The significance of breeding synchrony in the wildebeest

Abstract

Summary 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 rep...

Reproductive seasonality in African ungulates in relation to rainfall

“…The relationships with rainfall further support the suggestions that high but not excessive rainfall enhances conception and early fetal growth by promoting sufficient supply of high-quality forage, accumulation of fat reserves and attainment of peak physical condition necessary for conception (Talbot and Talbot 1963;du Plessis 1972, Estes 1976Prins 1996;BroJørgensen 2001). The positive rainfall influence at the time of parturition portrays increased demand for high-quality forage during late pregnancy and early lactation when energetic demands peak.…”

“…The results suggest that rainfall controls the timing and success of conception not only through its influence on adequate nutrition before and during conception (Talbot and Talbot 1963;Estes 1976) but also during gestation and at the time of birth, depending on species. Direct support for this suggestion comes from Kenyan Samburu elephants followed for 7 years, 86% of which conceived after the peak in vegetation productivity, with the highest conception rate occurring during the unusually high El Niño rains in 1997-98 and no conception occurring during the subsequent La Niña drought phase in 2000 (Wittemyer et al 2007a).…”

“…Moreover, the timing of birth peaks in the resident wildebeest population of the western Serengeti occurs in January, a month earlier than in the main migratory population (Ndibalema 2009), being adjusted to the earlier onset of the rainy season. In the nearby Ngorongoro Crater, there is a difference of up to 3 weeks in the timing of calving peaks in the resident subpopulations of wildebeest on opposite sides of the crater, reflecting microclimatic differences (Estes 1976).…”

“…Counteracting this is the benefit of predator-swamping for species with offspring that follow the mother after birth, penalising early or late-born young. Species that hide their young should thus show a broader spread of births than those with follower young (Estes 1976;Rutberg 1987).…”

Endocrine patterns of the estrous cycle and pregnancy of wildebeest in the Serengeti ecosystem

“…The evolutionary benefit of this approach has been attributed to the species having a 'follower' calf strategy that increases neonate survival through predator swamping and predator confusion (Estes, 1976;Rutberg, 1987). It is unlikely that synchronous reproduction in wildebeest evolved from an aseasonal breeding pattern, but rather a seasonal reproductive pattern that optimized resource availability was likely adjusted to yield a high degree of reproductive synchrony.…”