The timing of gonadal refractoriness in the female Turkish hamster (Mesocricetus brandti) is not dependent on the timing of gonadal regression.

When prepubertal female Turkish hamsters are reared on 12L:12D, individuals respond reproductively in two ways; the majority exhibit a delay in the onset of regular vaginal estrous cyclicity (first vaginal estrus; FVE), and some individuals exhibit FVE at a time not different from females reared on 16L:8D. Soon, however, these females also become reproductively quiescent. This study addresses the following questions: (1) At what age do these two subpopulations become refractory to 12L:12D? (2) Is the onset of refractoriness in adults reared on 12L:12D dependent on the time when they become anovulatory? All females reared on 12L:12D become refractory at a similar age (169.1 +/- 4.4 days in those that exhibit delayed FVE vs. 165.3 +/- 6.8 days in those that exhibited an early FVE and then became anovulatory; p much greater than 0.05). Also, there is no correlation between the age at which vaginal cycles ceased and the age at which those individuals became refractory (r2 = 0.12; slope not different from 0). On the basis of these data, we propose that in the female Turkish hamster the onset of photorefractoriness is timed, not by the interval of reproductive quiescence, but by a preceding event, perhaps the initiation of short photoperiod exposure or the onset of photosensitivity.