Forgotten fathers: paternal influences on mammalian sex allocation

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.