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Maternal dominance rank and secondary sex ratio in domestic swine

Abstract Abstract. Maternal dominance rank may influence the reproductive potential of male and female offspring differently. Recent models of adaptive variation in secondary sex ratio (offspring sex ratio at birth) predict that parents will invest more in the sex with greater reproductive potential. The Trivers & Willard (1973, Science, 179, 90-92) model predicts that under certain conditions high-ranking females will give birth to a greater proportion of sons than will low-ranking females. The local-resource competition model predicts that under other conditions high-ranking females will give birth to a greater proportion of daughters than will low-ranking females. Agonistic encounters in groups of multiparous domestic swine were observed and each sow was assigned a social dominance rank. The relationship between the rank status of sows and several reproductive parameters were analysed. High-ranking females gave birth to a greater proportion of sons than did low-ranking females. Neither secondary sex ratio nor maternal rank were related to litter size, maternal age, offspring weight, parental breed, parity or social group membership. These results, in conjunction with observations of the social behaviour of wild and feral Sus scrofa, support the Trivers & Willard model.