Photoperiodic control of annual reproductive cycle in subtropical brahminy myna, Sturnus pagodarum.

The aims of this investigation were to: (i) study the effect of the length of photoperiod on the timing of onset of gonadal recrudescence and postreproductive decline in photoresponsivity, and (ii) identify the mechanism(s) involved in the initiation of gonadal recrudescence in the subtropical brahminy myna (Sturnus pagodarum). Two series of experiments were performed using adult birds: (1) Groups of birds were subjected to 12L:12D, 14L:10D, and 16L:8D in January and to 12L:12D and 14L:10D in March. Those exposed to 12L and 14L in March were transferred to longer daylengths (16L:8D, 20L:4D) after 180 days. In the January birds, 14L and 16L induced testicular growth and regression earlier than 12L; also, 16L caused gonadal recrudescence earlier than 14L. The 14L photoperiod, however, induced gonadal development in March birds within 30 days, similar to that found in January birds under 16L:8D. The response of March birds to 12L:12D was, however, similar to those of January birds, except that the time of maximum testicular response and subsequent regression was advanced in the former group. It is concluded that the refractoriness to long day photostimulation is fully dissipated in March but not in January. (2) Groups of unstimulated and stimulated birds were exposed to "resonance" and "interrupted-night" light: dark cycles for 5 weeks. Resonance LD cycles of 12 (8L:4D)- and 36 (8L:28D)-hour induced day responses, while groups under 24 (8L:16D)- and 48 (8L:40D)-hr cycles behaved as if exposed to short days. In night-interruption experiments, a 1 hr light pulse given 12 and 16 hr, but not 20 hr, after dawn induced the long day response. These results suggest that in brahminy myna the time to the onset of gonadal recrudescence and subsequent regression (photorefractoriness) is dependent on the length of photophase as well as time of the year when exposed to stimulatory photoperiods, and that a light-sensitive rhythm with a period of about 24 hr is involved in the photoperiodic induction of gonadal recrudescence.