Entrainment of circadian system under variable photocycles (T‐photocycles) alters the critical daylength for photoperiodic induction in blackheaded buntings

Abstract

AbstractThe present study was designed to investigate the photoperiodic responses of blackheaded buntings to various T‐photocycles [T =the period length of light:dark (LD) cycle]. Two series of experiments were performed. In series I, groups of buntings (n =8‐12) were exposed to LD cycles of 22 to 26 hr with photoperiods of 6, 10, or 11 hr (T22 =6L:16D, 10L:12D, 11L:11D; T24 =6L:18D, 11L:13D; T26 =6L:20D, 11L:15D). Among these groups, birds subjected to 11L:11D showed heavy deposition of fat, weight gain, and testicular recrudescence, a response that is characteristic of long day (≥ 12 hr photoperiod per day) photostimulation, indicating that a T‐photocycle of less than 24 hr (T =22 hr) was photoinductive in buntings when presented with a photophase long enough to interact with the photoinducible phase (ϕi), located in the middle of the photocycle. Experiments in series II included an interrupted‐night lighting protocol with a main photophase of 6 hr and 1‐hr light break in photocycles of T =22 hr, T =24 hr, and T =26 hr. The results revealed that a 1‐hr light pulse was inductive when it was introduced in the middle of the photocycles, and, thus, was coincident with the ϕi. It is suggested that the critical daylength for photoperiodic induction in buntings is altered as a result of entrainment to various T‐photocycles. © 1995 Wiley‐Liss, Inc.