Trophic Level II - The Herbivores Wildebeest Page 31

  Connochaetes taurinus

Length: 170-240 cm (67-94")

Height: 115-145 cm (45-57")

Tail: 60-100 cm (24-39")

Weight: 140- 260 kg (308-573 lb.) female, 165-290 kg (364-639 lb.) male

Status:  Common; most of Africa’s 1.7 to 2 million wildebeest live in the Serengeti-Mara ecosystem

Habitat: Wildebeest are found in open bushlands and on grassy plains across the eastern third of Africa, south of the Sahara desert. The preferred habitat is short grasslands. They select ranges which are no more than 20 kilometers (12 miles) from water. They drink once a day when possible. Wildebeest are herd animals but the size of the herd depends on the quality of their range. The largest herds are found in the Serengeti-Mara ecosystem.

Food: Wildebeest have wide mouths which allow them to take large mouthfuls of grass. They do not feed selectively on grass the way topi or Thomson's gazelles do. They prefer to harvest short green grass. The short grass plains are located primarily in the Ngorongoro Conservation Area where it meets the south-eastern boundary of Serengeti National Park. These plains are an ideal environment for this species to feed. The alkaline and volcanic soils here create conditions that allow the preferred types of grass to grow while preventing the deep-rooted (and less desirable) grass species from taking hold.

One and a half million wildebeest feed here during the wet season. They trample the hard-packed volcanic soil, breaking it up and creating enough loose soil for the short grasses to take root. Their tonnes of droppings enrich the soil. It is estimated that each day the herd produces 1 800 kilograms (4,000 pounds) of droppings! Their urine returns needed nitrogen to the land.

The wildebeest herd defines this ecosystem. It is their travels over this range that sets its borders. They effect all other creatures living here. In sheer numbers, they overwhelm the other species. There are between 200,000 and 250,000 zebra, 500,000 Thomson's gazelles, 60,000 buffalo, 8,000 elephants and perhaps another 100,000 other antelope on these plains. In total, these other species do not equal the wildebeest population.

Grass and Gnu

The success of the herd here is due to the availability of so much suitable grass. In the 1960s, the herd numbered 350,000 animals. It topped 1,000,000 in the late 1960s and early 1970s. By 1975, it reached its present size of 1.3 to 1.7 million animals. It has stayed between these two figures since then, dropping to the lower figure in times of drought and reaching the higher shortly after calving season in good years.

The decline of the herd around 1900 was caused by the introduction of rinderpest. This disease was carried by European and African cattle and reached East Africa near the turn of the century. It killed hundreds of thousands of ruminants. Especially hard hit were buffalo and wildebeest. Zebra, because they are a non-ruminant, were not effected by the disease.

It is interesting to note that the zebra population has remained stable since accurate scientific records were begun in the late 1950s. While the wildebeest population rebounded due to efforts to eradicate rinderpest and the re-emergence of the grassland (see Fire) the zebra population remained between 200,000 and 250,000 animals. The Serengeti-Mara ecosystem had reached its carrying capacity for them. The types of grass that grew well in the "fire, elephant and seasonal rainfall" ecosystem that favored wildebeest did not benefit the zebras. In other words, there was no increase in the types of grasses the zebras eat. There was a considerable increase in the grasses that wildebeest eat.

It now appears that the wildebeest population has reached its carrying capacity and that the population is as large as it is going to get.

Social System: Ngorongoro Crater Non-migratory Herd

Like topi, wildebeest exhibit a number of variations in their social system. In some places, males defend territories year round. This happens when the herd remains within the same general range year round. Ngorongoro wildebeest exhibit this form of behavior. There, within the confines of the crater walls, the females move from place to place following the best grazing.

Males will stake out territories that may be 100 meters or more (328 feet) across during the time when females are not breeding. However, during the peak of breeding activities, the territories in prime locations may be as small as 30 meters (98 feet) across.

Bulls stake out the territory by horning the ground and rubbing their preorbital glands on the soil. They also create dung piles and urinate at the borders. Their best advertisement is their stance. Bulls will stand in the center of their territory in an alert position. This visual sign is understood by other bulls.

