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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.
February marks the start of this great migration,
preceding the long rainy season when wildebeest
spend their time grazing and giving birth to
approximately 500,000 calves within a 2/3-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
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 will
last 30 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 the responsibility of 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 will actively reject all
others. This means, the calves that become separated
from their mothers, are doomed to starvation or
predation.
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 their prime
condition. Wildebeest bulls become territorial at
4/5 years (Estes 1969), when during the rut they
will undergo dramatic behavioral changes, becoming
the noisiest (C.t. mearnsi) 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. The gestation period 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
Masai Mara. This final push north results in a
massing along the banks of the swollen Mara River,
producing one of the world's truly most spectacular
sights.
After four long months trekking,
these animals are in desperate need of water to
drink and grass to graze. Many of the young find it
exceptionally difficult to come this far, sometimes
going for days without water. Many die from
exhaustion and thirst before they reach the Mara
River, but also many survive, just, only to find one
last huge obstacle in their way.
Depending on the recent rainfall
sometimes the chosen crossing place is shallow,
which allows the majority of the herds to 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 or the
buffeting from other wildebeest as they swim.
Hundreds of animals can die during the crossing,
littering the waters edge with corpses. These will
be dispatched by the numerous, hungry, and eagerly
waiting crocodiles. These Nile crocodiles are some
of the largest in Africa, measuring up to 15 feet in
length. Lion and hyena will also take the bodies
from the waters edge if they can pull them out on to
land.
When so many animals are massed at
rivers and waterholes, stampedes are common, causing
cows and calves to inevitably become separated. It
is possible for a calf to cross and re-cross the
river 2/3 times during the frantic search for its
mother. This is when the vulnerable animals can be
taken by crocodiles as they panic and become
exhausted. Luckily for the calves, most separations
are only temporary and when they are re-united the
calf will immediately dive for the udder.
Between July and October the wildebeest reside in
the Masai Mara, grazing the lush green grasses that
the earlier rains encouraged.
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. There are
just so many prey animals around that the predators
have to taken advantage to feed their own young, a
time of plenty for all. October to November is when
the short rains begin to fall in the south and east
Serengeti, so the herds start to leave the Masai
Mara, crossing the Mara River yet again, bringing
all the same perils as before. They head slowly back
to the Serengeti's eastern plains. By the time they
arrive it will be February 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 250,000 wildebeest die during
this 1,800-mile journey, the sick, the lame, old and
very young, but the next calving will produce around
500,000 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).
WHY THE MIGRATION
OCCURS
The entirety of the migration is
dependant upon the coming of the rains.
In Africa's hot and dry climate,
such huge numbers of animal life cannot be sustained
by one place. They have such a dramatic effect on
the grazing pastures, taking no time at all to
completely exhaust the available food supply.
As the rains come at different times
in the Serengeti and the Masai Mara, so the animals
are driven to search for the rain-ripened grasses
and water to drink.
Wildebeest need to drink every day
or at least every other day. So, this clockwise
constant search for favorable conditions pushes the
herds onwards, month by month in order to feed
themselves and their young.
There are a few sedentary populations of wildebeest,
but most are migratory. Even the process of birthing
has evolved to become unlike any other land mammals.
Wildebeest are the only land animal
to synchronize birthing to within a 2/3-week period.
At a time and place, when there is plenty of grass
and water to drink. Open plains away from the
majority of predators. The rains herald the birth of
many animals' young in Africa; such is their
dependence on water.
If the majority of young are born
around the same time and are able within a few
minutes to keep up with their mothers and the
constantly moving herds, they are more likely to
avoid predation.
The larger the herd, the smaller is
any individual members probability of being targeted
for attack by a predator. This is known as the
"dilution effect".
Note: All pictures © by Siggi
Hosenfeld
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