Effect of season and photoperiod on FSH in male great tits

The present study investigated (1) the photoperiodic threshold for an incre ase in FSH and LH secretion in male great tits (Parus major) from south Swe den and (2) seasonal changes in FSH secretion patterns in short day (SD; 8 L:16 D) and long day (LD; 20 L:4 D) exposed great tits. When fully photosen sitive great tits were exposed to a simulated vernal increase in day length s plasma levels of FSH started to rise at a shorter day length (10.5 h) tha n did LH (11 h). Once the photoperiodic threshold was exceeded FSH levels i ncreased slowly, but continuously, and reached maximum levels when day leng ths had reached 13-14 h, whereas LH levels rose to maximum levels more or l ess immediately. Great tits break photorefractoriness in September. Exposin g sham-operated and castrated great tits to SD or LD in October and Februar y resulted in clear seasonal differences in their FSH cycles. At both times the great tits responded to castration and exposure to long days. However, castration and photoperiodic FSH responses were not only more rapid in Feb ruary, but also of a higher magnitude. (C) 1999 Academic Press.