Sex Ratio Adjustment Under Food Stress: Maximization of Quality or Numbers of Offspring?

Abstract

Trivers and Willard (1973) proposed that females in poor condition should produce female offspring. The suggested reason for this is that poor-condition female young are more likely to be reproductively successful than are poor-condition male young. Evidence from the literature does not support this hypothesis, and cited studies can be interpreted in terms of differential mortality between the sexes. An alternative hypothesis that poor environmental conditions will lead to the production of the cheaper sex, is proposed. This allows for maximization of the number of young produced. A study of redwing blackbirds illustrates the hypothesis, and the information necessary for a rigorous testing is outlined. Confusion should be avoided between adjustment of the primary sex ratio, which allows modification without the loss of potential offspring, and sex ratio adjustment through differential mortality of the 2 sexes after conception. The latter requires a loss in reproductive potential, which is particularly serious in organisms which produce only 1 young/yr.