Wildebeest (Connochaetes taurinus)
A Keystone Species in the Serengeti Ecosystem
By Richard Estes, Ph.D.
Why are there so many wildebeest? What makes this antelope the keystone species of the Serengeti ecosystem?
The short answer is that the wildebeest (Connochaetes taurinus) is specialized for precisely the climatic and ecological conditions that distinguish the Serengeti arid savanna: extensive short grasslands
that are highly productive during the rainy season, and taller grasslands in the woodland zone in the west and northern part of the ecosystem where wildebeest can seek green pastures and water during the long dry season.
Wildebeest concentrate by the thousands and even hundreds of thousands on the Serengeti grasslands because the grasses they feed on cover the ground and enable the animals to feed close together. This is particularly true of the
short-grass plains, where over 90 percent of the ground is carpeted by herbage. These are the pastures the wildebeest prefers, which it is better equipped to crop and process than other associated grazers, having flexible lips,
a broad muzzle, and wide row of incisor teeth that enable it to take big bites of short grass.
But wildebeest can only utilize the short-grass plains as long as rains keep the grass growing and replenish pans and pools with drinking water. Under those conditions, countless dung beetles bury the dung pellets of the wildebeest
hordes with great efficiency, thereby fertilizing the wildebeest pastures and promoting nutritious re-growth that the wildebeest can crop over and over again.
After a week without rain, the short grasslands begin to lose their green and the wildebeest hordes begin leaving, threading their way along countless game trails in files sometimes 10 or even 20 km long, heading westward for medium
and long grasslands that are still actively growing. As soon as rain falls again on the short-grass plains, they return.
But typically in mid-May the long rains end, and now the wildebeest keep going. Less specifically directional than the migration to the short-grass plains when the short rains begin, wildebeest divide into armies that follow diverse routes to the southwest (Maswa Game Reserve), west along the Mbalageti River into the western corridor, and northward into the Northern Extension. Eventually, practically the whole population ends up in the northwest part of their range, where local storms induced by Lake Victoria produce flushes of new grass on ground left bare by wildfires in the long grasslands. The Kenya Masai Mara National Reserve is the northernmost reach of the wildebeest migration -- except in years of exceptional drought, when wildebeest formerly ascended the Isiria Escarpment into montane grasslands to the west and north of the Masai Mara (now largely cut off by fenced wheat farms).
The migration into the woodland zone at the end of the long rains coincides with the annual rut. In the space of one month, beginning typically in early to late June, possibly triggered by the full moon, some 80 percent of the half-million wildebeest cows are impregnated. Perhaps half the matings occur during a two-week peak. It is a glorious, tumultuous spectacle, made loud by the deep bullfrog calls of the bulls, which compete furiously to round up and hold as many cows as they possibly can on closely packed, temporary individual territories. Fighting, chasing, and herding make up 99 percent of the action; actual mating is quite uncommonly seen. However, once a bull detects a female in heat, he will copulate with her many dozens of times, until she either leaves or goes out of estrus, which lasts one day at most.
The wildebeest reproductive system is the second and most decisive advantage this antelope has over associated herbivores. It is the only antelope (together with the black wildebeest of South Africa) that has "follower" (mobile, precocious) young. All the rest have "hider" young, which remain concealed for periods ranging from a few days to over a month, during which they are easy prey for predators the size of jackals (small and medium antelopes) and any large predators lucky enough to find them. Wildebeest calves have lost the instinct to hide, and mothers perform none of the elaborate maternal behaviors that support the hider strategy.
How did it happen that the wildebeest alone broke with the ancestral hider strategy, and what makes this such an important advantage? Clearly it is linked with migratory habits and with the aggregation of thousands of animals on short grasslands. Such aggregations are perforce highly mobile. Natural -- i. e. predator selection -- would favor shortening and ultimately eliminating the hiding phase, both because mothers with young unable to travel would be left behind and because concealment of new calves on short green grass would be difficult.
Wildebeest calves have tan coats for the first two months, which are similar to the color of most savanna antelopes and arguably as well adapted to concealment. But since they don't hide, the natal (doubtless ancestral) color creates a problem. Their tan livery contrasts with the dark gray or brown coats and reverse counter-shading (darker below and lighter above) that wildebeest have evolved in the course of becoming highly gregarious, making young calves stand out in a crowd of adults.
The solution to the problem was to produce the annual calf crop in the shortest possible time. Hence the short rut. Eight months later 80 percent or more of the cows drop their calves, preferably on the short-grass plains, within a month and with a two-week peak in which perhaps half of all the calves are dropped, mirroring the rut. The most obvious benefit is that the predator market is glutted. Less obvious is that older calves serve as cover for newborn calves -- comparable to the way high grass conceals hiding antelopes. Within two days, wildebeest calves can keep up with their mothers, running beside them, so well that the effort of catching one -- and overcoming the mother's fierce defense -- might better go toward getting a more substantial meal.
The abbreviation of the feeble stage makes the wildebeest the most precocious of all antelopes and possibly of all hoofed mammals. This precocity is displayed from the moment the calf is born. It begins struggling to rise within a minute, can gain its feet and run within as little as three minutes; the average elapsed time is only seven minutes! The average time for the related hartebeest and topi, and other antelopes, is 20 minutes.
The final piece of the jigsaw puzzle that comprises the wildebeest's unusual reproductive system is the tendency of animals of the same age, sex, and reproductive status to associate with one another. This tendency is most pronounced during the calving season. Apart from the association of males of similar age, yearling females with one another if not with their mothers, pregnant females group together and non-pregnant females are mostly found in herds of bachelor males. Most important is the association of females with calves. Every morning pregnant females foregather on calving grounds (an area of short grass, wide open and preferably on a slope), and proceed to drop their calves. As soon as the calves are mobile, the mothers lead them into the nearest nursery herd, where the neonates can disappear in a group of calves that have survived the feeble stage. The more calves in the group the better the chances of surviving the first 24 hours, when calves are most vulnerable to both diurnal predators (cheetah, wild dog) and nocturnal ones (hyenas, lions). The survival rate of wildebeest calves up to one month in a normal year runs between 70 and 80 percent, which is about 20 percent higher than the rate of associated herbivores.
To complete this brief explanation of what makes the wildebeest a keystone species in the Serengeti ecosystem, it must be noted that this antelope actively creates and maintains its preferred open grassland habitat. In addition to fertilizing the soil and the destruction of seedling trees by the trampling of many hooves, wildebeest bulls habitually horn young trees as an expression of aggression, thereby inhibiting the regeneration of woodland. Together with wildfires and elephants, horning of trees by wildebeest bulls transformed most of the Serengeti woodlands into tree savanna and grassland during the 1970s and 80s, coinciding with the increase of the population from a quarter-million to over one million animals after being freed of the cattle-borne disease, rinderpest, in 1964.
Thus large populations of wildebeest, like that of the Serengeti, may have greater environment impact than any other African mammal apart from the elephant.