begin to sweep west and north towards the long grass plains and woodlands of the Serengeti’s western corridor. Here the herd divides when some swing further west than others who head northwest. They will meet up again in the Masai Mara of Kenya. The long rains have started and the southern grass plains they leave behind are depleted of all food, so the herds must press on.
By the end of May the rains peter out and the rutting season begins, a time when males are in prime condition. Wildebeest bulls become territorial at 4/5 years old (Estes 1969), when during the rut they will undergo dramatic behavioral changes, becoming the noisiest and most active of all African antelopes. Territories, mostly small can be fought, won and lost but usually only held for a matter of hours while the migrating herd is passing. Territories of sedentary populations are much larger and usually are held for longer.
The rut will occur when the animals are in prime condition so as to ensure an adult conception rate of more than 95%. If female yearlings are well nourished, first conception may occur at 16 months, but more usually they only conceive one year later, gestation being from 8 to 8.5 months.
Competition to gather and hold as many females as possible is great; on average one bull will hold in the region of 16 females as long as they are within his territory. During this peak of male activity the bulls will neither eat nor rest as they indulge in constant fights with neighboring males.
June to July is a transitional period between the rains and the dry season. As they concentrate on the few remaining green patches of savannah, these huge herds reform and push further north towards the Mara River. This final push north results in a massing along the banks of the swollen River, producing one of the world’s truly most spectacular sights.
Over several months the herds will mill around crossing from one bank to the other the grass always greener on the other side. Heavy lake basin storms fall locally nurturing new growth of nitrogen rich grasses. A dramatic time as herds approach the rivers. Swarming along the banks to determine the optimum crossing points a majority of the herds cross safely.
But often the river is deep and fast flowing, with steep banks either side. Many of the weaker animals are not able to cope with the strong river currents and are dispatched by the numerous crocodile. These Nile crocodiles are some of the largest in Africa, measuring up to 15 feet in length.
When so many animals are massed at rivers and waterholes, stampedes are common, causing cows and calves to inevitably become separated. It is common for a calf to cross and re-cross the river 2/3 times during the frantic search for its mother.
Between July and October the wildebeest reside in the Northern Serengeti and the Masai Mara. The mass of over 2 million animals cover the savannah and grasslands as far as the eye can see. Predation by leopard, lion and hyena at this time is great simply due to numbers. October to November is when the short rains begin to fall in the south and east Serengeti, so the herds start to leave the Mara area. Through November and December they head slowly back to the southern Serengeti’s plains. By the time they arrive at Lake Ndutu it will be Late December early January and the cycle is complete. Arrival at the calving grounds marks the end of this, and the start of the next year’s migration.
In all 3-500,000 wildebeest die during this 800 kilometre round journey, the sick, the lame, old and very young, but the next calving will produce around as many new calves who must take their chances along with the adults on the following ‘Great Migration’. (Being a natural event the timings of this migration may vary from year to year).