Summary

This chapter considers the factors influencing population birth sex ratios and variations of the sex ratio within populations, together with adaptive interpretations of these effects and the mechanisms underlying them. Adaptations and mechanisms for birth sex ratios are poorly understood, whereas variation in the sexual biasing of postnatal investment is more clearly adaptive. Social and demographic factors influencing birth sex ratios and postnatal investment include: parental status (the Trivers-Willard effect), father's age, birth order and local resource enhancement and competition. Complex effects are expected since many causal factors may interact at different times during offspring dependency. Possible mechanisms for control of the birth sex ratio include: the proportion of X and Y sperm in ejaculates; the relative success of X and Y sperm in fertilization, as determined by hormonal changes over the menstrual cycle interacting with the time of insemination and coital frequency; and embryonic mortality. Prospects for new work emerge from the following analyses: mechanisms for the Trivers-Willard effect (X and Y sperm proportions in ejaculates, coital frequency, embryonic mortality, family size and paternal age; predicting adaptation from proposed mechanisms and ancestral states; relationships between status, reproductive success and its variance; measuring status more realistically; and marginal return as the correct measure for predicting investment.

Introduction

The peculiar fascination with the human sex ratio at birth has a number of sources.