Dependency on Testosterone of Photoperiodically-Induced Vernal Fat Deposition in Female White-Crowned Sparrows

Vernal premigratory fat deposition in White-crowned Sparrows (Zonotrichia leucophrys gambelii) is induced by increasing day length. This induction can be prevented by castration before photostimulation. We investigated whether steroid hormone replacement during short days abolishes the consequences of castration. The inhibition by ovariectomy of long-day-induced fat deposition was reversed by testosterone, but not by 5α-dihydrotestosterone or estradiol-17β. The administration of an aromatase inhibitor (ATD) simultaneously with testosterone blocked its effect. It is suggested that low levels of testosterone present in both sexes during the wintering phase are required for vernal premigratory fattening. It appears that testosterone initiates a process that leads to fat deposition after photostimulation, and that this process most likely occurs in the CNS.