the Sex Ratio of Offspring
ROBERT L. TRIVERS AND DAN E. WILLARD
Abstract. Theory and data suggest that a male in good condition at the end of the period of parental investment is expected to outreproduce a sister in similar condition, while she is expected to outreproduce him if both are in poor condition. Accordingly, natural selection should favor parental ability to adjust the sex ratio of offspring produced according to parental ability to invest. Data from mammals support the model: As maternal condition declines, the adult female tends to produce a lower ratio of males to females.
Fisher (1) showed, and others (2) reformulated, that natural selection favors those parents who invest equally in both their sons and their daughters. When the parents invest the same in an average son as in an average daughter, natural selection favors a 50/50 sex ratio (ratio of males to females) at conception (3,4). (For simplicity, we assume here that parents are investing equally in average offspring of either sex. ) Individuals producing offspring in sex ratios that deviate from 50/50 are not selected against as long as these deviations exactly cancel out and result in a sex ratio at conception of 50/ 50 for the local breeding population. Such a situation is highly unstable, since random deviations from the 50/50 ratio in local populations rapidly favor those individuals producing their young in ratios of 50/50. We show here that under certain well-defined conditions, natural selection favors systematic deviations from a 50/50 sex ratio at conception, and that these deviations tend to cancel out in the local breeding population.
Imagine a population of animals (for instance, caribou) in which the condition of adult females varies from good to poor (as measured, for example, by weight). Assume that a female in good condition is better able to bear and nurse her calf than is a female in poor condition, so that at the end of the period of parental investment (PI), the healthiest, strongest, and heaviest calves will tend to be the offspring of the adult females who were in the best condition during the period of PI. Assume that there is some tendency for differences in the condition of calves at the end of the period of PI to be maintained into adulthood. Finally, assume that such adult differences in condition affect male reproductive success (RS) more strongly than they affect female RS. That is, assume that male caribou in good condition tend