Effect of different photoperiods on testicular weight changes in the vole, Microtus agrestis

The testicular weight response of 2-week-old male voles exposed to 10, 12, 14, 15 or 16 h light (L) per day for 4 weeks and of 5-week-old males exposed from birth to 12, 13, 13.5, 14, 14.25, 14.5, 14.75, or 15 h light per day was proportional to the photoperiod: 13L or less produced little growth but 13.5L or more produced progressively heavier testes. Only 14.5L and 15L gave full spermatogenesis. 'Night-break' schedules produced a marked testicular response with 7L :8D :1L :8D and 7L :6D :1L :10D, intermediate effects with 7L :4D :1L :12D and none with 7L :2D :1L :14D. Of various skeleton photoperiods with 0.75L only 0.75L : 12.5D :0.75L :10D and 0.75L :10.5D :0.75L :12D produced marked increases in testicular weight. The peak of photosensitivity appeared to be phase-shifted towards 'dawn' by reducing the initial light phase. Its position appeared to move from 16 h after 'dawn' in block photoperiods to 14 h after 'dawn' in the 'night-break' and 12 h after 'dawn' in the skeleton photoperiod experiments. Previous experience of 16L :8D or 8L :16D had little effect on the subsequent response to skeleton photoperiods (0.5L :9D :0.5L :14D, 0.5L :13D :0.5L :10D, 0.5L :15D :0.5L :8D). Animals reared in these photoperiods only showed a positive testicular response to 0.5L : 13D :0.5L :10D. The lack of response to constant darkness indicated that voles were capable of perceiving these skeleton photoperiods.