Abstract
In support of the Trivers & Willard model of adaptive variation in birth 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 differences in adrenal activity as the result of stress experienced during agonistic encounters are two possible mechanisms underlying these findings. This study examined the role that these mechanisms may play, and further investigated the relationship between social status and birth 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 individuals were assessed during the first and fourth pregnancies. Within each pregnancy, there was no relationship between social status and birth sex ratio. Despite cross-pregnancy consistency in social status, there was no consistency in the sex ratios of the females' consecutive litters. However, high-ranking females produced significantly fewer males across all four pregnancies. There was no linear relationship between a female's social status and adrenal activity, and no evidence that adrenal activity was related to subsequent birth sex ratio. Maternal weight gain during pregnancy (an indicator of nutritional status) was unrelated to birth sex ratio. Data from a study of maternal nutrition in pigs also failed to show a clear relationship between nutritional status and birth sex ratio. Differences between the findings of the present and previous studies are discussed from functional and causal perspectives.
| Original language | English |
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| Pages (from-to) | 1361-1370 |
| Number of pages | 10 |
| Journal | Animal Behaviour |
| Volume | 50 |
| Issue number | 5 |
| DOIs | |
| Publication status | Published - 1995 |
Bibliographical note
Funding Information:We are indebted to Phil and Grant Maltby for looking after the pigs at Cambridge, to Chloe Hudson and Sue Kirkpatrick for assaying samples, and to Nadja Brodmann and Bert Kleiboer for helping to collect data. We thank Alistair Lawrence, Rob Young and two anonymous referees for useful comments. Both studies reported here were supported by the Agriculture and Food Research Council. M.M. also received support from the British Veterinary Association Animal Welfare Foundation and the Scottish Office Agriculture and Fisheries Department, and A.J.Z. was supported by a Brazilian Research Council (CNPq) studentship.