**5.1 Brain dimorphism**

Naturally occurring dramatic sexual dimorphism in vertebrate brains are exemplified in canaries (*Serinus canaria*) and zebra finches (*Taeniopygia guttata*) where three vocal control areas in the brain are strikingly larger in males (Nottebohm & Arnold 1976). Sex steroid effects on sexually dimorphic brain development was originally investigated in mammals (Raisman & Field 1973) and have since been described in the broader central nervous system (Breedlove & Arnold 1983b; a) for all vertebrate classes (Norris & Lopez 2011a). Correspondingly, sexual dimorphisms of brain neurotransmitter systems are also now evident (De Vries et al. 1984) (Simerly et al. 1985). Note that while this classical view of sex-steroid involvement in the dimorphic brain development is very robust, the paradigm has shifted to incorporate sex differences due to gene expression which occur before gonadal differentiation and subsequent organizational effects of sex-steroids (Mccarthy & Arnold 2007; Arnold 2009).
