Wool growth cycles in Soay sheep

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Abstract
SUMMARY: The wool growth cycle in six rams and eight ewes was studied by taking monthly coat and skin samples for 3 years from each. Coat length was measured, and the percentage of inactive follicles was noted as well as the percentage of medullated fibres.In the spring moult ‘mainland’ sheep (stock removed from St Kilda 60 years ago) cast less completely than ‘St Kilda sheep’ (Soays removed more recently from St Kilda). Mainland rams cast less than the ewes, but St Kilda rams began casting earlier, and completed casting more quickly than the ewes. Despite differences between years in the rate of casting, similarities in the pattern of casting were observed between years in the same animal, and between parent and offspring.This apparent annual cycle was confirmed by graphs of follicle inactivity which showed that most inactivity occurred from September to March in the rams, and from October to April in the ewes. It was possible to fit sin and cos waves of 12-month duration to the graphs and further analysis revealed superimposed a 6-month cycle in one group, and a 4-month cycle in another group, which were associated with subsidiary peaks of inactivity in the summer. There were suggestions of a cycle about 2 months in length throughout the year, and sin waves of this duration could be fitted to the data from one of the groups of ewes. Possible control mechanisms for the annual and shorter cycles are discussed.Peaks of follicle inactivity were frequently followed by troughs in coat length which suggested actual loss of the longest fibres.Medullation was absent or minimal from December to March, and there were two peaks during the period of maximum medullation, that in the spring being greater than the one in the autumn.