Abstract
Experiments were conducted to determine whether gestation length in mice was determined by the number of daylight/darkness cycles or by an absolute length of time. C57BL strain mice were bred and raised under a 10 1/2-hour light (L):10 1/2-hour dark (D) cycle. Inurement to the cycle was demonstrated by measuring running activity. The animals were 1st bred (Period 1) under the 10 1/2 L:10 1/2 D cycle (21 pregnancies) then for about 3 weeks (Period 2) under a 12 L:12 D cycle (10 pregnancies) followed again (Period 3) by breeding under the 10 1/2 L:10 1/2 D cycle (9 pregnancies). The mode of gestation lengths was 20 days for Period 1 (range: 19-20 days) 20 days for Period 2 (range: 20-21 days) and 21 days for Period 3 (range: 20-22 days). In terms of absolute time the gestation length remained approximately constant in mice inured to a shortened light-dark cycle.
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