Activational effects of androgens and estrogens can not reverse sex differences in song behavior and the song control nuclei in adult male and female canaries

Treatments with testosterone (T) do not activate singing behavior nor promote growth of song control nuclei to the same extend in male and female canaries (Serinus canaria). Because T acts in part via aromatization into an estrogen and brain aromatase activity is lower in females than in males, we hypothesized that this enzymatic difference explains the differential response to T in the two sexes. To test this idea, three groups of castrated males and 3 groups of photoregressed females received either 2 empty 10 mm Silastic implants or one empty implant and one implant filled with T or one implant filled with T plus one with estradiol (E2). Songs were recorded and analyzed for 3 hours each week for 6 weeks before brains were collected and song control nuclei volumes were measured in Nissl-stained sections. We confirmed that multiple measures of song were induced more efficiently by T in males than in females. Co-administration of E2 did notimprove these measures and even inhibited some measures such as song rate and song duration. Similarly, the volume of three main song control nuclei (HVC, RA, and Area X) was increased by the two steroid treatments but they remained significantly smaller in females than in males irrespective of the endocrine condition. The lower response of females to T is thus not caused by a lower aromatization of the steroid; sex differences in canaries are probably organized either by early steroid action or by sex-specific gene regulation directly in the brain.