Involvement of Threshold Photoperiod in Control of Reproductive Rhythmicity in Migratory Blackheaded Bunting,Emberiza Melanocephala

Abstract

To explore the need for minimum threshold photoperiodicity in regulation of metabolic and reproductive activities of a migratory finch, various programmed light-dark (LD) schedules, such as P1 (3L/21D), P2 (6L/18D), P3 (9L/15D), P4 (12L/12D), P5 (15L/9D), P6 (18L/6D), P7 (21L/3D), and P8 (24L/0D), have been used on photosensitive female blackheaded buntings for 42 days. Results indicate that the photoperiodic thresholds of 3 h, 6 h, and 9 h completely failed to have any response on buntings, while threshold photoperiodicities of 12 h, 15 h, 18 h, 21 h, and 24 h had significant effect (P < .001) on body weight, ovarian weight, and circulating plasma estradiol concentration, suggesting the role of the photoperiod as a primary environmental source to regulate various metabolic and reproductive functions. Further, it has been suggested that the threshold photoperiod in this species appears to be of 12 h duration.

Photoperiod-Independent Hypothalamic Regulation of Luteinizing Hormone Secretion in a Free-Living Sonoran Desert Bird, the Rufous-Winged Sparrow <i>(Aimophila carpalis)</i>

“…In the majority of birds that have been studied, when photoperiod reaches a critical threshold in the spring (usually 12 h of light or more, termed long days), production and secretion of the hypothalamic decapeptide chicken gonadotropin-releasing hormone I (cGnRH-I) increases [Perera and Follett, 1992;Parry et al, 1997;Dawson, 1999;Dunn and Sharp, 1999;Meddle et al, 2006;Kang et al, 2006]. Follicle stimulating hormone (FSH) and luteinizing hormone (LH) concentrations increase in the blood, followed by gonadal development, increased sex steroid production, and reproductive behaviors [Hamner, 1968;Follett and Maung, 1978;Wingfield et al, 1993;Silverin et al, 1999;Mishra and Tewary, 1999;Dawson et al, 2001]. After several weeks of photostimulation, most birds become insensitive to photoperiod as a reproductive stimulus (i.e., absolutely photorefractory), cGnRH-I secretion and production decrease, gonads regress, and reproductive behaviors cease [Hamner, 1968;Saldanha et al, 1994;Parry et al, 1997;Dawson et al, 2001;Dawson, 2005;Meddle et al, 2006].…”