Abstract
The role of circadian mechanism in the photoperiodic induction of gonadal growth was investigated using 46 individuals of a subtropical migratory finch, the common Indian rosefinch ( Carpodacus erythrinus), by means of an asymmetrical photoperiodic regime. The regime consisted of a nonstimulatory basic photophase (6 h) in combination with a scotophase, in a 24-h cycle, the scotophase being interrupted by 1 h light at temporally fixed hours. The light interruption between hours 12 and 17 after the onset of the basic photoperiod led to testicular growth, while 1 h light as a continuation of the basic photophase or as an interruption 10, 19 or 21 h after the onset of the basic photophase was not stimulatory. Results clearly indicate that the rosefinch possesses a fine time-measuring device, which involves an endogenous circadian rhythm in photosensitivity. The photosensitive phase appears between hours 12/13 and 17/18 following the onset of the first light period.
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