MATERNAL SOCIAL-STATUS AND BIRTH SEX-RATIO IN DOMESTIC PIGS - AN ANALYSIS OF MECHANISMS

In support of the Trivers & Willard model of adaptive variation in bir th sex ratio, recent research showed that high-ranking female domestic pigs, Sus scrofa, gave birth to a greater proportion of sons than low -ranking females. Differences in maternal nutritional status caused by differential access to food of high- and low-ranking females, or diff erences in adrenal activity as the result of stress experienced during agonistic encounters are two possible mechanisms underlying these fin dings. This study examined the role that these mechanisms may play, an d further investigated the relationship between social status and birt h sex ratio in the pig, not only within, but also across pregnancies. The reproductive performance of a group of 37 female pigs was recorded across four pregnancies. Social status and adrenal activity of indivi duals were assessed during the first and fourth pregnancies. Within ea ch pregnancy, there was no relationship between social status and birt h sex ratio. Despite cross-pregnancy consistency in social status, the re was no consistency in the sex ratios of the females' consecutive li tters. However, high-ranking females produced significantly fewer male s across all four pregnancies. There was no linear relationship betwee n a female's social status and adrenal activity, and no evidence that adrenal activity was related to subsequent birth sex ratio. Maternal w eight gain during pregnancy (an indicator of nutritional status) was u nrelated to birth sex ratio. Data from a study of maternal nutrition i n pigs also failed to show a clear relationship between nutritional st atus and birth sex ratio. Differences between the findings of the pres ent and previous studies are discussed from functional and causal pers pectives. (C) 1995 The Association for the Study of Animal Behaviour