Birth sex ratios in sheep over six lambing seasons

Summary The birth sex ratio of a commercial flock of Suffolk × sheep, Ovis aries, were studied over six consecutive lambing seasons. In all, data from 1820 lambs were recorded and analysed. The overall birth sex ratio was 49.56% male lambs. Significantly more males (56.23%) were born to ewes with single lambs than to ewes with like-sex twins (47.73%). Significantly more female lambs were born to ewes with triplet births (41.79% male). A significant difference in the birth sex ratio was observed between the first and second half of the lambing season, among like-sex pairs of twin lambs. More males were born in the first half (51.43%) and significantly more females (43.59% male) in the second half of the lambing season. The results are discussed with reference to the theory of Trivers and Willard (1973).