Frequent fighting occurs. Wildebeest bulls fight by dropping to their front knees and then locking horns. It is a test of strength and injuries are rare. Social "fighting" is a feature of the bulls’ display. A bull will pay a visit to each territory that neighbors his and then he and the resident bull will engage in a short bout. They spend about 45 minutes a day doing this (see also Ngorongoro: July 8, A Day In The Life of An African Eden). They will also cavort and leap around to show off their stamina to other bulls and cows.

I Love Gnu

When a cow herd enters a bull’s territory, the bull tests them by sniffing their vulva and tasting their urine. He performs a flehem when he comes across a cow that is estrus. He then courts her by following her around until she stands for mounting. If she is not ready to breed, she simply lies down or moves out of the territory.

Moving around the crater, often near or even with the females, are bachelor bulls. These bulls never attempt to breed. It appears that owning a territory releases hormones that stir the breeding urge.

Social System: Serengeti-Mara’s Migratory Herds

In the Serengeti-Mara, there are two distinct herds of wildebeest. One calves to the north of the Mara and migrates into the reserve during the dry season. The other calves in the south of the Serengeti and moves north into the Mara in the dry season. Both are migratory, though the northern population is smaller than the Serengeti herd. There are between 15,000 and 20,000 animals.

Bulls in these herds do not have the option of maintaining a fixed territory. They must set up territories when the herd stops to feed. There is no doubt that they are territorial. Bulls will not follow the cows once they leave the territory. Their behavior mirrors that seen in Ngorongoro except that it is a moveable territory. However, the male’s territories are even more compact than the Ngorongoro’s bulls. There may be as many as 700 breeding bulls within 2.5 square kilometers (1 square mile)! The average width of their territories is about 15 meters (49 feet).

By the height of the dry season, territorial behavior all but vanishes as bulls try to cope with the stresses imposed by poor grass and little water.

The Wildebeest Migration

Wildebeest Migration JANUARY	Wildebeest Migration MARCH	Wildebeest Migration MAY	Wildebeest Migration JULY	Wildebeest Migration SEPTEMBER

The rough pattern of the Serengeti’s wildebeest migration is well understood but no year sees exactly the same pattern. In general though, they follow a route.

December through March, the herd is in the south on the short grass plains. The herd, or more properly herds, are spread out from horizon to horizon. Densities of up to 100,000 animals have been seen in 256 square kilometers (100 square miles). Calving occurs in February or March. About 80% of the cows deliver at that time.

By April, the herds are generally moving west into the southern half of Serengeti National Park. May sees them moving into the western extension. The dry season is beginning.

In June, the herds are moving northerly but are usually well south of the Mara. Sometimes, in late July or August, they cross into Kenya. They stay in the Mara until October and by November they have crossed back into Tanzania.

The pattern is anything but fixed (see African Diary). Sometimes the short rains fail and the herds move further than they normally would. Other times, the long rains last into June or July and the herds may never cross into Kenya.

Behavior: The basic family unit of wildebeest is the cow and her present year’s offspring. Cows give birth in a three week period. Predation on the newborns is highest at the beginning and end of this period. A calving cow is easy prey and so are the calves. Many are taken by spotted hyenas, lions and wild dogs. However, if the herd was to spread their calving season over the year the carnage would be worse. By presenting so many victims they actually overwhelm the predators to the point where they simply can’t kill all of the newborn. The result is that four out of five survive. (In smaller herds, like those in Ngorongoro, half the calves may be lost in the first few days.)

Cows will put up a spirited defense of their babies but the odds are stacked against them, especially if a pack of dogs or a spotted hyena clan attacks. She can’t chase off all of them. She will not attempt to fight off a lioness. Fleeing is the only option. Fortunately, the calf is able to run about seven minutes after birth and within two days it can keep up with the herd.

The calf stays close to its mother’s side. When it is born, she licks it and consumes the birth membrane. The mother regains needed nutrients from the protein-rich birth sack but more importantly she learns to recognize her calf’s distinctive scent. This is called imprinting. Should the calf be separated and the cow be prevented from imprinting on her offspring, she will not allow it to nurse.

She will suckle only her calf. Should mother and calf be separated, the calf is doomed. When they move, the cow moos constantly to her calf. It bleats back. If they should become separated during a stampede or river crossing, she will call for a long time in the hopes her calf will find her. Sometimes the pair get lucky and find each other again.

