Photoperiodic Control of Thyroid Function and Wool and Horn Growth in Rams and the Effect of Cranial Sympathectomy

Abstract

Eight adult Soay rams (four control and four superior cervical ganglionectomized) were housed in an artificial lighting regimen of alternating 16-week periods of long days [16 h of light, 8 h of darkness (16 L:8D)] and short days (8L:16D) for nearly 3 yr, and the long term variations in growth of the wool and horns were recorded along with measurements of the plasma concentration of T₄, T₃, and testosterone. In the control rams all of the parameters varied in relation to the imposed lighting regimen. An increase in the growth of the wool and horns occurred during each period of long days at a time when the circulating concentrations of T₄ and T₃ were high; at this time the animals were reproductively quiescent, with low plasma levels of testosterone. During each period of short days the rate of wool and horn growth declined, as did the plasma concentrations of T₄ and T₃, while the rams became sexually active with high plasma levels of testosterone. Moulting of the old coat occurred during the periods of long days, coinciding with the phase of increasing wool growth. The variations in the ganglionectomized rams were quite different from those in the controls, bearing no clear relationship to the imposed lighting regimen. In these operated animals there were long term changes in all of the parameters, but these were generally less pronounced than those in the controls. The unusual pattern in the growth of horns and wool was correlated with long term changes in the plasma levels of T₄, T₃ and testosterone. Moulting of the old coat did not occur in the normal manner in the animals after the first year.