Photoperiodic control of nutritional and reproductive cyclicity in the Soay sheep
Conspicuous photoperiodically entrained, seasonal pattern of appetite, growth reproductive status and moult, have previously been identified in several species of ruminants inhibiting temperate latitudes. The principal objectives of this study were to characterise seasonal rhythmicity in the physiology of the Soay ram, and thereby investigate the mechanisms governing its control. A distinct 14&'37 sinusoidal cycle of predicted energy requirements for maintanance of body weight (264-204 kJ/kg W. 0.75 daily) was found to underlie the Autumnal 75&'37 fall in V.F.I. exhibited by the Soay ram. The cycle of metabolic rate thus revealed preceded that of appetite, suggesting a causal relationship. A 10&'37 seasonal change in digestibility inversely accompanied the seasonal cycles of V.F.I. and predicted maintenance requirements and was principally effected by a change in gut transit time which fell from 41.3 6.0 h during the rut to 25.8 3.6 h when V.F.I. was high. Pineal mediation of reproductive change is well documented. This study demonstrated the role of this gland in the photo-entrainment of the appetite cycle. Superior cervical ganglionectomy did not immediately result in the dissolution of nutritional or reproductive photo-induction although degeneracy of the system became apparent. Amplitude of the seasonal cycles studied was reduced following sympathectomy but synchrony with intact controls was retained during post-operative re-entrainment to a novel photoperiod. Expression of the appetite cycle is largely independent of reproductive change. Autoimmunisation of rams against LHRH produced gonadal involution in some of them, abolishing reproductive response to photoperiod, but the photoperiodically entrained cycle of appetite persisted, synchronous to that of intact controls. Immunocastrates demonstrated a 20&'37 diminution of the fall in V.F.I. from the zenith to nadir of the cycle. This was effected by the maintenance of a higher level of intake by the immunocastrates at the nadir of the cycle, in the absence of the superimposure of testosterone-related curtailment of V.F.I. during the rut.