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Research on mammalian sex allocation has focused almost exclusively on mothers under the implied assumption that the male contribution is neutral, with an equal proportion of X‐ and Y‐chromosomes resulting from meiosis during sperm production. Although early studies on sperm traits suggested that sex ratios were at parity, technological advances have made analysis cheaper and more reliable. Subsequently, more studies have shown variation in the X‐/Y‐chromosome‐bearing spermatozoa (CBS) ratio, usually in relation to unusual circumstances, like environmental contamination. Nonetheless, these are assumed to be abnormal, and studies on sex allocation still assume that biases in the sex ratio at conception arise maternally. We investigated sperm sex ratios in a mammalian model species, and found that sperm sex ratios at both the sample and population level show significant variation. We also found significant heterogeneity in the population, confirming that samples vary from each other. This challenges the current assumption that sperm sex ratio are meitoically controlled and shows variation under conditions where we would not expect it. These results raise the possibility of adaptive paternal control of sex allocation. We discuss the implication of this and the possible interaction with maternal sex allocation. This study confirms that complete sex allocation research requires the investigation into paternal contributions, as well as those of mothers.

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