Extraocular Control of Photorefractoriness in American Tree Sparrows (Spizella Arborea)1

Photorefractoriness, a reversible state of unresponsiveness to daylengths of gonadostimulatory duration, terminates seasonal breeding in many photoperiodic species of birds. Whether the eyes are components of the mechanism that triggers photorefractoriness is an important, but heretofore unresolved, question. Although a role for extraocular photoreception in the mechanism of photoinduced gonadal growth is well documented, the eyes may be important in the mechanism of photorefractoriness if, as some evidence suggests, they are gonadoinhibitory. With American tree sparrows (Spizella arborea), I here confirm that the absence of eyes does not impede photoinduced testicular growth and establish that an extraocular mechanism mediates the transition from photosensitivity to photorefractoriness: Tree sparrows blinded by bilateral ocular enucleation, when photosensitivity to long days or by miniature self-powered lights implanted atop the skull, showed marked testicular growth and then, as evidenced by spontaneous testicular regression, became photorefractory, as did sighted controls.