Testosterone treatment increases the metabolic capacity of adult avian song control nuclei

In songbirds, the size of brain nuclei that control song learning and produ ction change seasonally. These changes are mainly controlled by seasonal ch anges in plasma testosterone IT) concentration. One hypothesis to explain w hy it may be adaptive for these areas to regress in the fall is that this w ould decrease the metabolic demand of maintaining a large song system when singing is reduced or absent. We used a marker for cellular metabolism to e xamine birds with regressed song nuclei and compared them to birds whose so ng nuclei were induced to grow by administration of exogenous T. Photorefra ctory male Gambel's white-crowned sparrows were captured during their autum nal migration and kept in outdoor aviaries on a natural photoperiod. We imp lanted birds with Silastic (TM) capsules containing T or with empty implant s. Three weeks later the birds were sacrificed. We assayed the brains for c ytochrome oxidase (CO) activity and measured the volume of four song nuclei : HVc, RA, 1MAN, and area X. All four nuclei increased in volume in respons e to T treatment. T treatment increased the metabolic capacity of area X, H Vc, and RA relative to surrounding tissue but had no effect on the metaboli c capacity of 1MAN. These results support the hypothesis that song nuclei a re more metabolically active under the influence of T than they are when pl asma T levels are low. (C) 2001 John Wiley & Sons, Inc.