SUMMARY (1) Demographic parameters of a free-ranging population of gelada baboons were determined during a field study in Ethiopia. (2) The annual birth rate varied inversely with the severity of the rainfall around the period of conception. The distribution of births within the year, however, was timed so as to minimize the exposure of the neonates to the severe wet season conditions. (3) Females within reproductive units were in close reproductive synchrony, partly due to the environmental control of conceptions; however, social factors also played an important role in bringing females into synchrony. (4) After the first year of life, mortality fell most heavily on the oldest age classes; exposure to severe wet season conditions, old age and parasitic infestations were the main causes of death. Age-specific mortality and fecundity rates were estimated. (5) The population was found to be increasing at a fairly steady rate of about 120 per annum. Migration by entire sub-sections of the population at aperiodic intervals helped to maintain the actual density of animals within the study area around a longterm mean value.
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