The Growth Dynamics of Sexually Dimorphic Birds and Fisher's Sex Ratio Theory: Does Sex-Specific Growth Contribute to Balanced Sex Ratios?

Abstract

From Fisher's (1958) theory which states that the sex ratio at the termination of parental care should reflect total cost of producing each sex, a biased secondary sex ratio towards the smaller sex has often been expected for sexually dimorphic birds. The absence of such a sex ratio bias in sexually dimorphic birds has motivated studies which attempted to show, under the so-called sex-specific growth hypothesis, that the differential growth dynamics of males and females counters the difference in energetic costs expected from the size disparity and equalizes the energetic expenditure on males and females. These studies show that the smaller sex (1) shows faster feather growth, (2) attains adult measurements relatively faster, (3) fledges earlier and (4) shows lower variability of growth parameters than the larger sex, and argue that these developmental differences between males and females render the smaller sex as costly as the larger sex. It is shown here that the arguments used in support of the hypothesis that sex-specific growth equalizes the costs of males and females are invalid, and that some of the proposed mechanisms actually increase the cost difference between the sexes. In addition a review of male and female chick growth with respect to weight, tarsus and feathers showed no evidence in support of the sex-specific growth hypothesis in any of the dimorphic species. The energy investment in males and females may be a poor representation of parental cost. The available sex ratio data from stressed and unstressed populations suggest that the reproductive value sensu Williams (1966) is a more precise correlate of parental cost and hence is a better predictor for the