Birth sex ratios relate to mare condition at conception in Kaimanawa horses

Several hypotheses have been proposed to explain variation in birth sex ratios, based on the premise that variation is expected when the profitability of raising sons and daughters varies between individual parents. We tested the Trivers-Willard hypothesis that mothers in better condition produce relatively more sons and that mothers in poorer condition produce relatively more daughters when male reproductive success is more variable. We examined birth sex ratios in relation to mare body condition at conception in horses in which male reproductive success is differentially helped by slight advantages in condition. Horses meet the assumptions of the Trivers-Willard hypothesis better than many species on which it has been tested and in which sex ratio biases are not confounded by sexual size dimorphism such that one sex is more likely to die in utero in females in poor condition. Mares that had a female foal were in poorer condition at conception than those that had a male foal, and mares that had foals of different sexes in different years were in significantly poorer condition when they conceived their female foal There was no relationship between offspring sex and mid-gestation condition, and there was no difference in foaling rates in relation to body condition at conception. Consequently, sex ratio deviations are not explained by fetal loss in utero. Furthermore, differential fetal loss of the less viable sex cannot explain the greater proportion of males produced by mares in better condition. Therefore, our results suggest that sex ratio modification occurs at conception in wild horses.