Summary. Long-term cycles in diameter of the testes, colouration of the sexual skin and plasma concentrations of testosterone, FSH and prolactin were monitored in groups of pinealectomized (PINX), superior cervical ganglionectomized (SCGX), and control Soay rams living near Edinburgh (56°N). In Exp. 1, PINX, SCGX and control rams were kept outside for 4 years, and well defined seasonal cycles in each of the reproductive parameters were evident in all 3 groups (e.g. testosterone cycle length assessed by sine-wave analysis: 12·08 ± 0·17, 12·39 ± 0·14 and 12·15 ± 0·10 months for PINX, SCGX and control rams respectively). Qualitative differences, however, were apparent between the groups in the timing and amplitude of the reproductive cycle. The seasonal peak in reproductive function occurred from July to September in the PINX and SCGX rams, some 2 months earlier in the year than in controls, while the amplitude of the cycle was less marked in the PINX and SCGX rams. There were no significant differences between the experimental groups in the seasonal cycle in the plasma concentrations of prolactin.
In Exp. 2, SCGX and control rams were kept indoors under an artificial environment with a 32-week light cycle and constant nutrition for 4 years. Compared to the controls, in which the reproductive changes were synchronized to the driving light cycle, the SCGX rams showed poorly defined reproductive cycles of lower amplitude and longer period (e.g. testosterone cycle length: 57·8 ± 6·1 and 32·1 ± 0·2 weeks for SCGX and control rams, respectively). There was evidence of a cycle in some of the reproductive parameters in the SCGX rams with a period close to 32 weeks during the second half of the study (e.g. testosterone cycle 32·4 ± 0·8 weeks), which was taken to indicate social induction from the neighbouring control rams. In two further short-term experiments, SCGX rams showed a decline in testicular activity in response to receiving a restricted diet (60% of controls) and an increase in testosterone secretion in response to exposure to oestrous ewes.
The overall results illustrate that PINX and SCGX rams can generate long-term synchronized cycles in pituitary and testicular activity. The animals are apparently unable to respond to changes in daylength due to the loss of the functional pineal gland but they remain competent to respond to other environmental cues such as changes in nutrition, temperature and social factors. In PINX and SCGX rams living outside under a seasonal environment, these non-photoperiodic cues are sufficient to induce a clearly defined seasonal reproductive cycle. In normal intact animals, the additional influence of the annual change in daylength results in a delay in the timing of the cycle; the consequence is the late onset of the mating season in October and November which is a characteristic feature of the Soay sheep originating from 58°N.