SEXUAL DIFFERENTIATION OF AVIAN BRAIN AND BEHAVIOR: Current Views on Gonadal Hormone-Dependent and Independent Mechanisms

Abstract

Gonadal hormones are known to act during development to establish permanent sex differences in the anatomy and function of the vertebrate brain. They also act on the adult brain to activate reproductive behaviors. However, there are wide gaps in our understanding of how sexually dimorphic neural circuits translate into sex differences in behavior and other CNS functions. Moreover, not all sexually dimorphic properties of the adult brain can be attributed to known effects of gonadal hormones during development or adulthood, and factors other than gonadal steroids may contribute to these sex differences. This paper reviews sexual differentiation and the role of gonadal steroids and non-gonadal factors on sexually dimorphic development of the avian brain.

Activities of 3β-HSD and aromatase in slices of developing and adult zebra finch brain

“…These results stand in contrast to expectations that the male brain synthesizes and responds to greater levels of estrogens than females (Holloway and Clayton, 2001;Schlinger, 1998). Two possibilities most likely account for this discrepancy.…”

“…Estrogens are implicated in the masculine development of neural structures controlling song and in the seasonal growth and activation of song circuits in adults (Schlinger, 1998;Schlinger and Brenowitz, 2002). There is considerable evidence suggesting that these estrogens are formed in the brain (Holloway and Clayton, 2001;Schlinger and Arnold, 1991;1992a).…”

Testosterone and its effects on courtship in golden-collared manakins (Manacus vitellinus): Seasonal, sex, and age differences

“…In the present study, over 108 h of filming, three females produced 111 wingsnaps, 8 rollsnaps, and 62 snips clearly demonstrating that females can perform these masculine courtship behaviors when given T implants. These results suggest that neural circuits underlying the expression of at least some of the male courtship behaviors are not organized differently during development in males and females, as are copulatory behaviors in many vertebrate species (Breedlove, 1992;Morris et al, 2004) and as is courtship song in the zebra finch (T. guttata) (Schlinger, 1998). Rather they suggest that the expression of masculine courtship is dependent solely upon the activational effects of adult T, much like T can activate masculine copulation and courtship in females of some species (Balthazart and Ball, 1995) and song in female canaries (Serinus canaries) and budgerigars (Melopsittacus undulates) (Nottebohm, 1980, Nespor et al, 1996.…”

Genetic basis of male sexual behavior

“…Attempts have been made to establish the significance of sex-specific differences in brain morphology by documenting correlations with sexual orientation in humans (Swaab and Hofman, 1995). The most striking vertebrate brain sexual dimorphisms involve the song nuclei of songbirds (Arnold, 1997;Brenowitz, 1997;Schlinger, 1998;Nealen and Perkel, 2000). In zebra finches and other songbirds where only males sing a courtship song, learned from older male tutors, brain nuclei present in males but not in females are responsible for both the acquisition of the courtship song and its execution.…”

“…Thus the distinction between developmental and functional categories may be blurred. Developmental influences of hormones have been extensively documented (see reviews by Kelley, 1988;Arnold, 1997;Schlinger, 1998;Hull et al, 1999;Pfaus, 1999;Gorski, 2000). Thus it remains an open question how plastic and continuously responsive to hormones the brain is.…”