Cross-fostering of voles demonstrates in utero effect of photoperiod.

Postweaning body growth and reproductive tract weight of montane voles raised from birth in 14 h light/day are modulated by the photoperiod to which the voles' mothers were exposed while pregnant. This effect could result from factors acting in utero or during lactation, as a result of a change in photoperiod experienced by the mother on the day she gave birth. To distinguish between these hypotheses, male voles exposed to short or long photoperiods during gestation were raised by foster mothers that had been exposed to different photoperiods while pregnant. The differences in body weight, total length, and reproductive tract weight between voles at 74 days of age can be attributed to factors acting in utero. The effects of the gestational photoperiod are not manifested in the patterns of growth until after weaning.