Timing the breeding season of the ewe: what is the role of daylength?

Photoperiod is the proximate cue which times the reproductive period of the sheep and the role of daylength as a "zeitgeber" is evaluated in this report. Although sheep are regarded as a short-day breeding species, the reproductive period of the Suffolk ewe begins on daylengths which are on average 2.5 hours longer than those on which it ends. The reasons for this apparent anomaly are considered. Experimental evidence is presented which demonstrates that the transitions between the breeding season and anoestrus are not actively generated by changing ambient photoperiod. Rather the ewe expresses an endogenous circannual rhythm of reproduction and the role of daylength is to entrain this reproductive rhythm to a period of 365 days. It appears that only portions of the annual cycle of photoperiod are necessary to synchronize the breeding season, however, additional photic cues may be required to produce a reproductive period of normal duration.