Fig. 4: Testosterone treatment stimulate the early development of secondary sexual characteristics (crowing, comb)  in heterozygous (AR^+/−) male chicks but not in homozygous (AR^−/−) ones.

In AR^+/− males (upper panels), testosterone treatment induced precocious crowing (high-pitched vocal trills) (b) and comb growth (c), whereas in AR^−/− males (lower panels), these sexual phenotypes did not develop (e, f). Chicks from both groups display distress calls that show individual variations, which were not affected by testosterone treatment (a, d). This suggests that the development of sexual phenotypes depends on the presence of at least one functional allele of the androgen receptor (AR) gene. Testosterone treatment was through a subcutaneous hormone depot implanted at seven days post-hatching. Chicks began to crow in the first week after testosterone treatment, while comb development started in the second week. The sound amplitude (dB) is color coded.