Estradiol and Flutamide Effects on the Song System of Developing Male Zebra Finches

Abstract Estradiol (E 2 ) masculinizes the developing song system of female zebra finches (Taeniopygia castanotis) if it is administered in early life, but the effects of E 2 on the developing song system in males have not been thoroughly explored. Also, reports of early antiandrogen administration in developing male zebra finches differ in their findings. Therefore. this study was conducted to further explore the effects of administering E 2 alone, flutamide (Flut) alone, or the two in combination during early development. Brains and testes were examined after day 100. The results showed definite demasculinizing effects of early E 2 on the song nucleus HVC (proper name)—its volume and neuron number were markedly reduced. Nonetheless, early E 2 hypermasculinized HVC neuronal size. Flut slightly hypermasculinized RA volume (Robust nucleus of the Arcopallium), which replicates a previous study, but the absence of any additional effects is at odds with other studies. Arguments are put forward suggesting that early E 2 action on HVC development could be a consequence of differential steroid action on the male versus female genotype and/or a disruption of the temporal sequence of gene regulation in developing males. Early E 2 resulted in markedly reducing testes size, which is likely to be a consequence of hijacking endogenous endocrine feedback mechanisms. The reduction in testes size suggests low steroid secretion, which also could contribute to the demasculinization of HVC.