Photoperiodism, pineal clock and seasonal reproduction in the Indian Weaver Bird (Ploceus philippinus)

Abstract

Previous studies show that, in birds, pineal melatonin is important for circadian rhythmicity, and circadian rhythms mediate photoperiodic effects. The effect of pinealectomy or melatonin administration in photoperiodic induction of testicular growth is not found in many bird species. This is inconsistent with the fact that avian pineal is a self-sustained circadian biological clock and decodes both the daily and annual photoperiodic information. Does this mean that the pineal clock in birds regulates circadian rhythms, but not the one that is involved in the photoperiodic induction of the seasonal response? We have examined this in experiments on the subtropical Indian Weaver Bird (Ploceus philippinus). We investigated the effects of the absence of the pineal gland or exogenous melatonin administration on circadian rhythmicity in activity, and on photoperiodic induction of testicular growth, androgen-dependent beak pigmentation and luteinizing hormone-specific plumage coloration. Weaver birds were subjected for several weeks to short day length (8 h light: 16 h darkness, 8L:16D), long day length (16L:8D) and to light–dark cycles that tested the involvement of circadian rhythmicity in photoperiodic time measurement. The results show that circadian pineal clock may regulate the circadian rhythm in activity, but is not essential for the expression of photoperiodism associated with the photoperiodic induction of testicular growth. This suggests that the circadian activity and photosensitivity rhythms are outputs of different circadian oscillators or of the same set of circadian oscillators, but that they are not closely coupled.