The Gnu's World: Serengeti Wildebeest Ecology and Life History

11. The Calving Season: Birth and Survival in Small Herds and on Calving Grounds

239 Eight months after the rut, the cows that became pregnant during the three-week peak produce their calves. Assuming a 95 percent pregnancy rate and that 80 percent of the cows bred during the rut, over 400,000 calves would be expected in a population of 1.3 million wildebeest. Given a carnivore population of about 3,000 spotted hyenas and 2,000 lions, plus a few hundred cheetahs, leopards, and wild dogs—the only predators that actively hunt wildebeest calves—it is obvious that they can only consume a small percentage of the calf crop. Glutting their predators is rightly considered the primary advantage of having as restricted a breeding season in the tropics as ungulates in the North Temperate Zone with a short growing season. This extraordinary shortening of the birth season has evolved because wildebeest calves do not hide, yet retain the tan color inherited from hider ancestors that is conspicuous against the gnu’s dark uniform. To understand how the two wildebeests came to be the only antelopes to entirely abandon the ancestral hider-calf strategy, first the waypoints in the transition from hider to follower calves have to be considered . As discussed in chapter 3, the fact that hartebeest calves depend on concealment like all other antelopes indicates that the transformation to a follower system happened after the Alcelaphini became a distinct tribe in the late Pliocene a few million years ago. Stages in the transition can be seen in the topi’s genus (Damaliscus), in which the blesbok has follower calves but retains elements of the hider system, notably calves that Chapter 11 The Calving Season Birth and Survival in Small Herds and on Calving Grounds 240 | Chapter 11 are somewhat slower to become mobile and do not follow as closely as wildebeest calves (see chapter 3). It is interesting to note that all alcelaphine calves retain the ancestral tan color and, in fact, look remarkably alike considering the pronounced contrast between adults. The wildebeest’s habit of forming large, mobile aggregations favors shortening the hider stage, as mothers guarding hidden young can be left behind when an aggregation changes location. The difficulty is compounded if the population is migratory. The herding instinct is so strong that I have known wildebeest to abandon calves that were unable to accompany them when their aggregation moved away. The absence of cover for calf concealment would be another major problem for wildebeest . Mothers of hider species seek isolation and need cover to conceal newborns. Migratory wildebeest congregate on the shortest, greenest pastures during the calving season, where the locations of hiding tan calves would be pinpointed. Natural selection favoring survival of the most precocious calves led to an ever shorter hiding stage, culminating in offspring capable of following their mothers within an hour of birth. The eight-month gestation, adapted to peak growing seasons at both ends, meant that most calving would occur while the migration was aggregated on short green pastures. Further shortening of the birth season to the present three-week peak could have come about because survival of calves born during the peak would be higher than those born before or after the peak. As the following account of the calving season shows, the explanation is simple: the presence of calves in their most vulnerable newborn stage is obscured in nursery herds containing older calves; the more, the better. In other words, calves serve as substitutes for the vegetation that conceals hider calves. From this viewpoint, the wildebeest’s follower-calf system can be understood as a consequence of its adaptation to an ecological niche entailing migration between distant wet-season and dry-season pastures in arid and semiarid African savannas. As research has shown that survival rates in small herds are much lower (discussed below), resident populations should be considered offshoots of migratory populations rather than the other way around. The association of wildebeest of the same sex, age, and reproductive status, described in chapter 8, is of greatest reproductive significance for the species in the gathering of pregnant females on calving grounds during the birth peak. Instead of seeking isolation like hider species, cows of follower species seek one another’s company. The tendency of pregnant cows to associate and to remain in company during parturition The Calving Season | 241 leads to the establishment of calving grounds (fig. 11.1). These are not specific, traditional locations, although migratory wildebeest...