Male Bias In Sex Ratio Research Articles

The global male-bias in sex ratio at birth is sustained by the sex ratio genotypes of replacement offspring

Throughout the world, there is a male-bias in the sex ratio at birth (SRB). It is not known whether this phenomenon has a genetic basis, though there is tentative evidence from genealogical and genomic studies that it may have. It has been proposed that the higher rate of male childhood mortality in humans is linked to the male-bias in SRB through parental investment, but this may only apply to facultative not genetic sex ratio adjustment. In previous population genetic models, elevated mortality in one sex (prior to breeding) has been shown not to affect the SRB, but these models did not consider the role of replacement births (i.e. births that only occur because a sibling died prematurely). In a set of population genetic modelling simulations, in which sex ratio is controlled by an autosomal gene expressed in the male line, this study shows that when there is replacement of dead offspring, this leads to a sustained bias in the SRB in the direction of the sex suffering the highest mortality. In the example of higher male mortality, this occurs, because replacement offspring are disproportionately drawn from fathers who were genetically predisposed to have initially had sons (because sons were more likely to die prematurely), and more likely to pass on male-biasing alleles to replacement offspring. To test the empirical basis for replacement births, an analysis of birth data from the Demographic and Health Survey program was conducted, which shows that parents do indeed tend to replace children who die prematurely.

Offspring sex ratio in red-winged blackbirds is dependent on maternal age

In a marsh-breeding population of red-winged blackbirds, the sex ratio of offspring that survived to leave the nest varied with maternal age. Old mothers produced an excess of male fledglings, middle-aged mothers produced almost equal proportions of males and females, and young mothers produced nearly twice as many females as males. More males than females hatched from the eggs of old mothers, whereas among newly hatched progeny of middle-aged and young mothers the sex ratio did not differ from unity. The hatching rate of eggs of old mothers was unusually low, suggesting that the biased sex ratio of their hatchlings may have been caused by more frequent death of female embryos, although other possibilities can be imagined. Starvation of nestlings after hatching also affected the sex ratio among young that left the nest. When starvation occurred, it fell principally on young produced by the last and next-to-last eggs laid in the clutch. Because old mothers allocated relatively more energy to those eggs than to earlier-laid eggs, whereas young mothers apportioned energy equally to their eggs, few nestlings of old mothers but many nestlings of young mothers starved. Most nestlings that died were male. It followed that the male bias in sex ratio of progeny of old mothers did not change between hatching and nestleaving, but the ratio among progeny of young mothers shifted after hatching to a strong bias favoring females at nest-leaving.