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

Summary. 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.