Does a Father's Social Environment Influence Their Sons' Sperm Sex Ratio? Potential for the Epigenetic Transmission of a Sex-Allocating Mechanism.

Recent investigations have demonstrated that males exposed to other males during development produce lower numbers of Y-chromosome bearing sperm. Despite the potential for legacy effects, the multigenerational implications of variation in the paternal social environment for the sperm sex ratio have not been investigated. Here, we exposed male house mice (fathers) to high-male or high-female density conditions during their sexual development and quantified the sperm sex ratio of their sons. Our analysis revealed that the sons of fathers reared under high-male density conditions, produced, on average, higher numbers of daughter-producing sperm compared to sons of fathers reared under high-female conditions. As environmental and genetic influences in sons were controlled for (common-garden breeding and family-based design), this result can be attributed to nongenetic inheritance. Although our experiment produced a significant result, we acknowledge that the difference in the sons' sex ratio was small and that further investigation with the application of a more sensitive sperm sex ratio quantification method may produce a more robust outcome. Nevertheless, our investigation demonstrates the potential for the intergenerational transmission of the sperm sex ratio. We discuss the intergenerational nature of the sperm sex ratio as an adaptive strategy for increasing paternal fitness within different social environments and highlight mechanisms that could account for this result.