Variation in the production of sons and daughters is a key life history variable and it is predicted that parents would be advantaged if they could adjust the sex ratio of offspring when fitness returns vary between the sexes in a condition, environment or gene-specific way. Investigating the possibility for adaptive sex allocation in mammals has focused 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 investigated sperm sex ratios in a mammalian model species, the laboratory mouse, using fluorescence-in-situ-hybridization and found that sperm sex ratios at both the individual and population level do not reflect the expected 50:50 ratio. We discuss the possibility for adaptive paternal control, and the subsequent interaction that this may have with maternal sex allocation. Sex allocation theories that are applied to mothers may equally pertain to fathers, and in some cases may be more parsimoniously explained through the possibility of paternal sex allocation.