Birth seasons of ungulates in tropical regions show a complex pattern varying from asynchronous to highly synchronous and at different times of year. We examine the factors determining the phenology and synchrony of birth seasons of 13 species of ungulates in the Serengeti ecosystem, Tanzania. We propose that phenology of births (time of year) is determined by food supply, whereas birth synchrony (degree of coordination or spread) is an antipredator adaptation that functions in two different ways. High synchrony may occur through “predator satiation” in species with precocious newborn (“followers”), whereas asynchrony may occur through “predator avoidance” in species with nonprecocial young (“hiders”). We used green biomass of grass or tree shoots and percentage crude protein as measures of food supply. Births were determined from monthly sample counts covering the period 1967–1997. The frequency distribution of births was compared to that predicted by the abundance of green biomass and percentage protein, and by an even (asynchronous) monthly distribution. Wildebeest, topi, warthog, and Grant's gazelle births differed from all predicted distributions. Another group showed birth distributions similar to that of green biomass food (buffalo, oribi) or the distribution of percentage protein (giraffe, waterbuck, kongoni, zebra). Also giraffe, waterbuck, and Thomson's gazelle showed births spread more evenly through the year. For grazing species the lag in months between birth peak and protein peak is a positive function of metabolic body size whereas the lag with biomass is a negative function of body size. We suggest that small grazers produce their young early in the wet season ahead of the high protein peak, whereas large species produce their young in phase with high biomass and after the protein peak consistent with metabolic requirements. In terms of synchrony, large species in large herds with precocial young (wildebeest, topi, buffalo) have highly synchronized birth seasons consistent with the “predator satiation” hypothesis. Small species living in small groups with nonprecocial young (impala, Thomson's and Grant's gazelle, waterbuck, oribi) have births less synchronized than the food supply, as predicted by the “predator avoidance” hypothesis. In general, food supply determines the phenology of the birth season. Predation appears to shape the synchrony of births through two opposite adaptations. However, no single feature predicts all species' birth distributions. A combination of the phenology of food supply plus antipredator adaptations accounts for most but not all these distributions.

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