Life-history analysis of the Trivers and Willard sex-ratio problem

Phenotypic quality, such as condition or size, often varies between individuals. For species with extensive maternal care, the quality of offspring may partially be determined by the quality of their mother. Trivers and Willard (1973) predicted that high quality females should prefer offspring of the sex whose reproductive success is most strongly influenced by maternal care, which in many cases will be sons. Correspondingly, low quality females should prefer daughters. However, this prediction is not based on a proper analysis of variation in reproductive value. Using state-dependent life-history theory, I show here that high quality females should prefer offspring of the sex whose reproductive value is most strongly influenced by maternal care. I also show that when offspring quality is strongly determined by their mother's quality, but not influenced by their father's quality, high quality females can have higher reproductive value than high quality males, even though their reproductive success may be much lower. In such cases, high quality females should prefer daughters and, correspondingly, low quality females should prefer sons.