This chapter explores the biological basis of female hierarchies. It discusses the genetic basis of sex determination and the developmental and biochemical mechanisms by which differences characterizing the two sexes are produced. The chapter discusses the relative ranks of males and females in social organizations. It explains the social hierarchies present amongst females, both in societies consisting of males and females, and in unisexual societies. The chapter also discusses the population-genetic and evolutionary implications of sexual dimorphism and of a hierarchic organization among females, and make some remarks on the possible importance of the resulting mating patterns in human evolution. S. Ohno has pointed out that sexual differentiation is genetically a very simple character not requiring an elaborate mechanism of genetic control. The society of the barnyard chicken consists typically of one male and a flock of females; it is in the main the social organization of the females that has been investigated.