Evidence for a circadian component in the photoperiodic mechanism of the blackheaded bunting, Emberiza melanocephala

Abstract

Photosensitive male blackheaded buntings were exposed to three different light-dark cycles of 22 to 30 h duration, in which a 3 h photophase was coupled with varying durations of scotophases, for a total duration of 75 days. Gonadal growth was initiated in the 30-h cycle (3L:27D), whereas the same photoperiod (3 h) in 22 h and 24 h cycles (3L:19D, 3L:21D) did not induce testicular recrudescence. The results are consistent with the Bünning hypothesis (external coincidence model), and can be interpreted to mean that the positive ultra-short-day photoperiodic response is the result of an advance in the photosensitive phase of the response system so that it becomes coincident, at least in part, with the environmental photophase. In a further experiment, buntings long-day (20L:4D)-photostimulated for 30 days were transferred to continuous darkness (DD) and short days (8L:16D) for 40 days. When examined at the end, birds in both experiments showed testicular regression. The results are explainable in terms of the external coincidence model, but could be explained also by a relatively simple internal coincidence model. Nevertheless, application of the Law of Parsimony may favour the external coincidence system in the photoperiodic response mechanism of this species. © 1982.