Winning the Genetic Lottery: Biasing Birth Sex Ratio Results in More Grandchildren.

Population dynamics predicts that on average parents should invest equally in male and female offspring; similarly, the physiology of mammalian sex determination is supposedly stochastic, producing equal numbers of sons and daughters. However, a high quality parent can maximize fitness by biasing their birth sex ratio (SR) to the sex with the greatest potential to disproportionately outperform peers. All SR manipulation theories share a fundamental prediction: grandparents who bias birth SR should produce more grandoffspring via the favored sex. The celebrated examples of biased birth SRs in nature consistent with SR manipulation theories provide compelling circumstantial evidence. However, this prediction has never been directly tested in mammals, primarily because the complete three-generation pedigrees needed to test whether individual favored offspring produce more grandoffspring for the biasing grandparent are essentially impossible to obtain in nature. Three-generation pedigrees were constructed using 90 years of captive breeding records from 198 mammalian species. Male and female grandparents consistently biased their birth SR toward the sex that maximized second-generation success. The most strongly male-biased granddams and grandsires produced respectively 29% and 25% more grandoffspring than non-skewing conspecifics. The sons of the most male-biasing granddams were 2.7 times as fecund as those of granddams with a 50:50 bias (similar results are seen in grandsires). Daughters of the strongest female-biasing granddams were 1.2 times as fecund as those of non-biasing females (this effect is not seen in grandsires). To our knowledge, these results are the first formal test of the hypothesis that birth SR manipulation is adaptive in mammals in terms of grandchildren produced, showing that SR manipulation can explain biased birth SR in general across mammalian species. These findings also have practical implications: parental control of birth SR has the potential to accelerate genetic loss and risk of extinction within captive populations of endangered species

Grandparents who bias the sex of the offspring, have more successful offspring, gaining more grandchildren.

<p><b>A</b>) Granddams and <b>B</b>) grandsires who biased birth SR towards males had greater total success measured as total grandchildren produced (<i>P</i><0.0001; <i>P = </i>0.0108, respectively). Birth SR is shown as a <i>Z-</i>score, to control for number of F1 offspring (the X-axes also give examples of male biases for a given <i>Z-</i>score). <b>C</b>) Granddams, and <b>D</b>) grandsires, who biased birth SR towards males had greater success specifically via F1 males (for both, <i>P</i><0.0001). <b>E</b>) Granddams who biased birth SR towards females had greater success specifically via F1 females (<i>P = </i>0.0272), but no effects were found for female-biasing grandsires (<i>P</i> = 0.9426), (nor did they have more total grandchildren overall; see text). For clearer data visualization, the data were split into 10th percentiles by <i>Z-</i>score, and plotted values are least-squares means and standard errors within those percentiles. The solid line indicates the least-squares regression line partialled for the controlling variables. In <b>A</b> and <b>B</b>, the Y-axes shows F0 success as total grandchildren born. In <b>C</b>–<b>F</b>, granddam and grandsire success is shown as the grandchildren (F2) born per each of their F1offspring born of a given sex (i.e. the mean reproductive output of the F1 children of each sex).</p

Species with notably skewed Birth Sex Ratios.

<p>The variance in birth SR was figured for each species. The five species with the greatest variance (i.e. standard deviation²) in F1 birth SR for granddams and grandsires are listed. Because birth SR is expressed as <i>Z-</i>score, the expected variance for any species = 1. The observed variances are tested against the mean within-species variance in <i>Z-</i>score.</p