Ovulatory activity and plasma prolactin concentrations in wild and domestic ewes exposed to artificial photoperiods between the winter and summer solstices

Mouflon and domestic Manchega sheep differ in the timing of their reproductive season under natural photoperiod (NP) conditions. This study examines whether they also differ in their reproductive responses to artificial photoperiod cues. For this, mouflons (n = 24) and ewes (n = 24) were exposed between the winter and summer solstices to artificial long days (LD: 16 h light/day), to short days (SD) simulated via the use of melatonin implants, or to NP conditions (controls), and their ovulatory activity monitored. The effects of these treatments on annual changes in prolactin concentration were also recorded. In the LD mouflon ewes, the offset and onset (7 March 5 and 2 October +/- 4, respectively) of cyclic ovulatory activity occurred earlier (P < 0.001) than in the NP animals (26 April +/- 6 and 20 October +/- 2, respectively), but no differences were seen (P > 0.05) between the SD and NP mouflon ewes in either the onset of anoestrus (12 May +/- 6 and 26 April +/- 6, respectively) or the onset of subsequent ovulatory activity (13 October +/- 8 and 20 October +/- 2, respectively); however the duration of the anoestrus period was significantly reduced in the SD. In LD Manchega ewes, the onset of anoestrus was advanced (2 February +/- 5 vs 15 March +/- 11), but ovulatory activity started at the same time as in NP Manchega ewes (16 July +/- 4 vs 5 July +/- 8). In the SD Manchega ewes, two animals showed continuous cyclic ovulatory activity over the course of the experiment while the remainder entered anoestrus two months later (16 May +/- 6, P < 0.001) than their NP counterparts. In these SD ewes, the onset of cyclic ovarian activity was very variable. An annual rhythm of plasma prolactin concentration was seen in both the mouflon and Manchega ewes under all three photoperiod conditions. However, the amplitudes of the changes seen in prolactin concentration were smaller in both the LD and SD animals than in the corresponding NP animals (P < 0.001). In conclusion, the results show that these two types of Mediterranean sheep differ in their ovulatory response when subjected to artificial photoperiods. The results also suggest that refractoriness to SDs may be the most important physiological mechanism regulating the onset of anoestrus in highly seasonal breeds, but not in less seasonal breeds.