Photoperiodism research has relied on static day lengths and abrupt transitions between long and short days to characterize the signals that drive seasonal rhythms. To identify ecologically relevant critical day lengths and to test the extent to which naturally changing day lengths synchronize important developmental events, we monitored 9 cohorts of male Siberian hamsters (Phodopus sungorus), born every two weeks from 4 weeks before to 12 weeks after the summer solstice in a simulated natural photoperiod (SNP). SNP hamsters born from 4 weeks before to 2 weeks after the solstice underwent rapid somatic and gonadal growth; among those born 4-6 weeks after the solstice some delayed puberty by many weeks whereas others manifested early puberty. Hamsters born 8 or more weeks after the solstice failed to undergo early testicular development. The transition to delayed development occurred at long day lengths, which induce early puberty when presented as static photoperiods. The first animals to delay puberty may do so predominantly on the basis of postnatal decreases in day length, whereas in later cohorts, a comparison of postnatal day length to gestational day length may contribute to arrested development. Despite differences in timing of birth and timing of puberty, autumn gonadal regression and spring gonadal and somatic growth occurred at similar calendar dates in all cohorts. Incrementally changing photoperiods exert a strong organizing effect on seasonal rhythms by providing hamsters with a richer source of environmental timing cues than are available in simple static day lengths.