Data from: A father effect explains sex-ratio bias

Sex ratio allocation has important fitness consequences, and theory
predicts that parents should adjust offspring sex ratio in cases where the
fitness returns of producing male and female offspring vary. The ability
of fathers to bias offspring sex ratios has traditionally been dismissed
given the expectation of an equal proportion of X- and
Y-chromosome-bearing sperm (CBS) in ejaculates due to segregation of sex
chromosomes at meiosis. This expectation has been recently refuted. Here
we used Peromyscus leucopus to demonstrate that sex ratio is explained by
an exclusive effect of the father, and suggest a likely mechanism by which
male-driven sex-ratio bias is attained. We identified a male sperm
morphological marker that is associated with the mechanism leading to sex
ratio bias; differences among males in the sperm nucleus area (a proxy for
the sex chromosome that the sperm contains) explain 22% variation in
litter sex ratio. We further show the role played by the sperm nucleus
area as a mediator in the relationship between individual genetic
variation and sex-ratio bias. Fathers with high levels of genetic
variation had ejaculates with a higher proportion of sperm with small
nuclei area. This, in turn, led to siring a higher proportion of sons (25%
increase in sons per 0.1 decrease in the inbreeding coefficient). Our
results reveal a plausible mechanism underlying unexplored male-driven
sex-ratio biases. We also discuss why this pattern of paternal bias can be
adaptive. This research puts to rest the idea that father contribution to
sex ratio variation should be disregarded in vertebrates, and will
stimulate research on evolutionary constraints to sex ratios—for example,
whether fathers and mothers have divergent, coinciding, or neutral sex
allocation interests. Finally, these results offer a potential explanation
for those intriguing cases in which there are sex ratio biases, such as in
humans.