Dispersal and the sex ratio at birth in primates.

The females of many species of primates settle for life within the home ranges of their mothers, whereas males disperse as immatures. According to the theory of sex allocation, the costs incurred by mothers through local competition for resources with their philopatric daughters should favour the evolution of male-biased sex ratios at birth. I report here two tests of this hypothesis based on data from 15 genera of primates. First, I show that the intensity of competition for resources within kin groups is strongly and positively correlated with sex ratios at birth. Second, I show that sex ratios at birth are higher in genera with female-biased philopatry than in genera in which philopatry is not female-biased. These analyses suggest that local resource competition among kin powerfully influences the evolution of sex ratios in primates.