Orphan calves are all too common a sight. They will be chased off by other cows and there is even a documented case of zebras kicking a calf to death. Why they did this is uncertain. Perhaps it was to stop the plaintive bleating that might attract a predator. In any event, such brutality by one herbivore species directed at another is rare. The calf will most likely be taken by one of the many predators that constantly are alert for such mishaps.

Serengeti wildebeest bring onto the plains over 250,000 calves each year. Most, perhaps as many as 75%, will not survive their first year. Those that do survive will replace adults lost to old age, accidents and predation. At the start of the next calving season, the population will be about what it was the year before. The young and the old are the most common victim of the hunters.

Predators: Spotted hyenas, lions and leopards are the main predators of wildebeest. Wild dogs are capable of bringing down an adult wildebeest but prefer calves and yearlings. Lions will take all age groups but they do show a marked preference for young wildebeest if they are available. Hyenas prey on all ages too. Leopards and cheetah take mainly young calves. An adult is too big.

Crocodiles take numbers of wildebeest when they come to cross rivers or to drink. Such predation is limited to short periods of the year however.

Close-up: Impact on the Ecosystem

Impact on Predators

The wildebeest has a profound impact on the ecology of the Serengeti-Mara ecosystem.

It is often true that predators control or limit the population of their prey species. For example, Ngorongoro Crater has very high numbers of spotted hyenas and lions that prey on the crater's wildebeest and zebra. Studies have shown that they take enough young and old animals to keep the herd numbers fairly constant. When the lions suffered a die-off several years ago, the population of ungulates increased (see Ngorongoro, July 8: A Day In the Life of an African Eden).

However, in the Serengeti-Mara, the opposite is true. The migration of the three main prey species (wildebeest, zebra and Thomson's gazelles) actually limits the population of their predators. Lion prides are especially affected by this since they are more territorial than spotted hyenas and wild dogs. Nomadic lions actually benefit from the migration because they can avoid spending too long in any one area controlled by dominant males.

Hyenas will journey several kilometers away from their den site in order to follow the herds. The adult may cross through other hyena territories but it is usually not bothered or attacked by resident clans. There are times when a mother hyena might be away from her pups for a day or two. This could have a negative effect on the pups’ chance of survival but it is likely minimal.

Wild dogs are confined to a specific location only when they have pups too young to travel with them. This happens for about three months of the year. If they have to travel far to find prey, it could cause hardship for the pups. Today, there are so few wild dogs in the ecosystem that the movement of the wildebeest seems to have little effect. They are more likely to die from distemper and other diseases they contract from the local Maasai dogs.

Leopards are least affected by the migration of wildebeests because they hunt prey species that live in and around the woodlands. Leopards do not migrate. Lions are territorial and stay within their boundaries. Adjacent prides will attack transgressors if they try to cross through their ranges. Male lions attack other males. Lionesses attack other lionesses. Should a pride be forced by starvation to abandon their territory, they may well find it occupied by other lions when they return.

Confined by Territories

Confined to their territories in many parts of the open range, lion prides must turn to smaller, resident prey: warthogs, Grant's gazelles and topi. Woodland lions fare better. They will have buffalo to hunt when the easier prey moves on. Half of the lion cubs born in the Serengeti-Mara starve to death. The lesson here is a simple one. It is not the number of wildebeest that is important. One and a half million wildebeest could support much higher numbers of lions if the herd did not migrate. What is important is the length of time the prey stays in a specific territory. The migration patterns of the great herds in the Serengeti deprive the predators of their food. Many wildebeest actually survive long enough to die of old age in this ecosystem because there are so few predators to cull the weak and the infirm. Indeed, it is not uncommon to come across the body of an animal that died of sickness or accident. Often these carcasses are not accompanied by vultures or other scavengers and show no visible cause of death.

Comparison Chart of Predator Prey Populations: Serengeti/Ngorongoro Ecosystem

The above chart shows the number of predators supported by the prey species in the two ecosystems. Ngorongoro supports far more lions and hyenas than does the Serengeti because the herds in Ngorongoro are resident all year round. If the Serengeti herds supported the same ratio of predators to prey, the population of predators would be approximately 42,500. This figure is theoretical and could never be reached, but it does clearly illustrate the limitations imposed by migrating herds.

