Modelling sub-binomial variation in the frequency of sex combinations in litters of pigs

In some samples of pig litters, the number of males per litter exhibits significant sub-binomial dispersion: there are fewer unisexual and more "sex-balanced" litters than expected under a binomial model. It is argued here that the explanation is that the sex of zygotes may be subject to hormonal control with the consequence that, within a litter, the zygotes do not all have the same probability of being male. This variation in probability is approximated by a quadratic function of the time during oestrus that a zygote is formed so as to parallel the variation in maternal hormone levels. Other explanations are examined briefly, but each seems to have serious weaknesses.