What determines phenology and synchrony of ungulate breeding in Serengeti?

Birth seasons of ungulates in tropical regions show a complex pattern varyi ng from asynchronous to highly synchronous and at different times of year. We examine the factors determining the phenology and synchrony of birth sea sons of 13 species of ungulates in the Serengeti ecosystem, Tanzania. We pr opose that phenology of births (time of year) is determined by food supply, whereas birth synchrony (degree of coordination or spread) is an antipreda tor adaptation that functions in two different ways. High synchrony may occ ur through "predator satiation" in species with precocious newborn ("follow ers"), whereas asynchrony may occur through ''predator avoidance" in specie s with nonprecocial young ("hiders"). We used green biomass of grass or tre e shoots and percentage crude protein as measures of food supply. Births we re determined from monthly sample counts covering the period 1967-1997. The frequency distribution of births was compared to that predicted by the abu ndance of green biomass and percentage protein, and by an even and Grant's gazelle births (asynchronous) monthly distribution. Wildbeest, topi, warthh og, differed from all predicted distributions. Another group showed birth d istributions similar to that of green biomass food (buffalo, oribi) or the distribution of percentage protein (giraffe, waterbuck, kongoni, zebra). Al so giraffe, waterbuck, and Thomson's gazelle showed births spread more even ly through the year. For grazing species the lag in months between birth pe ak and protein peak is a positive function of metabolic body size whereas t he lag with biomass is a negative function of body size. We suggest that sm all grazers produce their young early in the wet season ahead of the high p rotein peak, whereas large species produce their young in phase with high b iomass and after the protein peak consistent with metabolic requirements. I n terms of synchrony, large species in large herds with precocial young (wi ldebeest, topi, buffalo) have highly synchronized birth seasons consistent with the "predator satiation" hypothesis. Small species living in small gro ups with nonprecocial young (impala, Thomson's and Grant's gazelle, waterbu ck, 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 synchron y of births through two opposite adaptations. However, no single feature pr edicts all species' birth distributions. A combination of the phenology of food supply plus antipredator adaptations accounts for most but not all the se distributions.