Forgotten fathers: paternal influences on mammalian sex allocation

• A growing body of evidence suggests that the assumption that males produce an unbiased proportion of X- and Y-chromosome-bearing spermatozoa is not always valid with significant between- and even within-individual variation.
• Several factors result in consistent biases, including environmental contamination and rate of mating.
• Paternal sex allocation could operate antagonistically or synergistically with maternal sex allocation, potentially constraining maternal allocation, or extenuating biases.
• Studies on sex allocation should include paternal as well as maternal predictions.

Sex allocation research in mammals has focussed almost exclusively on mothers under the assumption that the male contribution is genetically determined during meiosis and therefore not under adaptive control. Although early studies on sperm traits suggested that sex ratios were at parity, technological advances have made analysis more reliable and cheaper. Subsequently, more studies have shown variation in the production of X-/Y-chromosome-bearing spermatozoa. We review the evidence that the X/Y ratio varies between and within individuals in mammals, and test whether there are consistent predictors of these sex ratio biases and finally discuss their implications for future studies on sex allocation.