Unlike most ungulates, whose offspring remain concealed for some time after birth, the offspring of the wildebeest and other alcelaphine antelopes (except harte-beest) accompany the mother from the very first hour. Most other ungulates that have follower-young either protect them effectively through a maternal or group defence (e.g. cattle, muskox, equids, and rhinoceroses), or are able to flee to sanc-tuaries (e.g. goats, sheep and goat-antelopes). In the Alcelaphini, primary reliance is placed on flight and on concealment among conspecifics; there is no effective defence against predators and no sanctuary. The spotted hyena is the primary predator on wildebeest calves.
Over 80% of all wildebeest calves are born within an annual 2–3 week calving peak. During three different calving seasons (1 963-65) in Ngorongoro Crater, Tanzania, mortality in small, dispersed herds was found to be far higher than in large aggregations. The evidence suggested that this antelope's unusual reproductive strategy is adapted to the mobile-aggregated distribution pattern characteristic of migratory populations, rather than to the sedentary-dispersed pattern seen in resident populations. Further study on this specific question was undertaken during the 1973 calving season, with particular attention to reproductive success in small herds.
Calf survival in small herds averaged just over 50% at the end of the 1973 birth peak, compared to 84% in aggregations. Daily sampling showed that calving peaked at different times in different parts of the Crater; this resulted in an extended season, although the peak in each population unit was of approximately equal duration. In some areas, calving was virtually finished before the peak began in the aggregations. Mortality ranged from very high among early calves to low in herds whose peak coincided with that of a nearby aggregation. The lack of synchrony between Ngorongoro subpopulations apparently reflects microclimatic differences, there being a pronounced rainfall and vegetation gradient within the caldera despite its small (265 km2) floor area.
Low calf density combined with a lack of synchrony between small herds in different localities makes wildebeest living in the sedentary-dispersed pattern subject different localities makes wildebeest living in the sedentary-dispersed pattern subject to high calf mortality by the spotted hyena; in effect, resident subpopulations that calved ahead of the main population were providing a steady protein windfall for Ngorongoro's 470 hyenas. Tn the large aggregations, there were enough older calves to provide ‘cover’ for neonates during the first crucial day or two, making it much harder for hyenas to single out easily caught individuals. Therefore, lack of synchrony between aggregations did not apparently increase calf mortality to the same extent as it did in small herds. However, the highest survival rate was found among wilde-beest that left the Crater altogether and calved in an area with few hyenas.
An evolutionary model of the wildebeest's reproductive system is presented, in An evolutionary model of the wildebeest's reproductive system is presented, which it is postulated that the system developed as part of the migratory habit of species. Given a feeding specialization for short, green grass and large, necessarily mobile aggregations, several synergistic selection pressures against concealment in favour of a follower strategy can be readily imagined. Breeding synchrony large aggregations are seen as the keys to the success of this strategy and an alternative to group defence, whereby predator populations are glutted without being sustained and the most vulnerable neonates are protected through the confusion effect created by many calves. Such a strategy is much less adaptive in a sedentary-dispersed population–which is the distribution pattern common to most antelopes-suggesting resident wildebeest populations represent a secondary adaptation to stable environments. Yet calf survival even in resident populations may equal the survival rates hidden young of sympatric antelopes, and be adaptive to the more fixed resources available in such situations.