Crocodiles and Wildebeest

The Serengeti-Mara is home to some of Africa's largest Nile crocodiles which are up to 6.3 meters (21 feet) long. These crocodiles attain their huge size due to the abundance of seasonal prey that the herds supply. Unlike lions, the cold-blooded (endothermic) reptiles do not have to eat every few days. When the wildebeest come to the Kirawira pools of the Grumeti River, or cross the Mara, they bring a new supply of meat for the hungry jaws.

The crocodiles wait in ambush. Wildebeest wade in and the trap is sprung. Sometimes the crocodiles do not need to do anything. Panicked and anxious to cross the river, many wildebeest drown in these crossings and their corpses provide more food than even the giant reptiles can eat. When the herds move on, the resident crocodiles feed on the occasional fish or bird that falls prey to their jaws. These are mere snacks. The crocodile's metabolism slows and it may even aestivate in a den dug into the river bank. There, some will await the coming of the rains and the return of the herds.

Ecology of a Live Wildebeest

It is clear that the wildebeest herd affects the region's predators. They are a source of food when they arrive and a cause of famine when they leave. But they also affect the other animals around them, whether they are herbivores, beetles or fish. The herds must drink once a day. They make treks to the seasonal ponds, marshes, streams, lakes and rivers. Wildebeest droppings enrich the waters and their hooves trample the mud and sometimes even the fish. Storks fly in to take advantage of fish forced to gulp air at the surface due to the muddy waters left by the herd. Wildebeest follow the zebra herds which eat the coarser grass and leave the more tender shoots behind. Thomson's gazelles will follow the wildebeest and eat the short blades they leave behind. Again, the droppings and urine enrich the soil so that when the rains return it will bloom with green grass again. The trampling hooves of the herds also kill young saplings and help maintain the grassland.

Flies lay their eggs in the wildebeests’ nostrils, and these enter the stomach. Eventually, a new generation emerges from the dung left by the herd. Dung beetles and other insects use the droppings as a source of food. The beetles are, in turn, eaten by dwarf mongooses who may themselves become food for eagles or small cats. The Maasai burn the dead grass to force the wildebeest to move on. The burning isn't so much to get rid of the herds as it is to get rid of the clouds of flies that accompany them. Some of these flies inflict themselves on the Maasai cattle and are unwanted visitors. When the herds come into an area, they take the hunting pressure off of resident animals such as topi, impala and buffalo. When they leave, the predators turn their attention back to the residents.

Ecology of a Dead Wildebeest

A wildebeest is dead. It may have been killed by a predator, died accidentally or succumbed to disease. No matter how it happened, the remains are about to be recycled into the ecosystem. Scavengers (or predators) such as lions, spotted hyenas, vultures and jackals will begin by cleaning up most of the soft tissues. Lions and spotted hyenas might eat much of the meat. Vultures and jackals will clean off the remaining scraps. If the carcass is not discovered by these large animals, smaller ones will clean it up. Flies will lay their eggs and their maggots will consume the body. The maggots will then metamorphose into flies and the flies may feed birds. These’ in turn, will feed larger predators such as birds of prey or servals. The strongest bones will be cracked open by the spotted hyenas if they find the remains. Their stomach juices are so powerful they can even digest teeth! Any bones that remain will be trampled by the herds or eroded by weather. Some might be gnawed upon by rodents like the unstriped groundsquirrel or even warthogs.

A moth (Tinea deperdalla) will lay its eggs in the horns. The grubs (or larva) will eat the horn material (called keratin) and then enter a pupae stage. These hatch as moths and fly off to find another dead wildebeest to repeat the cycle. Possibly, a little bee eater may catch the moth and feed on it. The bee eater, in turn, might fall victim to a pygmy kestrel.  Moss may grow on the skull too and derive nourishment from it. Hair will be picked up by wagtails and other birds and used for nesting material.

In the end, the entire animal will be recycled and its remains returned to the soil. There is nothing left to mark its passing. Then, its molecules begin a new journey. Perhaps they will nourish the grass that feeds the wildebeest that feeds the lion. Perhaps they will follow another route.

(See also: Appendix III: Maasai and Serengeti-Mara Ecosystem and Appendix IV: Comparison of Average Density of Lions, Spotted hyenas & Prey in Ngorongoro and the Serengeti)

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Dave Taylor's African Safari - Book 3: Trophic Level II - The Herbivores (Standard Version)
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