PHOTOPERIOD AND TESTOSTERONE INDEPENDENTLY AFFECT VOCAL CONTROL REGION VOLUMES IN ADOLESCENT MALE SONGBIRDS

Previously, we found that, unlike adults, adolescent male dark-eyed ju ncos (Junco hyemalis) maintained large Area X volumes despite having l ow plasma testosterone concentrations. Other studies indicate that pho toperiod may act independently of testosterone to modulate vocal contr ol region (VCR) volumes in adult songbirds. In the present study, we i nvestigated the effects of testosterone and photoperiod on the volumes of four VCRs in adolescent male juncos, To test the hypothesis that V CR volumes in these males are testosterone independent, we treated bir ds exposed to short days with testosterone and later compared their VC R volumes with those of birds exposed to short days without testostero ne, To examine whether photoperiod alone could affect VCR volumes inde pendent of testosterone, we measured these volumes in photorefractory birds exposed to long photoperiod without testosterone. Administering testosterone induced singing, yet increased the volume of only one VCR , the robust nucleus of the anterior neostriatum (RA), In contrast, lo ng photoperiod increased several VCR volumes (Area X, higher vocal cen ter, and RA) despite low testosterone levels, but did not induce singi ng. Our results suggest a limited role for testosterone, but an import ant role for photoperiod, in controlling VCR volumes in adolescent mal e juncos, In addition, the results demonstrate that singing behavior c an be induced in adolescent males without a concomitant increase in mo st VCR volumes. (C) 1998 John Wiley & Sons, Inc.