Independent differentiation of sexual and social traits.

In the process of development in mammals and birds, an individual becomes either male or female. The dogma of sexual differentiation explains this development in terms of gonadal secretions, which direct the growth of an organism along masculine or feminine lines. However, this type of bimodality is not found in all traits that are displayed in reproductive contexts. For example, aggressive behaviors displayed during the breeding season can be expressed by both males and females. Furthermore, this reproductive aggression is highly variable between individuals, regardless of sex. The development of variable traits like reproductive aggression is therefore likely to be influenced by factors in addition to gonadal secretions. We propose that social traits associated with reproduction can differentiate along pathways that are distinct from the developmental pathways of primary sexual traits.