LIFE-HISTORY ANALYSIS OF THE TRIVERS AND WILLARD SEX-RATIO PROBLEM

Phenotypic quality, such as condition or size, often varies between in dividuals. For species with extensive maternal care, the quality of of fspring may partially be determined by the quality of their mother. Tr ivers and Willard (1973) predicted that high quality females should pr efer offspring of the sex whose reproductive success is most strongly influenced by maternal care, which in many cases will be sons. Corresp ondingly, low quality females should prefer daughters. However, this p rediction is not based on a proper analysis of variation in reproducti ve value. Using state-dependent life-history theory, I show here that high quality females should prefer offspring of the sex whose reproduc tive value is most strongly influenced by maternal care. I also show t hat when offspring quality is strongly determined by their mother's qu ality, but not influenced by their father's quality, high quality fema les can have higher reproductive value than high quality males, even t hough their reproductive success may be much lower. In such cases, hig h quality females should prefer daughters and, correspondingly, low qu ality females should prefer sons.